chemical purification of peat harvesting runoff water
TRANSCRIPT
Process and environmental engineering department
Water resources and environmental engineering laboratory
Master‟s thesis
Chemical purification of peat harvesting runoff water
Oulu 21.04.2011
Author: _____________________________
Elisangela Heiderscheidt
Supervisor: _____________________________
Prof. Bjørn Kløve
University of Oulu
Advisor: _____________________________
D.Sc. (Chem.) Jaakko Saukkoriipi
Finnish Environment Institute - SYKE
Advisor: _____________________________
D.Sc. (Tech.) Anna-Kaisa Ronkanen
University of Oulu
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UNIVERSITY OF OULU Abstract of thesis
Faculty of technology
Department Laboratory
Process and environmental engineering department
Water resources and environmental engineering laboratory
Author Supervisor
Heiderscheidt, Elisangela Kløve, B., professor
Name of the thesis
Chemical purification of peat harvesting runoff water
Subject Level of studies Date Number of pages
Water protection engineering Master‟s Thesis 21/04/2011 102 + 5
Abstract
Peat production activities such as drainage of the peatland area and the exposal of peat layers are known to cause an
increase on the runoff water discharging from the production sites and an increase on the leaching of pollutant substances
into water bodies located downstream. The leaching of pollutant substances such as suspended solids, nutrients, toxic
metals and organic matter may result in the eutrophication and siltation of the receiving water bodies causing water
quality deterioration and a series of negative impacts to the local aquatic ecosystem.
Among the treatment methods developed over the past decades for the treatment of peat harvesting runoff water is the
chemical purification via application of metal salt coagulants. The high costs connected to the implementation of
chemical purification via addition of liquid pre-hydrolyzed coagulants have until now rendered this treatment method
economically feasible only to large production sites. Nevertheless, recent reports have estimated that the costs for the
implementation of chemical purification via addition of solid metal salts coagulants are 50 to 75% lower which would
also enable the application of this treatment method to smaller peat production sites.
The study carried out for this thesis work aimed on developing the chemical purification of peat harvesting runoff water
via application of solid metal salts coagulants. The objective was to evaluate the treatment efficiency achieved by the
application of four metal salt coagulants (aluminium sulphate, aluminium chloride, ferric sulphate and ferric aluminium
sulphate) in the purification of water samples from two different locations via a series of laboratory (jar test) experiments.
The goal was to identify among the tested chemicals the best coagulant, optimum dosages, and the influence of process
parameters such as pH, temperature and mixing, on load removal efficiency. The obtained purification efficiencies were
evaluated based on the percentage removal of total nitrogen (tot-N), total phosphorous (tot-P), phosphate phosphorous
(PO4-P), suspended solids (SS) and organic matter (TOC) from the water samples being purified.
All tested solid coagulants were found to produce high load reduction levels which are in agreement with the reported
purification efficiencies achieved by the chemical purification treatment method. The purification efficiencies achieved
regarding the removal of concerning substances from the tested water samples were: tot-N (15 – 44%), tot-P (67 – 90%),
PO4-P (63 – 93%), SS (48 – 96%) and TOC (20 – 62%). It was also determined that the purification of peat harvesting
runoff water is highly dependent on a series of process parameters such as: coagulant type and dosage, pH, temperature,
mixing and the physicochemical characteristics of the water.
Ferric sulphate (Ferix-3) proved to be the best performing coagulant. It required, under optimized mixing conditions,
around 15% lower dosages (60 to 70 mg/l) than the other tested coagulants (70 to 80 mg/l) for the purification of both
water samples. Furthermore, even at lower dosages, it achieved slightly higher load removal efficiencies. Ferric sulphate
also produced the best settling characteristics among the tested coagulants. It presented the fastest sedimentation rates and
the best final clarification of the supernatant water. However, solid ferric sulphate presented a very narrow optimum
dosage range and appears to have suffered to a higher extend the effects of low temperature (5°C) applied to the
purification process.
Library location
University of Oulu, Science and Technology library Tellus
Additional information
3
Acknowledgements
My deepest gratitude goes to those who made this thesis possible; the support I received
from my work colleagues, friends and family members has been nothing short than
unbelievable. First and foremost I would like to thank my supervisor Prof. Bjørn Kløve for
the opportunity of participating in this project and the guidance he has provided. A huge
thank you to my advisers Jaakko Saukkoriipi and Anna-Kaisa Ronkanen who have
throughout this process offered me priceless advises and moral support. My sincere
gratitude goes also to Vapo Oy the commissioner of this project. Vapo‟s financial support
has not only brought this project to its very existence but assured that I could concentrate
all my efforts to this thesis work.
An especial thank you to: Laëtitia Depre, Tiina Leiviskä and Tuomo Reinikka without
whom the laboratory phase of this project would not have succeeded. Thank you also to my
colleagues and friends in the Water Resources and Environmental Engineering laboratory
who helped me in the thesis writing process. Your feedback and tips have enriched the
contents of this work.
Last but not least, thanks to my Brazilian and Finnish families. To my mom, dad and three
beloved sisters who are geographically so far but so close to my heart. Thank you for every
word of encouragement and for your understanding of my distance. Thank you to my
partner for believing in me when I mostly didn‟t. There are no words to describe my love
for you, thank you for your support and for making me feel at home here. I cannot forget to
mention my four legged boy, thank you for not eating my shoes when I didn‟t have time to
take you for walks!
Elisangela Heiderscheidt
April, 2011
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Table of contents
Acknowledgements ............................................................................................................... 3
Symbols and abbreviations ................................................................................................. 6
1 Introduction .................................................................................................................. 8
2 Peat harvesting industry and related environmental impacts ............................... 10
2.1 Peat harvesting industry in Finland ....................................................................... 10
2.2 Environmental impacts of peat harvesting and usage ........................................... 11
2.3 Peat harvesting runoff water .................................................................................. 12
2.3.1 Peat harvesting runoff water treatment methods ............................................ 13
2.3.2 Chemical treatment of peat harvesting runoff water ...................................... 15
3 Processes involved in chemical purification ............................................................ 17
3.1 Coagulation process .............................................................................................. 17
3.1.1 Removal of particles ...................................................................................... 17
3.1.2 Removal of dissolved substances ................................................................... 21
3.1.3 Coagulation via addition of metal salts .......................................................... 21
3.2 Flocculation process .............................................................................................. 25
3.3 Sedimentation ........................................................................................................ 27
3.4 Chemical removal of phosphorous ........................................................................ 31
3.5 Chemical removal of dissolved organic matter ..................................................... 33
3.6 Parameters influencing chemical purification ....................................................... 34
3.6.1 Influence of temperature and pH.................................................................... 35
3.6.2 Influence of mixing ........................................................................................ 39
4 Environmental impacts of metal residual discharge .............................................. 43
5 Materials and methods .............................................................................................. 45
5.1 Coagulants ............................................................................................................. 45
5
5.2 Sample collection .................................................................................................. 46
5.3 Laboratory analyses ............................................................................................... 47
5.4 Jar test experiments ............................................................................................... 48
6 Results ......................................................................................................................... 55
6.1 Optimum dosage range and purification efficiency .............................................. 55
6.1.1 Optimum dosage range of tested coagulants .................................................. 55
6.1.2 Purification efficiency .................................................................................... 59
6.2 Settling characteristics ........................................................................................... 65
6.3 Influence of temperature........................................................................................ 67
6.3.1 Influence of temperature on purification efficiency ....................................... 68
6.3.2 Influence of temperature on settling characteristics ....................................... 71
6.4 Influence of mixing ............................................................................................... 74
7 Discussion .................................................................................................................... 78
8 Conclusion and future aspects .................................................................................. 93
References ........................................................................................................................... 97
6
Symbols and abbreviations
Al Aluminium
AlCl3.6H2O Aluminium chloride
Al2(SO4)3.14H2O Aluminium sulphate
BAT Best available techniques
C Colour [mg l-1
Pt]
ca Circa, meaning around, approximately.
Ca Calcium
C0 Initial concentration of particles in suspension
COD Chemical oxygen demand
D Diameter of mixer impeller [m]
d Effective particle diameter [cm]
DOC Dissolved organic matter
Fr Fraction of particles removes from solution
Fe Iron
Fe2(SO4)3.8-9H2O Ferric sulphate
g Acceleration due to gravity [cm s-2
]
G Velocity gradient [s-1
]
L Length of rectangular basin [m]
Me Metal element
n Mixer rotational speed [revolutions per second]
Nav. Navettarimpi sample
NOM Natural organic matter
P Power consumed by mixing device [Nm s-1
]
Piip. Piipsanneva sample
PO4-P Phosphate phosphorous
Q Flow rate [m3 s
-1]
rpm Revolutions per minute
SS Suspended solids
sol Coagulant in solution form
t Retention time [s]
7
T Turbidity [NTU]
ti Elapsed time [s]
TOC Total organic carbon
tot-N Total nitrogen
tot-P Total phosphorous
Tt Turbidity at time t [NTU]
ν Horizontal flow velocity [m s-1
]
V Volume of tank [m3]
vi Particle settling velocity for elapsed time ti [cm s-1
]
νs Settling velocity [cm s-1
]
W Dissipation function [N m-2
s-1
]
Yi Fraction of particles in suspension at time ti
YT Total amount of particles removed from solution
Z Depth [cm]
Dimensionless power number (mixer and tank geometry)
α Dimensionless factor reflecting particle shape
ρs Density of the particle [g cm-3
]
ρw Density of the water [g cm-3
]
µ Dynamic viscosity [g cm-1
s-1
] or [Ns m-2
]
↓ Formation of precipitate
↑pH Sample with raised pH
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1 Introduction
The quality of the water of our rivers and lakes has a direct impact in our environment and
water protection measures are an important factor on the road to sustainable development.
Several industrial activities produce point and diffuse load sources which discharge into our
streams resulting in water quality deterioration and the environmental impacts related to it.
It is extremely important that sound research coupled with the best available technologies is
applied in the development and optimization of water protection and pollution control
measures, so that good water quality characteristics can be maintained or returned to our
water systems.
Peat production is an important industry to the Finnish economy. However, the drainage of
peatlands and other peat harvesting activities are known to increase the amount of water
discharging from the catchment areas as well as the amount of pollutant substances being
leached from the sites into water courses. (Heikkinen & Ihme, 1995; Kløve, 2001) Over the
past decades several treatment methods have been developed and are now applied in the
purification of peat harvesting runoff water. Throughout this period, improvements to the
different methods surfaced from the increasing awareness within the industry regarding the
environmental impacts of the imposed loads and from stricter emission limits imposed by
Finnish authorities. Nevertheless, due to factors such as load concentration and volumetric
discharge variations, the purification levels achieved by the applied treatment methods do
not reach, in all production sites, the requirements set by the Finnish legislation. (Silvan, et
al. 2010) Further developments to all peat harvesting water treatment methods are required
to ensure the appropriate level of load reduction and the protection of water resources
surrounding peat production sites.
The aim of this Master‟s thesis project was to develop the chemical purification of peat
harvesting runoff water via application of solid metal salt coagulants. Although chemical
purification is considered one of the best available technologies for the treatment of peat
harvesting runoff water, it is mostly applied to large production sites due to its economic
viability. (Kløve, 1997) Nevertheless, the high investment costs linked to chemical
purification process are mainly due to the special treatment structures and facilities required
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in conventional chemical treatment using liquid pre-hydrolyzed metal salt coagulants.
According to Vapo Oy inter-company project report prepared by Alatalo and Peronius
(2004) the costs of chemical purification treatment implementation are 50 to 75% lower for
smaller scale treatment structures using solid coagulants. The optimization of chemical
purification via addition of solid coagulants can then render this treatment method feasible
also to smaller production sites.
The objective was to evaluate and optimize through a series of laboratory experiments the
purification efficiency achieved by the application of four different metal salt coagulants to
water samples collected from two different locations. The evaluation of the purification
efficiency was based on the percentage removal of concerning substances such as
phosphorous, nitrogen, suspended solids and organic matter from the water samples being
purified.
Of interest were the identification of the best chemical among the tested coagulants, the
optimum dosages to be applied and the influence of process parameters on load removal.
The optimization of the purification process was based on analyzes of the influence of
different process parameters on the purification efficiency. Including the influence of
coagulant dosage, the mixing effect applied upon and after coagulant addition and the
temperature and pH of the water. The main goal of the performed laboratory experiments
was the development of guideline information that could be used for the design and
implementation of new treatment structures and for the improvement of already existing
treatment facilities.
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2 Peat harvesting industry and related environmental impacts
Peatlands are estimated to cover around 10 % of the globe surface area. The drainage of
peatlands for forestry, agriculture and peat harvesting is an activity which has been
performed for centuries resulting in economically important increase in land use for e.g.
wood and bio-energy fuel production. However, despite the economical advantages
peatland drainage produces a series of negative impacts on the environment. (Marttila,
2010, p. 15)
2.1 Peat harvesting industry in Finland
Peatlands cover around 30% of Finland‟s surface area summing up to 9.3 million ha and
although 1.2 million ha are technically suitable for the peat industry and contain 29.6
billion m3 of peat in situ, less than 1% of the total peatland area is used for industrial
purposes. Nevertheless peat is a very important fuel source in Finland, 17 to 20% of the
district heat and combined heat and power energy is produced with peat. About one million
Finns live in areas where district heat is generated by combined peat and wood combustion.
Peat is also widely used in horticulture as a growing medium. Furthermore peat products
are suitable for many other purposes such as litter or absorbent peat, composting peat, frost
insulation, landfill structures and soil improvement. (Association of Finnish Peat Industries,
2010)
The peat harvesting industry is worth hundreds of millions of Euros to the Finnish
economy. At the moment, there are eight large and middle-sized peat producing companies,
about 250 small-size family businesses and hundreds of private contractors and
entrepreneurs in peat harvesting and logistics (Association of Finnish Peat Industries,
2010). Peat harvesting and usage employs, directly and indirectly, more than twelve
thousand workers a year. (Paappanen & Leinonen, 2010) Vapo Oy, the commissioner of
this thesis work, is the world‟s leading supplier of peat. The company is owned mostly by
the Finnish state (50.1%) and by Suomen Energiavarat Oy a consortium of Finnish energy
companies (49.9%). (Vapo, 2010)
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Regarding the usage of peat as a fuel source, Finland is in the up most positions in Europe
and in the world. The use of peat as a fuel source in Finland during the past decade has
fluctuated but was on average around 25 TWh, value which it is expected to increase to
about 29 TWh by 2030. Peat provides over 6% of Finland‟s primary energy requirements
decreasing Finnish dependency on energy production from imported fuels such as coal and
natural gas. Peat‟s role in the security of energy supply in Finland is of most significance.
(Paappanen & Leinonen, 2010)
2.2 Environmental impacts of peat harvesting and usage
According to the World Wide Fund for Nature (WWF) the burning of peat in the
production of power in Finland is responsible for approximately 10 million tons of
greenhouse gas emissions per year. The emissions caused by the burning of peat are almost
as large as the carbon dioxide emissions of the whole traffic sector in the country. (WWF,
2008)
In addition to green house gases emissions of peat usage, peat harvesting activities are also
responsible for a series of negative impacts imposed to the surrounding environment of peat
production sites. The immediate environmental impacts of peat production projects are
local direct impacts due to preparation of the mire and the production. These may include
for instance; removing of vegetation from the mire, alterations in the landscape,
disappearance or changing of bird nesting environments, disturbance of hydrological
balance, as well as noise and dust emissions. (Sopo, et al., 2002)
As a result of these direct impacts, indirect impacts are also observed. The removal of
vegetation results for example in changes to the area landscape causing loss of local
biodiversity and has a direct impact on the population living in the area. The drainage of the
site via ditching and the exposal of peat layers cause an increase in run-off water from the
site as well as changes in ground water supply in the area. The water discharging from peat
production sites, if left untreated, will carry with it organic substances, toxic metals,
nutrients and particulates. These pollutants will result in the eutrophication and siltation of
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the receiving water bodies causing among other negative effects water quality deterioration,
loss in fishing resources and increased risk of floods. (Sopo, et al., 2002)
Due to the importance and the scale of the peat harvesting industry in Finland, the
environmental impacts related to peat production have not only been observed in the past
decades but have also received special attention from the Finnish environmental protection
authorities. Legislation has been put into place in order to minimize the impacts, encourage
the use of more sustainable and environmentally friendly extraction methods as well as
pollution control measures. (Silvan, et al., 2010)
2.3 Peat harvesting runoff water
Drainage and other peat extraction activities are known to increase the amount of water
discharging from the catchment area as both base-flow and storm-flow (Foundation for
Water Research (FWR), 1993). Peat extraction activities are also responsible for the
increase in the leaching of suspended solids (SS), dissolved organic carbon (DOC) and
nutrients, especially phosphorous (P) and nitrogen (N) into watercourses located
downstream of the extraction site. (Silvan, et al., 2010) Although the phosphorus and
nitrogen load caused by peat harvesting is only about 1% of the total load to water systems
in Finland, locally it can have a significant effect on water quality. (Vieltojärvi, 2005) The
leaching of nutrients and suspended matter into sensitive water bodies can cause adverse
impacts such as eutrophication, siltation, loss of biodiversity and other symptoms of water
quality deterioration. (Heikkinen & Ihme, 1995; Kløve, 2001)
Finnish national water protection authorities proposed back in 1998 that by year 2005 a
65% reduction in SS and a 30% reduction in nutrients loads should be achieved over
emission levels of 1993. (Vieltojärvi, 2005) But according to Silvan, et al. (2010) overall,
only a reduction of ca 30% in SS load and 20% in nutrient load has actually been achieved.
This leads to the conclusion that although advances have been made improvements to all
applied pollution control measures or runoff water treatment methods are still needed.
13
2.3.1 Peat harvesting runoff water treatment methods
Water and wastewater treatment methods are purification methods designed to remove
substances from the water that are harmful to the environment and to human health. Most
common harmful substances or contaminants found in natural waters in general are: SS,
oxygen demanding substances, nutrients and heavy metals (Lindquist, 2003, p. 36). As
previously mentioned discharge waters from peat harvesting areas contain SS, nutrients and
dissolved substances. In order to prevent the deterioration of the water quality in the
receiving water bodies it is necessary to efficiently treat or purify the runoff waters from
peat harvesting areas.
The contamination load imposed by peat harvesting areas in watercourses is somewhat
difficult to predict due to the large load fluctuations observed in the monitored sites.
However, studies have linked the load dependence to: the moisture content of the peat in
the extraction area, variation in runoff and peaks in discharge (Kløve, 1997); peat types and
their degree of humification (Svahnbäck, 2007); and peat extraction methods (Silvan, et al.,
2010). Most important it is to note that different sites will require different treatment
methods designed to satisfy the requirements of the peat production process, site hydrology
and geology, the sensitivity of the receiving water bodies and the current legislation.
According to Kløve (2001), over the past decades several methods have been developed to
reduce the SS and nutrient load from peat harvesting runoff waters. However, Kløve also
affirms that the nutrient load being leached from peat harvesting areas, even with the
developed treatment methods in place, is still high and does not always meet the
requirements established by the Finnish authorities. Some of the developed and most used
treatment methods for peat harvesting runoff water are: constructed sedimentation ponds;
overland flow fields; peak runoff control dam and chemical purification. (Kløve, 2001;
Central Finland Regional Environment Centre (CFREC), 2004)
- Constructed sedimentation pond: consists of a pond dug in the proximities of the
peat harvesting area into which the runoff water is discharged. The retention time is
14
designed to allow the removal of suspended solids (and nutrients attached to it)
from the water. It has an average SS reduction of 30 to 40%. (CFREC, 2004)
- Overland flow field: consists of directing the discharge water of a peat harvesting
site into the surface layer of a natural bog or a peat bog. The vegetation of the
surface layer works as a mechanical filter separating solids and sludge from the
water. Dissolved nutrients are believed to be removed in the peat layer as a result of
chemical and biological processes. The average load reductions are: 55% (ditched
wetlands area) to 92% (natural wetland area) of SS, up to 49% of total nitrogen (tot-
N) and up to 46% of total phosphorous (tot-P). (CFREC, 2004)
- Peak runoff control dam: consists of a weir structure which controls the volume of
water discharged from the production site. While controlling water discharge rates
this structure also traps peat particles, erosion generated substances and nutrients as
well as acting as an auxiliary in ditch erosion reduction. Marttila and Kløve (2009)
reported the following load reductions for peak runoff control dams: 61-94%
reduction in SS, 45-91% reduction in tot-N and 47-88% reduction in tot-P.
- Chemical purification: chemical treatment method consists on the addition of
chemicals used for treatment of drinking water to the runoff water of a peat
harvesting area. The chemicals cause sedimentation of solids and dissolved
substances which deposit in the bottom of the sedimentation pond. The average load
reductions are: 30 - 90% of SS; 30 - 60% of tot-N and 75 - 95% of tot-P. (CFREC,
2004)
Peat harvesting companies are required to attain an environmental permit from Finnish
environmental authorities in order to be able to establish a new peat production site. The
permit is given on the basis of the Environmental Protection Act and the Water Act. For
sites over 150 ha an Environmental Impact Assessment (EIA) is carried out. The issued
environmental permit stipulates according to the best available technologies (BAT) and site
characteristics the water pollution control method to be applied and monitoring
requirements. (Hellsten, et al., 2008)
15
Overland flow field is the most used treatment method since 1985. (CFREC, 2004)
Chemical treatment is also considered one of the best available technologies for peat
harvesting runoff water purification and is now applied to sites where overland flow fields
cannot be constructed (e.g. lack of space) or do not achieve the required purification levels.
The author finds it important to emphasize here that although overland flow fields are
considered the most cost effective has the highest utilization rate, there is a huge potential
encased within the use of chemical purification treatment. High load removal efficiencies
are most certainly achievable if the process parameters are optimized. Chemical
purification can easily be combined to other treatment methods serving as the main load
reducer or as a final water quality polishing process.
2.3.2 Chemical treatment of peat harvesting runoff water
The chemical treatment of runoff deriving from peat production areas is based on the ability
of the chemicals to precipitate SS and dissolved substances, such as nutrients and organic
matter, present in the water. The development in the use of chemicals for the purification of
peat harvesting wastewater has been slow. Mostly to blame are the observed high costs
involved in the treatment implementation and maintenance, as well as its seasonal
application due to technical requirements such as the freezing of applied chemicals at
temperatures below 0 °C. (CFREC, 2004) Kløve (1997) affirmed that the chemical
treatment of peat harvesting wastewater, as developed to date, is economically viable only
for extraction areas larger than 200 ha.
All over the world metal salts of aluminium and iron are widely used in the chemical
purification of water and wastewater related to industrial and domestic usages. Specific
research in the application of metal salts coagulants for the purification of peat harvesting
wastewater is though scarce. A 2004 Vapo Oy internal project report regarding the use of
chemical coagulants (Alatalo & Peronius, 2004) was made available to this project. The
report contained results achieved by direct field application of two coagulants: aluminium
sulphate and ferric aluminium sulphate. Field experiments were carried out at Navettarimpi
16
peat production site over the summer periods of 2002 and 2003 with aluminium sulphate
and during summer 2003 with ferric aluminium sulphate. Reported dosages of 51 to 107
g/m3 of aluminium sulphate produced average removal efficiencies of 40 to 45% in SS, 27
to 32% in tot-N, 72% in tot-P and 48% in COD (chemical oxygen demand) concentrations.
Dosages of 66 to 165 g/m3 of ferric aluminium sulphate were required to achieve average
load reductions of around 78% in SS, 39% in tot-N, 88% in tot-P and 66% in COD. (Alatalo
& Peronius, 2004)
The use of iron based coagulants in field tests have also been reported by the Finnish
Environmental Institute (SYKE) (2005), but unfortunately no information is presented
about the field application conditions, applied dosages or achieved results. It is assumed
that the purification levels reported by CFREC (2004) and previously described in this
thesis work are also results obtained in field applications.
As previously stated, of interest here is the development in the use of solid metal salts
coagulants. According to Alatalo and Peronius (2004) the costs of implementation for the
application of solid coagulants is around 50 to 75% lower when compared to the
conventional purification stations using liquid coagulants. It is of the author‟s opinion that,
via the optimization of chemical purification process parameters it is also possible to create
process design guidelines which can reduce the chemical purification method maintenance
costs, by reducing chemical dosages and enhancing purification efficiency.
17
3 Processes involved in chemical purification
Coagulation, flocculation and sedimentation processes are chemically and physically
induced purification methods used to remove contaminant substances from the water.
Contaminant substances may occur as SS, including colloidal particles with diameter of
0.08 to 1 µm, and dissolved matter. (Lindquist, 2003, pp. 115-120)
3.1 Coagulation process
Coagulation process is a chemically induced destabilization process used in water and
wastewater treatment with the objective of removing from solution non settleable
contaminant substances occurring as particulates and dissolved matter. The coagulation of a
given solution is achieved via addition of chemical coagulants such as metal salts of iron
and aluminium, activated silica, clays, lime, natural and synthetic organic polymers, etc.
(Sincero & Sincero, 2003, pp. 549-552)
3.1.1 Removal of particles
There are two main classes of colloidal particles named hydrophobic (low degree of affinity
with water) and hydrophilic (high degree of affinity with water) colloids. The amount of
water bounded to hydrophilic particles can account to up to ten times the particles dry mass.
Although the hydrophobic term establishes no affinity with the water, hydrophobic particles
also possess a layer of water molecules strongly bounded to their surface. (Bratby, 2006,
pp. 9-10)
Colloidal and smaller size particles when in solution are capable of remaining in a disperse
state due to electrical repulsive forces acting between them and to some extend to the
hydration of the particles surface layer. The term stability refers to this ability of colloidal
particles to remain as independent entities within a given dispersion. (Bratby, 2006, pp. 3-
4)
18
The repulsive property of colloidal particles is due to the electrical forces that they posses.
The electrical forces are produced as a result of charges, called primary charges, that the
particles posses at their surfaces. The particles primary charges may originate due to two
factors; the dissociation of the polar groups and the preferential adsorptions of ions from
solution. The primary charges of hydrophobic colloids are mainly due to the adsorption of
ions from the medium while the primary charges of hydrophilic colloids are mostly related
to polar groups such as carboxylic and amine. Depending on the pH of the solution colloids
may attain positive or negative surface charges (Figure 1). (Sincero & Sincero, 2003, pp.
547-548)
Figure 1 – Primary charges of hydrophilic colloid as a function of pH (Sincero & Sincero,
2003, p. 548)
The colloidal particles, if their primary charges are strong enough, attract counter ions in
the solution which form a compact layer, called stern layer, around the particle surface.
Note that the stern layer also contains water molecules and adsorbed hydrated ions. The
ions forming the stern layer then attract their own counter ions from the solution and form a
looser layer called the diffuse layer (Figure 2). The stern and diffuse layers form the so
called electrical double layer of the colloidal particle. When the colloidal particle moves not
all charges move with it, only a part of the diffuse or outer layer moves with the particle
shearing at a shear plane. Because the surface charges are electrical they posses
electrostatic potential which is greatest at the particle surface (Nernst potential) and
decreases to zero at the bulk of the solution (Gouy Chapman layer). The electrostatic
19
potential at a distance from the particle surface at the location of the shear plane is called
zeta potential. The greater the zeta potential the greater is the force of repulsion between the
colloidal particles and more stable is the solution. (Sincero & Sincero, 2003, pp. 546-548)
To destabilize a colloid its zeta potential must be reduced. The reduction of the zeta
potential can be achieved by the addition of chemicals. The added chemicals should contain
counter ions of the colloidal particles primary charges, which upon addition will neutralize
these charges and consequently reduce the zeta potential and enable the occurrence of the
coagulation process. (Sincero & Sincero, 2003, pp. 550-551)
Figure 2 - Conceptual representation of the electrical double layer. (Bratby, 2006, p. 18)
A complete coagulation process is a combination of four destabilization mechanisms which
include; double layer compression, charge neutralization, entrapment in a precipitate and
intra-particle bridging. (Lin & Lee, 2007, p. 376) For the compression of the electric double
20
layer to occur and culminate on the coagulation of the colloidal particles, counter ions of
the primary charges must be added until the Van der Waals force of attraction between the
particles exceed the repulsion forces due to their primary charges. Direct charge
neutralization is triggered by the addition of ions of opposite charges that have the ability to
direct adsorb to the colloid surface. Entrapment in a precipitate, also known as sweep
coagulation occurs when cations of a metal salt forms hydroxide precipitates using colloidal
particles as nucleation sites entrapping the colloid in the precipitates. Furthermore as the
precipitate sediments it carries down with it a large number of other colloidal particles.
Intra-particle bridging or patch coagulation takes place when bridging molecules, mainly
polymeric molecules, attach a colloidal particle to one active site and a second colloidal
particle to another active site. If the active sites of the polymeric molecule are close to one
another, coagulation of the colloidal particles then occurs. (Faust & Aly, 1999, pp. 221-223;
Sincero & Sincero, 2003, p. 551)
Chemical coagulants are substances referred to as electrolytes and polyelectrolytes.
Electrolytes are materials that when placed in solution cause the solution to be conductive
of electricity due to the charges they posses. Polyelectrolytes are polymers possessing more
than one electrolytic site. Electrolytes and polyelectrolytes are able to coagulate and
precipitate colloids due to the charges they posses. In natural waters, due to their acidic
nature, most particles are negatively charged therefore they repel each other and remain
disperse in the liquid if no destabilizing substance or electrolyte is applied. Metal salts of
iron and aluminium are commonly used coagulants in water and wastewater treatment.
(Lindquist, 2003, pp. 121-124)
An important phase of the coagulation process is the addition of the chemical coagulant
into the stable solution or wastewater it needs to destabilize. The mixing of the coagulant is
an important operation for the coagulation process, rapid and throughout mixing provides
complete and uniform dispersion of the coagulant added to the water, enabling the four
destabilization processes to occur and effective coagulation to be achieved. (Sincero &
Sincero, 2003, p. 553; Lin & Lee, 2007, p. 377)
21
3.1.2 Removal of dissolved substances
Dissolved substances in water and wastewater include: orthophosphates; natural organic
matter (NOM) including humic substances and other organic dissolved material such as
carbohydrates and sterols, etc. (Lindquist, 2003, p.116) Although traditionally the
coagulation process is described in terms of the destabilization of colloidal solutions (as it
has been done in the previous section), coagulation process is also responsible for the
removal of dissolved substances via direct precipitation or adsorption onto precipitates of
metal hydroxide. (Lamsal, 1997)
The precipitation of particulates and dissolved matter from wastewater follow different
chemical rules. Consequently, different coagulants will present different relative
efficiencies on the removal of particulates and dissolved substances. Metal salts of
aluminium and iron have also been showed to efficiently remove dissolved substances such
as NOM and phosphates from wastewater. (Lindquist, 2003, pp. 122-124; Jiang & Wang,
2009) The efficiency observed on the removals of dissolved substances will depend on the
type and dosage of coagulants, coagulation pH, water temperature, concentration of NOM
and other wastewater characteristics such as alkalinity. (Omoike & Vanloon, 1999; Jiang &
Wang, 2009)
3.1.3 Coagulation via addition of metal salts
The most used chemical coagulants in water and wastewater treatment are salts of
aluminium and iron, not only for their effectiveness but also for their ready availability and
lower cost (Bratby, 2006, p. 32). Hence the effectiveness of selected aluminium and iron
salts as coagulant agents for the chemical purification of runoff water from peat harvesting
sites is the focus of this work; the mechanisms or reactions involved in the coagulation
processes of aluminium and iron salts are further detailed in the following sections.
22
Coagulation mechanism of metal salts
The mechanisms or reactions involved in the coagulation process with metal salts will be
represented here by reactions related to aluminium sulphate (alum) and ferric sulphate.
Alum is the most used salt of aluminium in water and waste water treatment and its
chemical formulation is: Al2(SO4)3 . xH2O, with x assuming values from 13 to 18 and
referring to the hydration of the salt. Ferric sulphate chemical formulation is: Fe2(SO4)3.
xH2O, with x assuming values from 7 to 9. (Sincero & Sincero, 2003, p. 568) For brevity
aluminium sulphate and ferric sulphate will be referred to as Me2(SO4)3 without the water
of hydration and with „Me‟ representing the salt‟s metal element.
When metal salts are added to water they dissociate and react with water molecules
(hydrolysis) according to equations (1) to (7). These reactions occur very quickly and all
reactions are completed within few seconds forming between other complexes and
polymeric species the precipitate metal hydroxides Me(OH)3↓ (the downward pointing
arrow represents the formation of the precipitate). (Sincero & Sincero, 2003, p. 554-561;
Gregory & Duan, 2001
Me2(SO4)3 → 2[Me(H2O)6]3+
+ 3SO42-
(1)
Note that in the subsequent reactions the H2O molecules of the formed complexes are
omitted for simplicity
Me3+
+ 3H2O ↔ Me(OH)3↓+ 3H+ (2)
Me3+
+ 2H2O ↔ Me(OH)2+ + 2H
+ (3)
Me3+
+ H2O ↔ Me(OH)2+
+ H+ (4)
Me(OH)2+ + H2O ↔ Me(OH)3↓ + H
+ (5)
Me(OH)3 + H2O ↔ Me(OH)4- + H
+ (6)
23
2[Me(OH)]2+
+ 2H2O ↔ [Me2(OH)2]4+
+ 2H+ (7)
Further polymerization of metal complexes also occurs
As it can be seen, these reactions are complex and involve dissolution, hydrolysis and
polymerization of the metal salt. It is important to note that the hydrolysis reactions are
primarily dependent on the pH of the solution (Figures 3 and 4). According to Saukkoriipi
(2010, p. 22) for example, pH affects not only to the speciation of the mononuclear
aluminium species but also to the speciation and formation of polynuclear aluminium
hydroxide complexes
The complete but simplified equation including the reactions of the metal salt with the
alkalinity present in the water is shown below in equation (8). According to this equation an
alkaline substance is required when metal salts are added to the water. The bicarbonate
alkaline is used since it is the alkalinity that is always found in natural waters. (Sincero &
Sincero, 2003, p. 568)
Me2(SO4)3. xH2O + 3Ca(HCO3)2 →2Me(OH)3↓ + 6CO2 + 3Ca SO4 + xH2O (8)
The complex ions Me3+
, MeOH2+
, Me(OH)2+, Me(OH)4
- together with the formed
polymeric metal species are effective charge neutralizers. Due to the fact that the hydrolysis
reactions occur in fast rates and that the likelihood of the trivalent metal cations finding and
reacting with water molecules is much greater than the likelihood of them first reacting
with contaminants particles or molecules, the coagulation mechanism which prevail under
natural water conditions is sweep coagulation via metal hydroxide (Me(OH)3↓) precipitate
formation. (Lindquist, 2003, p. 126)
Sincero and Sincero (2003, p. 555) affirmed that for the effective removal of colloids as
much metal sulphate as possible should be converted to the solid precipitate Me(OH)3↓ and
as much of the concentration of the complex and polymeric ions formed should neutralize
the primary charges of the colloids to induce their destabilization. The diagrams proposed
by Amirtharajah and Mills (1982, cited in Bratby, 2006, p. 85) and Johnson Amirtharajah
(1983, cited in Bratby, 2006, p. 86) shown in Figures 3 and 4 also suggest that the best
24
coagulation condition for conventional treatment with aluminium and iron salts would be in
the region of Me(OH)3↓ precipitation and optimum sweep floc formation.
Figure 3 - Coagulation domain diagram for aluminium sulphate (Bratby, 2006, p. 85)
Lindquist (2003, p. 20) wrote that in conventional water and wastewater treatment where
the objective is the removal of particulates the focus should be directed to the sweep
coagulation mechanism hence it is often difficult to achieve rapid and through mixing
required for the charge neutralization mechanism to occur. The aforementioned author also
established that the optimum pH for the sweep coagulation mechanism to occur with
aluminium and iron salts varies consecutively between 5.5 and 6.5 and 5.5 to 8.
According to Sincero and Sincero (2003, p. 569) it is nevertheless impossible to attain the
optimum coagulation pH and aluminium dosage for the purification of a particular water
from the presented metal salts reactions due to their complexity. Consequently these
coagulation parameters must be determined in laboratory via jar test experiments.
25
Figure 4 – Coagulation domain diagram for ferric chloride (Bratby, 2006, p. 86)
3.2 Flocculation process
The flocculation process follows the rapid mixing stage where the destabilization reactions
occur and the primary flocs are formed. Flocculation is a physically induced process which
aims to promote the growth of the primary flocs by enabling them to aggregate and form
larger agglomerates. These agglomerates can easily be removed by a subsequent separation
process such as sedimentation or flotation. (Vigneswaran & Visvanathan, 1995, p. 61;
Bratby, 2006, p. 240)
Flocculation is a very important process within chemical purification. An effective
flocculation process will produce flocs with good settling characteristics enabling an
effective subsequent solid liquid separation. There are two distinct processes within the
flocculation process; perikinetic flocculation which arises from thermal agitation of the
fluid and orthokinetic flocculation which arises from induced velocity gradient in the
liquid. (Bratby, 2006, pp. 240-241)
26
Perikinetic flocculation
Perikinetic flocculation is the aggregation of particles due to the random Brownian
movement of fluid molecules. Particles under Brownian motion move and collide with
other particles, forming progressively larger agglomerates until the flocs reach a size
beyond which Brownian motion has little or no effect. This flocculation process starts
immediately after destabilization and it is complete within seconds. (Vigneswaran &
Visvanathan, 1995, p. 61; Bratby, 2006, pp. 240-241)
Orthokinetic flocculation
Orthokinetic flocculation is the agglomeration of particles due to induced fluid motion. By
inducing gentle motion and creating velocity gradients within the water the suspended
particles are encouraged to make contact and form larger agglomerates. (Vigneswaran &
Visvanathan, 1995, p. 62) According to Bratby (2006, p. 243) the greater the velocity
gradients the more particle contacts there will be in a given time. However, velocity
gradients above a critical value will result in small flocs due to the higher rate of breakage
of the larger formed flocs. Bratby also affirmed that, the lower the velocity gradient the
larger will be the final floc size, although it will take longer for the larger flocs to form.
Velocity gradients can be induced by setting the liquid in motion using a wide range of
available mixers, among them are: mechanical mixers such as back mixers; hydraulic
mixers such as baffled channel and gravel bed mixers. (Vigneswaran & Visvanathan, 1995,
p. 64) Lin and Lee (2007, p. 380) reported that a mean velocity gradient ranging from 20 to
70 s-1
together with contact times from 20 to 30 minutes should be kept during the
orthokinetic flocculation process.
27
3.3 Sedimentation
Sedimentation is the removal of settleable solids by the effect of gravity. It is essentially, a
solid-liquid separation process which follows the coagulation and flocculation processes.
The process takes place in a sedimentation basin which design parameters are obtained
from purification process characteristics and requirements such as water inflow rates,
settleability of the suspended solids formed during flocculation and space availability.
(American Water Works Association (AWWA), 2003, p. 83) The seatleability of the
suspended solids is characterized by the settling velocity of the particles (flocs) in
suspension. Sedimentation aims to remove the flocs formed and produce a clarified
overflow liquid.
The sedimentation process or the settling of the suspended solids contained in the water is
directly influenced by the characteristics of the water, the system hydraulics and the
characteristics of the particles in suspension. These characteristics include the temperature
of the water (which influences its properties), the settling basin geometry and overflow rate,
the specific gravity of the material in suspension, and the size and shape of the suspended
particles. (AWWA & American Society of Civil Engineers (ASCE), 1990, p. 111)
The various regimes observed within the settling of particles are mostly referred to as
settling types 1 to 4 which are defined according to AWWA (1990, pp. 371-372) as follow:
- Type 1: Settling or sedimentation of discrete particles in low concentration, with
flocculation and other inter-particle effects being negligible.
- Type 2: Settling or sedimentation of particles in low concentration but with
flocculation. As flocculation occurs, particles masses increases resulting in faster
settling rates.
- Type 3: Zone settling or sedimentation under the condition where the particles
concentration cause inter-particles effects to the extent that the rate of settling is a
function of particle concentration. Zones of different particle concentration may
develop because of the differences in the particles settling velocity.
28
- Type 4: Compression settling or subsidence under the layers of zone settling. The
rate of settling depends on the residence time and weight of the solids in the above
layers.
The sedimentation of flocculent systems is a complex process where the settling velocities
of the particles in suspension change with time and depth as the particles agglomerate and
form larger flocs. An accurate theoretical analysis of the sedimentation process is also made
complicated by the fact that the particles involved are not regular in shape, density or size.
The theory related to the settling of particles in ideal systems is nevertheless applied and
can serve as a useful guide in the interpretation of such complex systems. (AWWA, 1990,
p. 372)
For most theoretical computation of settling velocity an ideal system of discrete particle
settling with flocculation and other interparticle effects being negligible is assumed.
(AWWA, 1990, pp. 371-383) The settling velocity of a discrete particles for Re <1 is given
by equation (9) known as Stokes‟ law. (Chapra, 1997, p. 300)
(9)
Where:
νs = settling velocity (cm s-1
)
α = dimensionless factor reflecting the particle‟s shape (for spheres α = 1)
g = acceleration due to gravity (cm s-2
)
ρs = density of the particle (g cm-3
)
ρw = density of the water (g cm-3
)
µ = dynamic viscosity (g cm-1
s-1
)
d = effective particle diameter (cm)
In practical applications when suspensions of non-uniform particles in flocculent systems
are concerned, where the particles or flocs densities are mostly unknown, settling velocities
cannot be determined via Stokes‟ law. Practical tests should then be applied in order to
determine the particles settling rate. The most common test performed is the column
29
settling test which proceeds as follows: The water sample or suspension of interest is placed
in a tall clear column and the descending level between the suspension and clear water
interface is recorded at frequent intervals. The results are then plotted producing what is
known as the suspension settling curve. (AWWA, 1990, p. 383)
Bratby (2006, pp. 289-295) affirms that jar tests are an important tool in determining unit
process design parameters. As aforementioned, jar tests may be used for the determination
of coagulant dosage, coagulation process pH, etc. Bratby also affirms that due to the
fragility of the flocs formed via chemical induced coagulation flocculation processes, the
results obtained by column settling tests are not as reliable, for design purposes, as the
results obtained from settling tests performed in jar tests reactor with samples taken from
one depth point only.
The procedure for evaluating particle settling velocities using a jar test reactor is as follows:
The jar test reactor (beaker) is filled with the correct volume of the water sample
(suspension). The selected coagulant is then added to the sample and optimized mixing
parameters for coagulation and flocculation processes are adopted. After the flocculation
time has elapsed and the mixing is stopped the sample is allowed to stand. Samples are
taken at the same depth at suitable periods determined by previous observation of the
suspension under study. The samples are then analyzed for turbidity and the mean settling
velocity of the particles (flocs) in the sample is determined by standard procedure as follow
(Bratby, 2006, p. 294):
- Samples are taken at one depth „Z‟ at different times „t‟
- Particles with settling velocity great enough to carry then passed the sampling point
within a time „ti‟ will not be present in the sample taken at time „ti‟. Hence all
particles in the sample have settling velocities „νi‟ less than or equal to:
(10)
- „Yi‟ is the fraction of particles in the original suspension with settling velocity less
than „νi‟ and is defined by equation (11) below where „Ci‟ is the concentration of
30
particles in the sample taken at time „ti‟ and „C0‟ is the initial concentration of
particles in the suspension
(11)
- If „Y0‟ is the fraction of particles with velocity less than „ν0‟then the fraction 1 – Y0
of particles will be completely removed. For slow settling particles they will be
removed in the ratio νi/ν0 and the fractional removal „Fr‟ of these particles will be:
(12)
- The total removal is thus:
(13)
- The settling test yields a distribution curve like the one presented in Figure 5. The
integral expression of equation (13) corresponds to the shaded area in the figure and
may be determined graphically. (Bratby, 2006, p. 294)
Figure 5 – Settling test distribution curve, settling curve. (Bratby, 2006, p. 294)
31
3.4 Chemical removal of phosphorous
Phosphorus is normally present in natural waters and it is often the limiting nutrient for
plants and microorganisms. Therefore the leaching of phosphorous into fresh water system
such as lakes and rivers can cause eutrophication. Phosphorous can be found in soluble or
particulate form in drainage waters and the aim of phosphorous precipitation is to convert
its soluble form into insoluble so that a separation process such as sedimentation can be
applied removing phosphorous from solution. Soluble phosphorous forms include
orthophosphates, polyphosphates, pyrophosphates and organic phosphates.
Orthophosphates and other condensed forms are known to form insoluble salts with a
number of metal ions including aluminium and iron. The actual form of the reactions
between metal ions and phosphorous depend on a number of factors such as: the
concentration of the metal and phosphate ions, pH of the solution, and presence of other
reactants such as sulphates, carbonates and organic species. Chemical precipitation removes
phosphorous as orthophosphates and particulates the easiest. Polyphosphates and organic
phosphorous are also know to participate in precipitation and adsorption reactions although
not as readily as the other phosphorous species. It is important to note that orthophosphate
is the type of phosphorous that plants can readily assimilate. (Bratby, 2006, pp. 124-125)
According to Sincero and Sincero (2003, pp. 631) when aluminium and iron salts are added
to the water, the dissociated trivalent metal cations (Al3+
and Fe3+
) will react with
phosphate ions to precipitate the metal phosphates while also reacting with water molecules
to precipitate the metal hydroxides and to form complexes. Significant direct precipitation
of metal phosphate salts via reaction with the trivalent metal cations is nevertheless
restricted under the slightly acidic pH of natural waters. As it can be seen in Figure 6 the
concentration of phosphate ions in solution is only significant at pH values well above
neutral.
32
Figure 6 – Distribution of phosphorous species as a function of pH. (Bratby, 2006, p. 124)
Georgantas and Grigoropoulou (2006) stated that the main mechanisms of phosphorous
removal are: 1) the incorporation of phosphate to the solids in suspension their subsequent
removal; 2) The direct adsorption of phosphate ions on the hydrolyzes products of the
added metal salt coagulants; 3) The formation of insoluble metal (Me) phosphates salts
where the basic reactions occurring are described in equation (14).
Me3+
+HnPO4n-3
↔ MePO4 + nH+ (14)
Lindquist (2003, p. 127) affirms that the efficiency of phosphate removal via adsorption by
metal hydroxides is considerably less than that of direct precipitation with Al3+
and Fe3+
and their hydrolysis products. Figures 7 and 8 consecutively illustrate the influence of pH
when precipitating orthophosphates with constant dosages of aluminium and ferric
sulphate. Orthophosphates were precipitated by adding concentrated aqueous solution of
metal salts (0.25 mmol of Al3+
and Fe3+
as aluminium and ferric sulphate) to solutions of
orthophosphate (0.25 mmol) and by adding solution of orthophosphate to 20 minute old
formed metal hydroxides. It is clear that more orthophosphates were precipitated via direct
precipitation than via hydroxides. Furthermore, orthophosphates were also more effectively
removed by both aluminium and iron salts under acidic conditions or at pH values lower
than 6. (Lindquist, 2003, p. 127)
33
Figure 7 – The influence of pH on the Figure 8 – The influence of pH on the
percentage of precipitated orthophosphate percentage of precipitated orthophosphate
via aluminium sulphate addition via ferric sulphate addition (Lindquist,
(Lindquist, 2003, p. 127) 2003, p. 127)
3.5 Chemical removal of dissolved organic matter
The presence of NOM in natural waters is mostly associated with humic substances
originating from the extraction of living wood substances, the solution of incomplete
degradation products in decaying wood and the solution of soil organic matter (Bratby,
2006, p. 87). These substances assign the water a characteristic dark brownish colour
referred to as organic colour.
The use of chemical induced coagulation processes for the removal of NOM is widely
applied especially in the purification of water for drinking purposes. Just as for the removal
of particulates, the most common coagulation agents used are salts of aluminium and iron.
The removal efficiency of NOM is dependent on factors such as: the nature and
concentration of the NOM, the type and dosage of coagulant and the pH and temperature of
the solution. (Libecki & Dziejowski, 2008)
34
The coagulation mechanisms which contribute to the removal of NOM include (Bratby,
2006, p 93):
- Charge neutralization-precipitation, consisting of the reaction between soluble
polynuclear metal coagulants species and humic substances.
- Simultaneous precipitation consisting of charge neutralization-precipitation
reactions, and reaction with metal hydroxides precipitates, occurring
simultaneously.
- Adsorption of humic substances to metal hydroxides surface by van der Waals
interactions, etc.
According to Jiang and Wang (2009) studies conducted on the efficiency of organic colour
removal via coagulation using metal salts have concluded that the optimum pH for ferric
salts are within the range of 3.7 to 4.2, the optimum pH for aluminium sulphate is within
the range of 5.0 to 5.5 and the optimum dosage for achieving the lowest residual colour, in
molar terms, are mostly the same for aluminium and iron. However, Bratby (2006, pp. 109-
110) emphasizes that optimum conditions should be determined via comprehensive jar test
experiments, so that the best combination of pH, chemicals and dosages can be found not
only for the removal of the NOM but also for achieving the overall desired purification
levels.
3.6 Parameters influencing chemical purification
It has become clear throughout the previous sections where individual aspects of the
coagulation, flocculation and sedimentation processes were evaluated that a series of
factors or process parameters can exert great influence on the purification process outcome.
Among these parameters are: temperature, pH of the solution, mixing effect applied upon
and after chemical addition and the type and dosage of added coagulant
35
3.6.1 Influence of temperature and pH
The pH of the water sample to be treated may be the single most influential parameter
affecting the chemical purification. As previously stated in this work, the primary charges
of particles and molecules present in water are directly dependent on the pH of the solution.
Under slightly acidic conditions such as in the case of natural waters, particles and
molecules posses mainly negative charges. When the metal salts coagulants are added to
the water they cause the pH of the solution to decrease due to the consumption of alkalinity
during the coagulant hydration. The pH of the solution upon metal salt coagulant addition
will affect the dissolution of the coagulant, the molar fraction of the different hydroxide
species formed and their charges and consequently will directly affect the occurring
coagulation mechanisms. (Bratby, 2006, pp. 42-85)
Low temperatures affect coagulation, flocculation and sedimentation processes by altering
the coagulants solubility, increasing water viscosity, and retarding the kinetics of hydrolysis
reactions and particle flocculation. (Bratby, 2006, pp. 171-173) As previously mentioned,
when metal coagulants are added to the water they dissociate and the trivalent metal cations
undergo hydrolysis forming not only the precipitate hydroxides Al(OH)3↓ and Fe(OH)3↓
but also other dissolved hydroxides. The molar fraction distribution of dissolved species in
solution is not only pH but also temperature dependent (Pernitsky & Edzwald, 2006). In
consequence the coagulation process which is dependent on the concentration distribution
of the species formed is also temperature and pH dependent. Figure 9 illustrates general
trends in the distribution of dissolved aluminium species as a function of pH and
temperature while Figure 10 presents the molar fractions of iron species as a function of
pH.
36
Figure 9 - Theoretical distribution of Al species in solution as a function of pH and
temperature. (Pernitsky & Edzwald, 2006, p. 124)
Figure 10 - Distribution of monomeric Fe hydrolysis products as a function of pH.
(Gregory & Duan, 2001, p. 2019)
37
It is important to highlight that the species distributions showed in Figures 9 and 10 are not
meant to be definite under all conditions. The actual distribution is affected by the degree of
polymerization of the primary formed monomeric species and the presence in the solution
of other aluminium and iron complexing species such NOM, phosphate and sulphate ions.
(Pernitsky and Edzwald, 2006)
The solubility of the formed aluminium and iron hydroxides species is as well dependent of
the pH and temperature of the solution. According to Bratby (2006, p. 172) with decreasing
temperature, the minimum solubility of aluminium hydroxide precipitate shifts to a higher
pH, just as the optimum coagulation pH also shifts towards a higher value. The shift in pH
for the minimum solubility of aluminium and iron hydroxides (Al(OH)3↓ and Fe(OH)3↓)
was also reported by Pernitsky and Edzwald (2006) and Lim-Seok Kang and John (1995)
where the theoretical solubility diagram for aluminium at 20°C and 5°C and for Iron (III) at
25°C and 5°C in deionised water were extracted and are presented in Figure 11.
The applied dosage of coagulant also influences the pH at which the lowest residual iron
and aluminium concentration is obtained. Increasing the amount of coagulant added
increases the pH range which gives the lowest residual concentration of aluminium or iron
and also shifts it in the basic direction. Therefore, the pH which gives the lowest residual
concentration of aluminium or iron should be determined experimentally for a specific
water type at the actual process temperature and applied metal salt dosage. (Lindquist,
2003, p. 145)
Regarding the effects of temperature, studies have shown that different coagulants are
affected differently by temperature fluctuations and, although overall low temperatures
affect both aluminium and iron salts coagulation performances, it appears that iron salts are
affected to a lesser extent than aluminium salts. (Bratby, 2006, p. 171)
38
Figure 11 – (a) Theoretical solubility diagram for aluminium at 5 and 20 °C (Pernitsky &
Edzwald, 2006, p. 124). (b) Theoretical solubility diagram for iron (III) at 5 and 25 °C
(Lim-Seok Kang & John, 1995, p. 894)
25 °C 5 °C - - - - - -
Fe(OH)3 (s)
-lo
g c
oncentr
atio
n
(b)
(a)
39
Morris and Knocke (1984, cited in Bratby, 2006, p. 171) affirmed that the effect of
temperature appears to be more pronounced when the coagulation mechanism relies on
enmeshment or sweep coagulation by the metal hydroxides precipitate. Bratby (2006, p.
171) complemented this statement by affirming that „low temperatures (1 °C) do not inhibit
the rate of metal-hydroxide precipitation but have a detrimental effect on floc formation
characteristics‟. Low temperatures result in smaller flocs inhibiting the enmeshment
mechanism of particle removal, which is especially important for low turbidity waters.
3.6.2 Influence of mixing
The effective mixing applied to the solution throughout the coagulation and flocculation
processes have direct impact on the purification levels achieved by the chemical
purification process. The mixing effect applied during the coagulation process is referred to
as rapid mixing while slow mixing is the term used for the mixing applied during the
flocculation process.
Velocity gradients within a liquid mass can be induced by setting the liquid in motion using
a variety of mixers such as: mechanical mixers, hydraulic mixers, pump mixers, etc.
(Vigneswaran & Visvanathan, 1995, p. 64)
The root mean square velocity gradient (G) for any given type of mixer is determined using
equations (15) and (16) presented below (Bratby, 2006, p. 261).
(15)
(16)
Where:
G = velocity gradient (s-1
)
V = volume of flocculation tank (m3)
40
P = power consumed by mixing device (Nm s-1
)
W = dissipation function (N m-2
s-1
)
µ = absolute viscosity, 10-3
Ns m-2
for water at 20 °C
For a mechanical mixer with rotating blades the power „P‟ drawn by the device is
determined by its rotational speed and the geometry of the tank in which it operates. The
power consumed by such devices was defined by Leentvaar and Ywema (1980, cited in
Bratby, 2006, p. 261) and is given by equation (17). Equation (18) is the resulting root
mean square velocity gradient equation for mechanical mixers with rotating blades.
(17)
(18)
Where:
G, V, P, W and µ are defined as above and;
= dimensionless power number related to mixing device and tank geometry
ρ = liquid density (kg m-3
)
n = mixer rotational speed, revolutions per second
D = diameter of mixer impeller (m)
The time of contact or retention time in the mixing unit or basin is given by equations (19)
and (20) described below. (Lin and Lee, 2007, p. 377)
(19)
For plug flow,
(20)
41
Where:
t = retention time of the basin (s)
V = volume of basin (m3)
Q = flow rate (m3 s
-1)
L = length of rectangular basin (m)
ν = horizontal velocity of flow (m s-1
)
Rapid (or flash) mixing is one of the most important stages involved in the chemical
purification process. It is the stage in which a coagulant is rapidly and uniformly dispersed
through a mass of water. (Lin & Lee, 2007, p. 377) During the rapid mixing stage the
destabilization reactions occur (coagulation) and the primary flocs are formed. The
characteristics of the formed primary flocs strongly influence the flocculation process
which follows. (Bratby, 2006, p. 219)
The time and intensity of mixing required for the fast mixing stage has been intensely
reported but different sources usually recommend different values. (Rossini, et al., 1999)
The required fast mixing time has generally been assumed to fluctuate between 30 and 60 s.
However, according to Bratby (2006, p. 220) due to the fast rate within which the
hydrolysis and destabilization reactions occur, fast mixing times over 5 s may not improve
the subsequent flocculation process efficiency. Furthermore Griffith and Willians (1972,
cited in Bratby 2006, p. 220) stated that, beyond a certain optimum rapid mixing time, a
detrimental effect on the flocculation efficiency may be observed.
In water and wastewater treatment plants the rapid mixing unit is specifically equipped with
the most convenient type of mixer to provide the mixing effect required. Bratby (2006, pp.
222-226) reported that depending on the desired coagulation destabilization mechanism
different types of mixers may be chosen and different retention times applied. The
aforementioned author also affirmed that, for an efficient rapid mixing stage to be achieved
high velocity gradients must be applied together with high turbulence effect. However,
there is an upper limit for the applied velocity gradient depending on the coagulation
process requirements. Too high velocity gradients during rapid mixing can cause a delay on
floc formation in the flocculation process which follows (Bratby, 2006, p. 225).
42
According to Lin and Lee (2007, p. 379) velocity gradient (G) values of 500 to 1000 s-1
are
required for the rapid mixing stage in order to produced effective flocculation. Lin and Lee
also affirm that the product G*t should produce values from 30000 to 60000 with time (t)
generally in the range of 60 to 120 s, a much higher range than that recommended by
Bratby (2006). However, Bratby (2006, p. 226) states that: „the best way of determining the
appropriate rapid mixing time for a particular water is to conduct laboratory scale and/or
pilot scale tests.‟
When slow mixing and the flocculation process are concerned suitable mixers are also
chosen according to the process requirements. The reported velocity gradient and retention
times to be applied are also conflicting. Nevertheless, Vigneswaran and Visvanathan (1995,
p. 64) and Lin and Lee (2007, p. 380) reported that a mean velocity gradient ranging from
20 to 70 s-1
together with contact times from 10 to 30 minutes should be kept during the
flocculation process. When optimizing the mixing parameters for the flocculation process
one must bear in mind the solid liquid separation process which follows. Larger and denser
flocs may be more suitable for sedimentation process while smaller and lighter flocs may
provide a more efficient flotation process. According to Bratby (2006, p.243) the greater
the velocity gradient the smaller will be the final floc size due to the higher rate of breakage
of the larger formed flocs. And the lower the velocity gradient the larger will be the final
floc size although it will take longer for the larger flocs to form.
43
4 Environmental impacts of metal residual discharge
Aluminium and iron are abundant substances in Finnish soils and are naturally leached into
our water systems. However human activities such as metal ore mining, intensified forestry,
peat production and agricultural draining have increased the load of iron and aluminium in
many of the Finnish rivers ecosystems. (Vuorinen, et al., 1998; Vuori, 1995)
The concentration of toxic aluminium and iron compounds in the aquatic system is directly
dependent on the pH as well as on the concentration and type of organic matter present in
the water. Under acidic conditions the predominant species of aluminium and iron are
soluble bio-available ionic metal hydroxides. Humic substances form stable complexes
with metals reducing the concentration of ionic bio-available metals hydroxide in the water,
therefore lowering their toxicity. (Vuorinen, et al., 1998)
The biggest concern regarding aluminium residuals is related to human health and the
possible link between aluminium and adverse neurological effects, specifically the adverse
effects manifested in Alzheimer‟s disease. (Bratby, 2006, p. 173) However, aluminium has
as well for a long time been recognized as toxic for aquatic systems when found in high
concentrations. According to Rosseland, et al. (1990) aluminium acts as a toxic agent on
gill-breathing animals such as fish and invertebrates and may also as an organically
complexed form, be absorbed by mammals and birds and interfere with their metabolic
processes. The above mentioned authors also affirm that some inorganic monomeric forms
of aluminium have adverse effects on plants root systems and that aluminium can
accumulate in invertebrates and plants making its way up to the terrestrial food chain.
According to Vuori (1995) high concentration of iron in fresh water systems have long
been considered a deteriorating factor. Increasing evidence is available suggesting that iron
concentration has a significant impact on the structure and function of river ecosystems.
Vuori also stated that the direct action of toxic iron compounds can cause impairment of
aquatic life survival, reproduction and growth rates. Edén, et al. (1999) reported that acidic
waters (4.5 < pH < 5.5) together with high contents of iron (> 1 mg/l) and aluminium (>
100 µg/l) may cause severe impact on the rivers biological system. Of special concern is
44
the impacts imposed by these conditions on several fish species which metabolism,
reproduction, growth and mortality rates have been affected by these deteriorated water
quality characteristics.
A study carried out by Poléo, et al. (1997) evaluated the sensitivity of seven common
Scandinavian fresh water fish species to aluminium rich waters (0-300 µg/l of inorganic
monomeric aluminium species). The investigations concluded that aluminium is acutely
toxic to freshwater fish species under neutral and acidified water conditions but more in
acidified conditions. Vuorinen, et al. (1998) affirm that aluminium and iron in
concentrations found in the Finnish fresh water systems (150 to 800 µg/l of total Al and
500 to 4000 µg/l of total Fe) may be toxic to fish such as trout and grayling and that the
toxic effect increases with increasing acidity of the water.
Chemical purification of peat harvesting runoff water has the potential to decrease but also
to increase the concentration of metal residuals discharging from peat production sites. The
coagulation and sedimentation of metal containing particulates and dissolved substances
may reduce the metal concentration of the discharge water. However, a considerable
amount of aluminium or iron is added to the water during the purification process. Most of
the added aluminium and iron during the coagulation process should be removed from
solution via precipitation of their respective hydroxides and other insoluble formed
compounds. (Bratby, 2006, p. 173) However, for this to occur, all process parameters
should be optimized. Effective coagulation and subsequent flocculation processes will
result in low metal residual in the discharging water. As previously stated in this work, the
coagulant dosage, the pH and the temperature of the water are directly linked to the
solubility of the various iron and aluminium compounds in solution. Therefore the
coagulation process should be closely monitored and controlled to assure as low metal
residuals as possible in the discharge water.
45
5 Materials and methods
The selection of the four coagulants to be tested and evaluated in this study was made based
on the analyses of previous research project report prepared by Vapo Oy (Alatalo &
Peronius, 2004) from which further developments were required. The water samples to be
purified were collected from two different locations with the objective of also evaluating
the influence of varying water quality characteristics on the purification levels achieved by
chemical treatment. Jar test experiments were used to evaluate the coagulation, flocculation
and sedimentation process parameters thus providing a way to access chemical treatment
purification efficiency under laboratory conditions.
The laboratory experiments were performed in four distinctive phases:
- Phase 1 – Optimum dosage range and purification efficiency
- Phase 2 - Settling characteristics
- Phase 3 - Evaluation of temperature influence on the purification efficiency and
settling characteristics
- Phase 4 - Evaluation of mixing parameters influence on the purification efficiency
5.1 Coagulants
Four solid metal salt coagulants were studied in this thesis project: aluminium sulphate,
aluminium chloride, ferric sulphate and a mixture of ferric (15%) and aluminium sulphate
(85%). The selected coagulants chemical composition, manufacturers, and main
characteristics can be found in Table 1. With the objective of evaluating the effects of
coagulants solubility, aluminium sulphate and ferric sulphate were also tested in pre-
dissolved (solution) form. For that, stock solutions of 10 g/l of coagulant were prepared and
used over a 3 day period.
It is necessary to highlight that the aluminium chloride (AlCl3.6H2O) utilized in our
experiments was of analytical quality with 99% purity while the other coagulants were of
46
commercial quality with around 90% purity. No corrections have been made to the
presented coagulant dosages to compensate the higher purity of aluminium chloride.
Table 1 – Tested coagulants and their main properties.
Coagulant
Chemical
composition Physical form Manufacturer Purity
Aluminium
sulphate (ALG) Al2(SO4)3.14H2O
Small to medium
size granules
Kemira Oyj,
Kemwater > 90%
Aluminium +
Ferric sulphate
(ALF-30)
Al2(SO4)3.14H2O +
Fe2(SO4)3.8-9H2O
Mixture of small
to large size
granules
Kemira Oyj,
Kemwater > 90%
Ferric sulphate
(Ferix-3) Fe2(SO4)3.8-9H2O
Medium to large
size granules
Kemira Oyj,
Kemwater > 90%
Aluminium
chloride AlCl3.6H2O
Small size crystal
granules
Alfa Aezar
Gmbh & Co ≈ 99%
5.2 Sample collection
With the objective of evaluating the influence of variations in water quality characteristics
on the purification efficiency to be achieved by the tested coagulants, water samples from
two different locations were used to conduct the laboratory experiments. The samples were
taken from two peat harvesting sites under exploration by Vapo Oy. The first sample was
collected from Navettarimpi peat harvesting site located between Vaala and Kestilä (E:
3475877, N: 7138725) in August 2010. The second was collected from Piipsanneva peat
harvesting site in Haapavesi (E: 3431984, N: 7116451) in October 2010. In both occasions
around 400 litres of water was pumped from ditches prior to chemical treatment into 35 l
plastic gallons which had previously been acid treated to eliminate any source of
contamination. The gallons were stored in cold room (5 – 10 °C). Prior utilization each
gallon was vigorously shaken to provide thorough mixing of the sample and to eliminate
changes in water quality due to sedimentation of particulate matter or adsorption of
substances to the plastic surface.
47
5.3 Laboratory analyses
The purification efficiency achieved by the tested coagulants under different process
parameters was firstly monitored and evaluated in the laboratory via measurements of
colour, turbidity, pH, temperature and conductivity. The standards followed and used
equipment is given in Table B of appendix 1. From the results of these measurements,
samples of the treated water were then selected and sent for further analyzes to the Finnish
Accreditation Service (FINAS) accredited Environment Measurement and Testing
Laboratory (T003 and T164). The performed outsourced analyzes (standard methods in
Table A of appendix 1) were: total organic carbon (TOC), total nitrogen (tot-N), total
phosphorous (tot-P), phosphate phosphorous (PO4-P), suspended solids (SS), aluminium
(Al) and iron (Fe). To obtain the prior treatment water quality characteristics (Table 2) or
initial conditions, samples of the untreated water were sent for the above mentioned set of
analyzes and in addition for analyzes of conductivity (Cond.) and pH. Furthermore, to
eliminate possible errors while evaluating the purification efficiency, samples of the
untreated water were sent for analyzes on the day of the sample collection and at regular
intervals during the testing period. Errors while evaluating the purification levels could
arise from possible changes in water quality during the storage period.
Table 2 – Water quality characteristics of Piipsanneva and Navettarimpi samples.
Water quality
parameter
Average value ± standard
deviation (Navettarimpi) n
Average value ± standard
deviation (Piipsanneva) n
pH 6.3 – 6.8 (range) 5 5.8 – 6.3 (range) 8
Cond. [mS/m] 6.2 – 7.4 (range) 5 7.2 – 7.4 (range) 8
SS [mg/l] 17.1 ± 3.9 9 18.8 ± 1.5 8
tot-N [µg/l] 1720 ± 148 9 2000 ± 0 6
tot-P [µg/l] 58 ± 5.6 9 61 ± 2.4 8
PO4-P [µg/l] 24 ± 2.5 9 21 ± 2.8 8
TOC [mg/l] 27 ± 1.5 9 27.5 ± 0.5 8
Fe [µg/l] 3830 ± 132.3 9 2150 ± 50 4
Al [µg/l] 4230 ± 59.6 9 730 ± 59.1 4 n = number of samples analyzed.
48
5.4 Jar test experiments
The jar test experiments were performed using a six jars paddle stirrer equipment from
Kemira Kemwater named Flocculator 2000 (Figure12). The rotational speed and mixing
time for each individual stirrer (mixer) is program-controlled allowing the application of
different parameters to each individual one litre cylindrical beaker.
Figure 12 - Jar test equipment
The dimensions of the individual beakers and stirrers are as follow:
- Beakers - Stirrers
Height = 18 cm Length = 15 cm
Diameter = 9 cm Width of paddle = 3 cm
Volume = 1 l Length of paddle = 5.6 cm
Based on the dimensions of the beakers and on the type and dimension of the mixer (stirrer)
an estimation of the velocity gradient (G) imposed to the liquid within the jars for the fast
and slow mixing stages was calculated using equation (18) were:
49
- (dimensionless power number related to mixing device and tank geometry
extracted from Bratby (2006, p. 264) according to jar test equipment dimensions
varies from 3.5 to 4.0)
- m (diameter of mixing impeller)
- m3 (volume of beaker)
- µ = 0.001 Ns/ m2 (absolute viscosity of water at 20 °C)
- ρ = 1000 kg/m3 (density of the water)
For the coagulation or fast mixing stage:
- n = 400 rpm = 6.667 rps (adopted mixer rotational speed)
- Calculated G = 756 s-1
- For contact time t = 60 s; G*t = 45360
For the flocculation or slow mixing stage:
- n = 70 rpm = 1.167 rps (adopted mixer rotational speed)
- Calculated G = 55 s-1
- For contact time t = 15 min = 900 s; G*t = 49500
The adopted rotational speeds and contact times for both mixing stages were based on
preliminary jar tests experiments and on literature recommended values cited in this work.
Jar test procedure for laboratory experiments phase 1 - determination of optimum
dosage range and purification efficiency
All four selected coagulants were tested in this phase of experiments according to the jar
test procedure which follows:
1- One litre of the untreated water was transferred to each of the six jars. For
establishing the tests initial conditions an extra sample of the untreated water was
separated and analyzed for turbidity, colour, temperature, pH and conductivity.
50
2- Constant and equal mixing parameters were applied to the six individual mixers.
Solid coagulants: 400 rpm for 60s, 70 rpm for 15 min followed by 30 minutes of
sedimentation time where no mixing was applied. Coagulants in solution form: 300
rpm for 10s, 50 rpm for 25 min followed by 30 minutes of sedimentation time
where no mixing was applied
3- Increasing dosages of coagulant was added in sequence to the water samples in the
jars simultaneously to the start of mixing.
4- After the mixing and sedimentation period elapsed samples of the supernatant water
were extracted from each jar and analyzed for turbidity. Based on the obtained
results an optimum dosage range composed of six different dosages for each tested
coagulant was determined.
The optimum dosage range of each coagulant was then applied on 3 consecutive
experiments for each solid coagulant and 2 consecutive experiments for each of the
coagulants in solution form following procedures 1 to 3 and 5 to 7:
5- The samples extracted after the sedimentation time had elapsed were analyzed for
turbidity, colour, pH and conductivity. Purification efficiencies were evaluated and
reported as the removal of colour and turbidity with applied dosage and were
expressed as the ratios: T/Ti (turbidity of the purified water or final turbidity) /
(turbidity of the raw water or initial turbidity); C/Ci (colour of the purified water or
final colour) / (colour of the raw water or initial colour).
6- From the dosage range applied the dosage which presented the best removal of
colour and turbidity was identified as the optimum dosage and the dosage which
resulted in the removal efficiency of 40 to 60% in colour and/or turbidity was
identified as the limit dosage or lowest working dosage. The procedure of the
identification of a limit dosage was a tentative of obtaining the lowest dosage to be
applied in order to achieve reasonable purification levels of ca 50% removal
efficiency of total phosphorus, phosphates and suspended solids.
7- Further 400 ml of supernatant water were extracted from the samples treated with
the optimum (coagulants in solid and solution form) and limit dosages (solid
51
coagulants) and sent to the laboratory for analyzes of TOC, tot-P, tot-N, PO4-P, Fe,
and Al.
Procedures 1 to 7 were carried out for both water samples at field pH conditions.
Piipsanneva water sample was also tested after it‟s pH was increased by one unit from 5.8
to 6.85 via the addition of slake lime (Ca(OH)2). After the addition of Ca(OH)2 the sample
was thoroughly mixed and left to stand for 12 hours before tests were performed.
According to analyzes of the results obtained in this first phase of tests, it was decided that
only three of the four coagulants would be tested in the second, third and fourth phases of
experiments: ALG, ALF-30 and Ferix-3.
Jar test procedure for laboratory experiments phase 2 – settling characteristics
For the phase 2 of laboratory experiments the procedures 1 and 2 of phase 1 (detailed
above) were followed and in addition:
1- Each experiment used only one jar. The outside wall of the cylindrical beaker was
marked across its circumference with a line at a point 8 cm from its bottom.
2- Optimum dosages of each coagulant were applied in individual tests. The coagulant
was added simultaneously to the start of mixing. After coagulation and flocculation
mixing period had elapsed eleven 30 ml samples were extracted from the jars with a
volumetric pipette inserted in the water until the 8 cm mark. The samples were
collected in sequence and in time intervals as follow: 13 minutes inside flocculation
time (or 2 minutes before mixing ceased) and then at 1, 2, 3, 4, 6, 8, 11, 13, 17 and
25 minutes inside sedimentation time.
3- The extracted samples were subsequently analyzed for turbidity. Since turbidity is a
measure which can be correlated to the concentration of particulates in the water it
was used to monitor how the concentration of particulates changed over time at a
constant depth point in the jars. The obtained results were expressed as the ratio
52
Tt/Tt0 = (turbidity at time t) / (turbidity measured during flocculation period 2
minutes before mixing was ceased).
Two replications of the settling test were performed for each of the three studied solid
coagulants tested in this phase of laboratory experiments. Furthermore two replications
were also performed for ALG and Ferix-3 in solution or pre-dissolved form.
Jar test procedure for laboratory experiments phase 3 – evaluation of temperature
influence on purification efficiency and settling characteristics
The tests performed in this phase of laboratory experiments were carried out in two stages.
First the influence of low temperature on the overall purification efficiency was evaluated
and in sequence the influence of temperature on the settling characteristics of the formed
flocs was also investigated. For the evaluation of temperature influence in the purification
efficiency the procedures followed mostly replicated the procedures described for
laboratory phase 1 and in addition:
1- The jars containing the untreated water samples were placed inside a temperature
controlled insulated water tank (Figure 13). The temperature of the pre-determined
volume of water in the tank was controlled via a Lauda RK KS low temperature
thermostat. The jars were left in cold water bath and slow mixing was applied to
keep the samples homogenized until a constant temperature of 5 (± 1°C) was
achieved in all jars.
2- Three increasing dosages of each coagulant were applied. The dosages were
selected from the optimum dosage range determined in phase 1 and consisted of the
lowest working dosage, the optimum dosage and a higher than optimum dosage for
each coagulant.
3- Only from the sample treated with the optimum dosage of each coagulant extra 400
ml of supernatant water was extracted and sent to the laboratory for further
analyzes.
53
Procedures 1, 2 and 3 were performed twice for all coagulants in solid and in solution form,
however, only samples from one repetition for the coagulants in solution form were sent to
the laboratory for analyzes.
Figure 13 – Temperature controlled insulated water tank.
For the determination of the influence of low temperature on the coagulants settling
characteristics, all procedures from laboratory phase 2 (where the settling characteristics of
the flocs formed at 20 °C was evaluated) were followed and in addition procedure 1
detailed previously in this phase of laboratory experiments.
Jar test procedure for laboratory experiments phase 4 – evaluation of mixing
parameters influence on purification efficiency
For the evaluation of the influence of mixing parameters a series of jar test experiments
were performed which basically followed the procedures applied in phase 1 except:
1- Each experiment used three jars. The three stirrers were programmed individually
where one of the four mixing parameters (fast mixing time and speed, slow mixing
time and speed) was different in each jar and the other three remained constant.
54
2- The optimum dosage of each coagulant was added and although two repetitions of
each jar test experiment were performed, only one sample from each variation of the
four mixing parameters was sent for analyzes.
3- The sequence of parameters variation and the actual values applied can be seen in
Table 3.
The results obtained by mixing parameters applied during the first phase of experiments
were also evaluated during this phase of testing. E.g. while evaluating the purification
efficiencies achieved under different slow mixing speeds not only the results obtained by
20, 50 and 90 rpm values applied in this stage were analyzed but also the results obtained
by the applied 70 rpm during phase 1 of testing. The chosen value which was taken as
optimum and kept constant for the evaluation of the other mixing parameters was the one
which provided in average the best removal efficiencies for all three tested coagulants.
Table 3 – Variation applied to each of the four mixing parameters while testing coagulants
in solid and solution form
Influence of mixing parameters on the purification efficiency of solid coagulants
Jar test
experiment
Slow mixing speed
(rpm)
Slow mixing
time ( min)
Fast mixing
time (s)
Fast mixing
speed (rpm)
1 20, 50, 90 15 60 400
2 70 5, 10, 25 60 400
3 70 25 10, 60, 120 400
4 70 25 60 100, 200, 300
Influence of mixing parameters on the purification efficiency of coagulants in
solution form
Jar test
experiment
Slow mixing speed
(rpm)
Slow mixing
time ( min)
Fast mixing
time (s)
Fast mixing
speed (rpm)
1 20, 50, 90 25 60 400
2 50 5, 10, 25 60 400
3 50 25 10,60,120 400
4 50 25 10 100, 200, 300
55
6 Results
The results obtained during the laboratory part of our study are introduced here. They have
been grouped according to the four laboratory experiment phases: 1) Optimum dosage
range and purification efficiency; 2) Settling characteristics; 3) Influence of temperature; 4)
Influence of mixing.
6.1 Optimum dosage range and purification efficiency
The findings of the laboratory experiments phase 1 are here introduced. The obtained
results are divided into two different sections for clarity. The first section (optimum dosage
range) describes how increasing dosages of the tested coagulants affected the removal of
substances which attribute turbidity and colour to the water samples being purified. In this
section the identified optimum dosages of the individual coagulants for the purification of
both water samples are also reported. Section two (purification efficiency) describes the
purification efficiency achieved by the tested coagulants regarding the removal of
concerning substances from the water samples.
6.1.1 Optimum dosage range of tested coagulants
Aluminium and iron based coagulants presented different patterns of colour and turbidity
removal when increasing dosages of the coagulants were applied. Although the removal
efficiency presented by the purification of Navettarimpi and Piipsanneva samples differ
from one another the behavioural patterns of the individual chemicals with increasing
dosage remained mostly the same. The graphs contained in Figures 14 and 15 present the
purification efficiency (regarding the removal of colour and turbidity) achieved with
increasing dosage of solid coagulants applied to Navettarimpi and Piipsanneva samples
consecutively.
56
Figure 14 - Removal of turbidity and colour for the applied dosage ranges of a) AlCl3, b)
ALG, c) Ferix-3 and d) ALF-30 for purification of Navettarimpi water sample.
The aluminium based coagulants, ALG and AlCl3 presented very similar patterns of colour
and turbidity removal, where colour and turbidity values steadily decreased with increasing
dosages of up to ca 50 mg/l (Figure 14 a and b). After this point, colour and turbidity
removal increased slightly or remained constant when dosages of over 200 mg/l were
applied.
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 50 100 150 200
Rat
io f
/i
Dosage mg/l
T/Ti C/Ci pH
pH
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 30 60 90 120
Rat
io f
/i
Dosage mg/l
T/Ti C/Ci pH
pH
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 20 40 60 80 100 120
Rat
io f
/i
Dosage mg/l
T/Ti C/Ci pH
pH
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 30 60 90 120
Rat
io f
/i
Dosage mg/l
T/Ti C/Ci pH
pH
a) b)
c) d)
57
Figure 15 - Removal of turbidity and colour for the applied dosage ranges of a) AlCl3, b)
ALG, c) Ferix-3 and d) ALF-30 for purification of Piipsanneva water sample.
The iron based coagulant; Ferix-3 presented a very different pattern of colour and turbidity
removal with increasing dosage when compared to the aluminium based coagulants. Little
removal or even slight increase in colour and turbidity was observed with dosages up to 50
mg/l (Figures 14 c and 15 c). After this point, when increasing the dosage by only 10 mg/l
the removal efficiency sharply increased to its optimum values. Optimum removal
efficiency values were though only observed for the narrow dosage range of 60 to 80 mg/l
for Navettarimpi and 50 to 70 mg/l for Piipsanneva water samples. Increasing the applied
dosage above the presented optimum range resulted in a sharp increase in turbidity and to a
less extent in colour values.
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 40 80 120 160 200
Rat
io f
/i
Dosage (mg/l)
T/Ti C/Ci pH
pH
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 40 80 120 160 200 240
Rat
io f
/i
Dosage (mg/l)
T/Ti C/Ci pH
pH
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 30 60 90 120
Rat
iio f
/i
Dosage (mg/l)
T/Ti C/Ci pH
pH
0,0
1,0
2,0
3,0
4,0
5,0
6,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 30 60 90 120
Rat
io f
/i
Dosage mg/l
T/Ti C/Ci pH
pH
b) a)
c) d)
58
ALF-30 presented a combination of the behavioural patterns of the aluminium and iron
based coagulants which make up its composition. It presented a rather steadily colour and
turbidity removal with increasing dosages up to 50 mg/l. After this point, removal
efficiencies remained mostly constant for dosages up to 90 mg/l. However, further
increases in dosage resulted on a steady increase in turbidity and to a less extend in colour
values.
It is clear that the addition of all coagulants greatly reduced the pH of the mixed solution
(Figures 14 and 15). The addition of iron based chemicals had a greater affect on the pH of
the water. The exact pH values obtained in the purified water are of course dependent on
the initial pH of the water sample. Nevertheless, the pH measured in the samples treated
with Ferix-3 were about half to a full pH unit lower than the pH values observed in samples
treated by the aluminium based coagulants.
Figures 16 and 17 contain the graphs which represent the patterns of removal of colour and
turbidity with increasing dosages of ALG and Ferix-3 in solution form. The behaviour of
the coagulants in solution mostly imitated the behaviour of the coagulants in solid form. It
is important to emphasize tough that the best removal efficiencies of colour and turbidity
were not only reached at lower dosages for both coagulants but also the efficiencies
achieved were overall higher when solution of the coagulants were applied. It is also of
interest to note that while Ferix-3 in solid form presented a narrow optimum dosage range
with a sharp increase in colour and turbidity with dosages above optimum, when applied in
solution form the observed increases in colour and turbidity can be described as steady but
not sharp.
59
Figure 16 - Removal of turbidity and colour for the applied dosage ranges of a) ALG and
b) Ferix-3 in solution form for the purification of Navettarimpi water sample.
Figure 17 - Removal of turbidity and colour for the applied dosage ranges of a) ALG and
b) Ferix-3 in solution form for the purification of Piipsanneva water sample.
6.1.2 Purification efficiency
The obtained results concerning the purification efficiency achieved by each of the tested
coagulants are presented in graphic form and are expressed as removal efficiency in
percentage. Removal efficiency represents how much has the chemical treatment reduced
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 30 60 90 120
Rat
io f
/i
Dosage mg/l
T/Ti C/Ci pH
p
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
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Rat
io f
/i
Dosage mg/l
T/Ti C/Ci pH
pH
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 30 60 90 120
Rat
io f
/i
Dosage (mg/l)
T/Ti C/Ci pH
pH
0,0
1,0
2,0
3,0
4,0
5,0
6,0
7,0
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 30 60 90 120
Rat
io f
/i
Dosage (mg/l)
T/Ti C/CI pH
pH
a) b)
a) b)
60
the concentration of the concerning analyzed substances (tot-N, tot-P, PO4-P, SS and TOC)
relative to their initial concentration in the runoff water samples.
The removal efficiency achieved by the addition of optimum and limit dosages of AlCl3,
ALG, ALF-30 and Ferix-3 in solid form to Navettarimpi water sample can be seen in
Figure 18. Although all tested coagulants achieved fairly high removal efficiency levels, it
is clear that overall at optimum dosage Ferix-3 was the best performing coagulant.
Optimum dosage of Ferix-3 (70 mg/l) removed twice as much (40%) of the tot-N
concentration compared to the other coagulants (20 to 24%), about 20% more of the TOC
and also 15% more of the SS concentration from solution. All solid coagulants achieved ca
80 % removal efficiency of tot-P and PO4-P concentration present in the water sample
when optimum dosages were applied.
Figure 18 - Average removal efficiency of tot-N, tot-P, PO4-P, SS and TOC for optimum
(coloured bars) and limit dosages (pattern filled bars) of solid coagulants for the
purification of Navettarimpi water sample.
When the limit dosage was applied ALG clearly outperformed the other coagulants. It is
important to emphasize that ALG and ALF-30 limiting dosages (40 mg/l) are lower than
that of Ferix-3 (50 mg/l). ALG removed over 20% more tot-P concentration than ALF-30
-60
-40
-20
0
20
40
60
80
100
tot-N tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
ALG 70 mg/l ALG 40 mg/lALF-30 70 mg/l ALF-30 40 mg/lFerix-3 70 mg/l Ferix-3 50 mg/lAlCl3 50 mg/l AlCl3 30 mg/l
61
and 10% more than Ferix-3. It also removed 55% of SS from solution while ALF and
Ferix-3 caused actually an increase in the SS concentration under limit dosage conditions.
Figure 19 contains the removal efficiency achieved by the addition of optimum and limit
dosages of all four solid coagulants for the purification of Piipsanneva water sample. The
identified optimum dosages for the purification of Piipsanneva sample are considerably
higher, at least for aluminium based coagulants, than those identified for Navettarimpi
sample. Although the possible reasons for these higher dosage requirements are further
discussed in the results discussion section, it is important to highlight that the optimum
dosages have been identified by using results of colour and turbidity removal. Hence the
removal of colour was a particular issue with Piipsanneva sample the dosages applied here
were overall higher than those applied for Navettarimpi water sample.
Figure 19 - Average removal efficiency of tot-N, tot-P, PO4-P, SS and TOC for optimum
(coloured bars) and limit dosages (pattern filled bars) of solid coagulants for the
purification of Piipsanneva water sample.
The higher applied dosages of aluminium and mixed based products have not though
produced satisfactory removal of colour which is confirmed by the low removal efficiency
of TOC (<30%) presented by ALG, AlCl3 and ALF-30. An optimum dosage of 70 mg/l of
0
10
20
30
40
50
60
70
80
90
100
tot-N tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
ALG 140 mg/l ALG 50 mg/l ALF-30 90 mg/l ALF-30 60 mg/l Ferix-3 70 mg/l Ferix-3 50 mg/l AlCl3 90 mg/l AlCl3 30 mg/l
62
Ferix-3 removed twice as much TOC concentration (ca 56%) than the 140 mg/l dosage of
ALG. Ferix-3 achieved overall the best removal efficiencies at optimum and limit dosages.
Note that although Ferix-3 required considerably lower optimum dosage it removed over
20% more PO4-P and 15% more tot-N than the other coagulants in their identified optimum
conditions. It is important to emphasize that the small differences observed between
removal efficiencies at optimum and limit dosages for the purification of Piipsanneva
sample direct to the fact that the identified optimum dosages, especially for aluminium and
mixed based coagulants, may have been overestimated.
Note that Ferix-3 limit dosage (50 mg/l), based on the obtained removal efficiencies is
clearly inside its optimum dosage range. Due to Ferix-3‟s narrow working dosage range
(Figure 15 - c) it is difficult to identify a limit dosage which can provide lower but
satisfactory purification levels. It is also interesting to note that the limiting dose for ALG
has been identified as 50 mg/l which is almost three times lower than its optimum dosage
of 140 mg/l.
When comparing the removal efficiencies achieved for the chemical purification of
Navettarimpi and Piipsanneva samples (Figures 18, 19 and Table 4) it is clear that the
aluminium based coagulants achieved much higher removal efficiencies when purifying
Navettarimpi sample at much lower dosages. Take as an example the removal efficiencies
achieved for tot-P and TOC by ALG which were consecutively over 10 and 20% higher in
the purification of Navettarimpi sample at half of the dosage applied to Piipsanneva sample.
The water quality characteristics of Navettarimpi and Piipsanneva samples were very
similar (Table 2) presenting only small variation regarding the load of undesirable
substances and in pH values. It was assumed that the overall lower removal efficiencies
achieved by aluminium based coagulants for the purification of Piipsanneva water sample
was strongly dependent on the small but significant difference on pH presented by the
samples. For testing this assumption a decision was made on raising the pH of Piipsanneva
sample by one unit by adding calcium hydroxide (Ca(OH)2) and running the purification
tests on the raised pH sample using three of the four solid coagulants; ALG, ALF-30 and
Ferix-3. Figure 20 presents the differences in the removal efficiency observed between
63
Piipsanneva field (5, 8) and raised (6, 85) pH water samples. Note also that the normal pH
presented by the discharging waters from Piipsanneva extraction site, according to the field
engineers, is in average ca 6.5. The lower pH observed in the tested sample may be due to
rain fall which occurred the day before the sample collection.
According to the obtained results (Figure 20, see also Table 4) the removal efficiencies
achieved were overall much higher for all concerning substances for the raised pH water
sample. ALG, for example removed around 12% more tot-P, 10% more TOC and 20%
more SS at raised pH conditions. It is important to note that the aluminium based chemicals
not only raised their removal efficiency levels but did so via the addition of much lower
dosages. ALG optimum dosage for the normal pH sample was 140 mg/l while it only
required 70 mg/l for purifying the raised pH water sample. It is fundamental to emphasize
here that the removal efficiencies achieved and the coagulant dosages applied for the
purification of the Piipsanneva raised pH sample followed very closely the efficiencies and
dosages obtained in the purification of Navettarimpi sample.
Figure 20 - Average removal efficiency of tot-N, tot-P, PO4-P, SS and TOC for optimum
dosage of solid coagulants for the purification of Piipsanneva water sample at normal
(pattern filled bars) and raised pH (coloured bars).
The purification efficiencies achieved via the addition of ALG and Ferix-3 in solution form
was also evaluated. The graphs contained in Figure 21 present a comparison between the
0
20
40
60
80
100
tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
ALG 70 mg/l pH= 6.85 ALG 140 mg/l pH=5.8ALF-30 70 mg/l pH= 6.85 ALF-30 90 mg/l pH=5.8Ferix-3 70 mg/l pH= 6.85 Ferix-3 70 mg/l pH=5.8
64
removal efficiencies achieved by the addition of ALG and Ferix-3 in solid and solution
forms. For the purification of both, Navettarimpi and Piipsanneva water samples the
removal efficiencies achieved by the coagulants in solution were consistently higher at
lower dosages than that of the coagulants in solid form (see also Table 4). The most
significant difference occurred in the removal of SS concentration. The SS removal
efficiency achieved by ALG and Ferix-3 in solution was consecutively 27 and 21% higher
for the purification of Navettarimpi sample and 18 and 28% higher for the purification of
Piipsanneva sample.
Figure 21 - Average removal efficiency for optimum dosage of ALG and Ferix-3 in solid
and solution form for the purification of a) Navettarimpi and b) Piipsanneva water samples.
Regarding the removal efficiencies obtained by the addition of aluminium chloride to both
water samples, no accurate and direct comparison can be made to the results obtained by
the other tested coagulants. It is possible to adjust aluminium chloride purity advantage by
including a 10% increase to its applied dosages. This would result AlCl3 dosages
comparable to that of Ferix-3 and slightly lower dosages than those required by the other
tested coagulants. However, this approach would not take into account the influence of the
impurities contained within the commercial quality coagulants. Based on this fact and the
preliminary decision that only three of the fours solid coagulants would be tested in
laboratory experiment phases 2, 3 and 4, aluminium chloride was then the chosen coagulant
to be eliminated from the subsequent tests.
0
20
40
60
80
100
tot-N tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
ALG solution 70 mg/l ALG solid 70 mg/lFerix-3 solution 60 mg/l Ferix-3 solid 70 mg/l
0
20
40
60
80
100
tot-N tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
ALG solution 80 mg/L ALG solid 140 mg/L Ferix solution 50 mg/L Ferix-3 solid 70mg/L a) b)
65
Table 4 - Purification phase observations. Final pH and average removal efficiency
obtained for the application of the identified optimum dosage of each tested coagulant.
Coagulant
Optimum
dosage
range
(mg/l)
Applied
dosage
(mg/l) Sample
Final
pH
range Average removal efficiency (%)
tot-N tot-P PO4-P SS TOC
AlCl 50-60 50 Nav. 4.6-4.7 37 79 77 76 42
ALG 60-80 70 Nav. 4.6-4.7 24 80 80 60 39
ALG sol. 60-80 70 Nav. 4.6-4.8 29 87 84 85 42
Ferix-3 60-70 70 Nav. 3.8-3.9 40 87 85 75 60
Ferix-3 sol. 50-70 60 Nav. 4.0-4.2 40 92 89 94 68
ALF-30 70-80 70 Nav. 4.5-4.6 29 81 83 65 40
AlCl 60-90 90 Piip. 4.0-4.2 18 73 74 74 23
ALG 90-140 140 Piip. 4.1-4.2 18 68 76 59 20
ALG sol. 70-90 80 Piip. 3.9-4.0 20 71 75 77 22
Ferix-3 50-70 70 Piip. 3.5-3.6 33 87 87 59 56
Ferix-3 sol 50-70 50 Piip. 3.7-3.8 33 89 89 88 63
ALF-30 80-90 90 Piip. 4.0-4.2 18 67 63 48 25
ALG 60-80 70 Piip. ↑pH 4.5-4.6 - 80 89 79 30
ALF-30 60-80 70 Piip. ↑pH 4.4-4.5 - 78 82 65 33
Ferix-3 60-80 70 Piip. ↑pH 3.6-3.7 - 90 91 76 59 The post-fix “sol” refers to the coagulant in solution form; Nav. = Navettarimpi sample; Piip. =
Piipsanneva sample; - analyze not performed on the particular sample.
6.2 Settling characteristics
The settling characteristics of the individual coagulants represent their ability to, not only
coagulate the pollutants present in the water but specially their ability to form suitable flocs
which will provide short sedimentation times and a clarified supernatant water. The settling
characteristics of ALG, ALF-30 and Ferix-3 in solid form when applied to Navettarimpi
water sample are presented in Figure 22 (a). As it can be seen, Ferix-3 presented the fastest
sedimentation rate and overall higher clarification of the supernatant water. It reached the
Tt/Tt0 = 0.2 mark or 80% of initial turbidity removal after 4 minutes of sedimentation,
while ALG and ALF-30 reached the same mark after 11 and 17 minutes consecutively.
A comparison between the settling characteristics of ALG and Ferix-3 in solid and solution
form is presented in Figure 22 (b). Settling characteristics of the coagulants in solution
66
mostly replicated that of the coagulants in solid form. Some distinction can be made at the
beginning of sedimentation, where the solid coagulants appear to present somewhat faster
settling rates, and at the end of sedimentation, where the coagulants in solution form
presented slightly better supernatant water clarification levels.
Figure 22 - Turbidity removal as a function of time expressed by the ratio Tt/Tt0 for
optimum dosages of applied coagulants to Navettarimpi water sample. a) ALG, ALF-30
and Ferix-3 in solid form; b) ALG and Ferix-3 in solid and solution form.
The settling characteristics of ALG, ALF-30 and Ferix-3 in solid form when applied to
Piipsanneva water sample can be seen in Figure 23 (a). It is once again clear that Ferix-3
presented the faster settling rates and the best final clarification of the supernatant water.
ALG and ALF-30 presented slightly different settling behaviours. ALF-30 showed faster
settling rates at the beginning of sedimentation when compared to ALG. ALF-30 reached
50% solids settlement after 4 minutes into sedimentation time while ALG needed around 7
minutes. However, as sedimentation time progressed ALF-30 settling rate slowed
considerably and it required 25 minutes to reach ca 70% turbidity removal while ALG
required only about 17 minutes.
The graph presented in Figure 23 (b) shows a comparison between settling characteristics
of ALG and Ferix-3 in solid and solution form. The solid form of Ferix-3 presented slightly
faster sedimentation rate than its solution, on the other hand the solution form presented
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 5 10 15 20 25 30
Tt/T
t0
Time (min)
ALF-30 70 mg/l ALG 70 mg/l
Ferix-3 60 mg/l
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 5 10 15 20 25 30
Tt/T
t0Time (min)
ALG solution 70 mg/l ALG solid 70 mg/l
Ferix-3 solution 60 mg/l Ferix-3 solid 60 mg/la) b)
67
better final supernatant water clarification. ALG solution presented much faster settling rate
and final clarification than ALG in solid form.
Figure 23 - Turbidity removal as a function of time expressed by the ratio Tt/Tt0 for
optimum dosages of applied coagulants to Piipsanneva water sample. a) ALG, ALF-30 and
Ferix-3 in solid form; b) ALG and Ferix-3 in solid and solution form.
It is important to add here that the dosages applied during the settling tests were inside of
the coagulants optimum dosage range but not necessarily their most optimum dosage was
used. It is the case of ALG in solid form which optimum dosage for Piipsanneva sample
has been identified as 140 mg/l and 80 mg/l was applied during the settling test. A point
was made to add the same dosage of a coagulant in solid and solution form so that straight
comparison could be achieved. Since the optimum dosages for the purification of
Piipsanneva samples via addition of ALG in solid and solution form differed greatly a
compromise was necessary and the 80 mg/l the optimum dosage of ALG in solution form
was selected.
6.3 Influence of temperature
The influence of temperature on the chemical purification process was evaluated in two
distinct stages. Firstly, the influence of temperature in the purification process as a whole
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 5 10 15 20 25 30
Tt/T
t0
Time (min)
Ferix-3 70 mg/l ALG 80 mg/l
ALF-30 80 mg/l
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 5 10 15 20 25 30
Tt/T
t0
Time (min)
Ferix-3 solid 70 mg/l Ferix-3 solution 70 mg/l
ALG solid 80 mg/l ALG solution 80 mg/la) b)
68
was investigated. The effects of low temperature on the purification levels achieved are
introduced here. Secondly, the influence of temperature on the settling characteristics of the
flocs formed during the coagulation and flocculation processes was monitored and the
observations made are also presented.
6.3.1 Influence of temperature on purification efficiency
Overall reducing the test temperature from 20 to 5°C had a negative influence on removal
efficiencies or purification levels achieved by all added coagulants in solid and solution
form. The magnitude of this influence depended on the coagulant, its physical form and
applied dosage, substances being removed and to a less extent the water sample being
purified.
As it can be seen in Figure 24 I (a, b, c) the lower temperature had little influence in the
removal of phosphorous by the three tested solid coagulants from Navettarimpi water
sample. A small negative influence of 10% or less was also observed in the removal
efficiency of tot-N and TOC. The lowering of temperature had nevertheless great effect on
the removal of SS by all solid coagulants. Ferix-3 suffered the biggest influence; its
removal of SS at 5°C was about 60% lower than at 20°C.
Figure 24 II (a, b, c) presents a comparison between the removal efficiencies of ALG,
Ferix-3 and ALF-30 at 5 and 20°C for the purification of Piipsanneva water sample. Note
that tot-N analyzes were not performed for Piipsanneva sample. Just as for Navettarimpi
sample and even to a greater extent, the biggest influence of lowering the temperature was
observed in the removal efficiency of SS. Ferix-3 presented 0% removal efficiency of SS
meaning that it did not reduce the initial SS concentration of the sample at low
temperatures. This represents a reduction in purification efficiency of about 60% compared
to the removal efficiency observed at 20°C. The applied dosage of ALF-30 at 5°C actually
increased the initial SS concentration of the sample in 15%. ALG‟s SS removal or
purification efficiency at 5 °C was about 40% lower than at 20°C.
69
Figure 24 - Influence of temperature on purification efficiency for solid ALG, ALF-30 and
Ferix-3 applied to Navettarimpi I (a, b and c) and Piipsanneva II (a, b and c) water samples.
It is also possible to observe in Figure 24 I and II (a, b, c) that the difference between the
coagulants removal efficiencies of all phosphorous fractions for Piipsanneva sample was
0
20
40
60
80
100
tot-N tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
5 °C 20 °C
0
20
40
60
80
100
tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
5 ̊ C 20 ̊ C
0
20
40
60
80
100
tot-N tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
5 °C 20 °C
-40
-20
0
20
40
60
80
100
tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
5 ̊ C 20 ̊ C
0
20
40
60
80
100
tot-N tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
5 °C 20 °C
0
20
40
60
80
100
tot-P PO4-P SS TOC
Ave
rage
re
mo
val e
ffic
ien
cy (
%)
5 ̊ C 20 ̊ C
I a) ALG 70 mg/l II a) ALG 140 mg/l
II b) ALF-30 90 mg/l I b) ALF-30 70 mg/l
I c) Ferix-3 60 mg/l II c) Ferix-3 70 mg/l
70
slightly more accentuated than for Navettarimpi sample, especially for aluminium based
chemicals. While for Navettarimpi no clear reduction in removal efficiency was observed,
for Piipsanneva sample at 5°C ALG and ALF-30 removed around 15 % less tot-P than at
20°C.
The influence of temperature in the purification efficiencies achieved by ALG and Ferix-3
in solution form can be evaluated by using the graphs presented in Figure 25. Graphs I and
II represent a comparison between the purification levels achieved at 5 and 20°C via
addition of the coagulants in solution to Navettarimpi and Piipsanneva water samples
consecutively. The observed changes in the purification efficiencies with the lowering of
temperature followed similar pattern to the previously observed with the addition of solid
coagulants. For the coagulants in solution form, the removal of SS was as well greatly
reduced at low temperature and Piipsanneva sample also presented a more accentuated
worsening on its overall purification levels. However, note that ALG solution and not
Ferix-3 (opposite to the solids) suffered the most negative influence in purification
efficiency for the applied change in temperature. ALG solution removed around 40% less
SS at 5°C than at 20°C while Ferix-3 solution removed around 10 % less. The removal of
tot-P and TOC by ALG was around 10% lower at 5°C for both water samples while Ferix-3
removal of tot-P and TOC was mostly not affected by temperature.
71
Figure 25 – Influence of temperature on purification efficiency of ALG and Ferix-3 in
solution form applied to Navettarimpi I (a, b) and Piipsanneva II (a, b) water samples.
6.3.2 Influence of temperature on settling characteristics
The lowering of temperature caused a negative or delayed effect on the settling rates of all
coagulants in both solid and solution form. It also caused a worsening of final supernatant
water clarification in both Navettarimpi and Piipsanneva water samples. (Table 5, Figures
26 and 27). It is worth to note that the observed delayed effects in settling rates and the
worsening of supernatant water clarification were similar for both water samples but
slightly more accentuated in the results obtained for Piipsanneva sample.
0
20
40
60
80
100
tot-N tot-P PO4-P SS
Ave
rage
re
mo
val
eff
icie
ncy
(%
)
5 °C 20 °C
0
20
40
60
80
100
tot-P PO4-P SS TOC
Ave
rage
re
mo
val
eff
icie
ncy
(%
)
5 ̊ C 20 ̊ C
0
20
40
60
80
100
tot-N tot-P PO4-P SS TOC
Ave
rage
re
mo
val
eff
icie
ncy
(%
)
5 °C 20 °C
0
20
40
60
80
100
tot-P PO4-P SS TOC
Ave
rage
re
mo
val
eff
icie
ncy
(%
)
5 ̊ C 20 ̊ C
I a) ALG solution 80 mg/l II a) ALG solution 80 mg/l
II a) Ferix-3 solution 60 mg/l II b) Ferix-3 solution 70 mg/l
72
Table 5 - Temperature influence on settling
Coagulant Dosage
(mg/l) Coagulant
form Tt/T0 = 0.5
20 °C (min) Tt/T0 = 0.5
5 °C (min) Tf/T0
20 °C Tf/T0
5 °C Water
sample
ALG 70 solid 4 - 6 8 - 11 0.10 0.15 Nav.
Ferix-3 60 solid 2 - 3 3 - 4 0.07 0.13 Nav.
ALF-30 70 solid 4 - 6 8 - 11 0.16 0.30 Nav.
ALG 70 solution 4 - 6 6 0.08 0.10 Nav.
Ferix-3 60 solution 2 - 3 4 - 6 0.04 0.11 Nav.
ALG 80 solid 6 - 8 8 - 11 0.16 0.22 Piip.
Ferix-3 70 solid 2 4 - 6 0.09 0.17 Piip.
ALF-30 80 solid 6 - 8 11 0.24 0.35 Piip.
ALG 80 solution 2 - 3 4 - 6 0.09 0.12 Piip.
Ferix-3 70 solution 2 - 3 4 - 6 0.05 0.09 Piip. Nav. = Navettarimpi sample; Piip. = Piipsanneva sample
The effects of low temperature in settling were the most evident in the first minutes and
most intense period of settling time; from 0 to 6 minutes. Ferix-3, in solid and solution
form, appears to have suffered these effects to a greater extent although it still provided at 5
°C the fastest settling rates between all tested coagulants (Figures 26 and 27).
ALG, ALF-30 and Ferix-3 in solid form required between half and twice as much time to
reach 50% removal of turbidity (Tt/Tt0 = 0.5) at 5 °C than at 20 °C. ALF-30 presented
reductions of 10 to 15% on final supernatant water clarification while reductions of 5 to
10% were observed for ALG and Ferix-3 (Table 5).
73
Figure 26 – Influence of temperature on the settling characteristics of ALG, ALF and
Ferix-3 in solid form (a, b), ALG solution (c) and Ferix-3 solution (d) applied to
Navettarimpi water sample.
As previously mentioned the coagulants applied in solution form (ALG and Ferix-3) also
presented delays on their settling rates and reductions in final water clarification when
tested at lower temperature. Furthermore, the observed delays were more accentuated while
as well more constant between experiments and purified water samples than when solid
coagulants were applied. Coagulants in solution form required in all experiments (except
for ALG applied to Navettarimpi sample) twice as much time to reach 50% removal of
turbidity at 5 °C than at 20 °C (Table 5). Nevertheless it was observed that ALG solution
while applied to Navettarinpi sample was only slightly influenced by the lowering of
temperature as it can be seen in Figure 26 (c).
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 10 20 30
Tt/T
t0
Time (min)
ALF-30 70 mg/l 20 C
ALG 70 mg/l 20 C
Ferix-3 60 mg/l 20 C
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 10 20 30
Tt/T
t0
Time (min)
ALF-30 70mg/l 5 C
ALG 70 mg/l 5 C
Ferix-3 60 mg/l 5 C
0,00
0,20
0,40
0,60
0,80
1,00
1,20
0 10 20 30
Tt/T
t0
Time (min)
ALG 70 mg/l 20 C
ALG 70mg/l 5 C
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 10 20 30
Tt/T
t0
Time (min)
Ferix-3 60 mg/l 20 CFerix-3 60 mg/l 5 C
a) Solid coagulants 20°C b) Solid coagulants 5°C
c) ALG solution at 5 and 20°C d) Ferix-3 solution at 5 and 20°C
74
Figure 27 - Influence of temperature on the settling characteristics of ALG, ALF and
Ferix-3 in solid form (a, b); ALG solution (c); and Ferix-3 solution (d) applied to
Piipsanneva water sample.
6.4 Influence of mixing
The objective of this phase of laboratory experiments was not only to identify the influence
of the mixing effect applied upon and after coagulant addition but also to establish some
guidelines to the mixing requirements of the individual coagulants. Table 6 contains the
optimum mixing parameters identified for each of the tested coagulants while purifying
both Navettarimpi and Piipsanneva water samples. These optimum parameters were
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 10 20 30
Tt/T
t0
Time (min)
Ferix-3 70 mg/l 20 C
ALG 80 mg/l 20 C
ALF-30 80 mg/l 20 C
0,00
0,20
0,40
0,60
0,80
1,00
1,20
0 10 20 30
Tt/T
t0
Time (min)
Ferix-3 70 mg/l 5 C
ALG 80 mg/l 5 C
ALF-30 80 mg/l 5 C
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 10 20 30
Tt/T
t0
Time (min)
ALG 80 mg/l 5 C
ALG 80 mg/l 20 C
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 10 20 30
Tt/T
t0
Time (min)
Ferix-3 70 mg/l 5 C
Ferix-3 70 mg/l 20 C
a) Solid coagulants 20°C b) Solid coagulants 5°C
c) ALG solution at 5 and 20°C d) Ferix-3 solution at 5 and 20°C
75
identified as the parameters which provided the highest removal efficiency for the most
number of analyzes (tot-N, tot-P, PO4-P, SS and TOC).
Table 6 – Optimum mixing parameters identified for all coagulants in solid and solution
form while testing both Navettarimpi (n = 1)and Piipsanneva (n = 2) water samples.
Dosage
Slow
mix
speed
Slow
mix
time
Fast
mix
speed
Fast
mix
time Average removal efficiency (%) Coagulant (mg/l) Sample (rpm) (min) (rpm) (s) tot-N tot-P PO4-P SS TOC
ALG 70 Nav. 70 25 400 10 39 89 91 95 52
ALF-30 70 Nav. 70 25 400 10 35 85 87 82 51
Ferix-3 60 Nav. 70 25 400 10 44 90 87 88 62
ALG sol 70 Nav. 50 25 100 10 33 88 89 85 39
Ferix-3 sol 60 Nav. 50 25 300 10 40 92 89 94 68
ALG 80 Piip. 90 15 400 60 15 75 83 68 20
ALF-30 80 Piip. 90 15 400 60 13 72 83 54 26
Ferix-3 70 Piip. 70 25 400 60 30 89 92 83 61 ALG sol 80 Piip. 50 25 300 10 20 71 75 77 22 Ferix-3 sol 50 Piip. 50 25 300 10 33 89 89 88 63
Bold values represent the best removal efficiency observed for the indicated substance and the
related coagulant. The post-fix “sol” refers to the coagulant in solution form. Nav. = Navettarimpi
sample; Piip. = Piipsanneva sample.
Tables C, D and E of Appendix 1 present the purification efficiencies achieved for all tested
variations of fast mixing (coagulation) speed and time as well as slow mixing (flocculation)
speed and time. A short analysis of the results presented on tables C, D and E highlighted
the fact that the applied mixing time and speed on both, coagulation and flocculation stages
of the purification process had some influence on the removal efficiencies achieved.
Furthermore the obtained results indicated that the influence of the mixing parameters
applied during the flocculation process was somewhat greater than the influence of the
mixing parameters applied during the coagulation process.
Some distinction was observed between the mixing requirements of coagulants in solid and
solution form. However, although the mixing parameters are somewhat coagulant specific,
similar optimum mixing requirements were found among the solid coagulants and
coagulants in solution form.
76
For the coagulation stage high mixing speeds of 400 rpm and mixing times from 10 to 60 s
provided the best removal efficiencies for all solid coagulants. The coagulants tested in
solution form required lower mixing speeds and shorter mixing times for the coagulation
stage compared to their solid counterparts. ALG and Ferix-3 in solution required no more
than 10 seconds of fast mixing time and mixing speeds from 100 to 300 rpm. The applied
variations of fast mixing time and speed during this experiment resulted on fluctuations
between 5 and 20% on removal efficiencies obtained for all analyzed substances (Tables C,
D and E of Appendix 1).
As it can be seen in Table 6, for the flocculation stage of the purification process, mixing
times from 15 to 25 minutes and mixing speeds from 70 to 90 rpm produced the best
removal efficiencies for all solid coagulants. While for the coagulants in solution form the
longest applied slow mixing time of 25 minutes combined with a middle range mixing
speed of 50 rpm resulted in the best removal efficiencies. The applied variations of slow
mixing time and speed during the jar test experiments resulted on fluctuations between 5
and 100 % on removal efficiencies obtained for all analyzed substances (Tables C, D and E
of Appendix 1).
For solid coagulants low mixing speed of 20 rpm, even when combined with mixing times
inside the optimum range, produced removal efficiencies up to 80% lower when compared
to efficiencies achieved with optimum speeds of 70 to 90 rpm. An example of this
reduction in purification efficiency is the removal of tot-P by ALG when applied to
Navettarimpi water sample which is around 75% lower at 20 rpm than at 70 rpm. Short
flocculation mixing time of 5 minutes appears to have had an even greater influence on
removal efficiencies especially for the removal of SS where the fluctuations in removal
efficiency compared with optimum mixing conditions were greater than 100%.
Noticeable distinction was observed between the individual solid coagulants regarding the
magnitude of the influence exert by short mixing times and low mixing speeds during
flocculation. Low mixing speeds appear to have affected more the purification levels
achieved by ALG and ALF-30, greatly reducing the removal efficiency of all analyzed
substances. While short mixing times had a more negative effect on the purification
77
efficiency of Ferix-3. Although it is clear that individually short mixing times and low
mixing speeds have a significant negative effect on the purification levels achieved by all
solid coagulants.
As aforementioned the mixing requirements presented by the coagulants in solution form
differed from the mixing requirements of solid coagulants. However, low mixing speeds
and short mixing times in the flocculation stage also produced significant fluctuations in the
removal efficiency of coagulants in solution form. Nevertheless the fluctuations were not as
pronounced as those observed for the solid coagulants. Low mixing speed of 20 rpm and
short mixing time of 5 minutes produce similar reduction in removal efficiencies for both
tested coagulants ALG and Ferix-3. Overall, these mixing parameters when applied
individually caused reductions of 40% in the SS removal efficiency and 10% in the removal
efficiencies of all other analyzed substances.
78
7 Discussion
Chemical purification as a water treatment method is the sum of three singular but strongly
linked processes: coagulation, flocculation and sedimentation. As expected, the results
obtained throughout the four phases of laboratory experiments performed in this thesis
work highlighted the fact that chemical purification of any given water is highly dependent
on a series of parameters such as: pH, temperature, coagulant type and dosage, applied
mixing and physicochemical characteristics of the water. It is important to emphasize that
these parameters affect not only individually the aforementioned processes but also the
relationship between them.
Purification efficiency
Overall, based on the presented results all tested coagulants achieved under optimum
chemical purification conditions high load reduction levels. The obtained average load
reductions were generally higher than those reported by Alatalo & Peronius (2004) regarding
Vapo Oy internal research project on field applications of metal salt coagulants (40 - 45%
SS, 27 - 32% tot-N, 72% tot-P and 48% COD). Furthermore, the achieved purification
levels were inside of the load reductions range expected for chemical purification of peat
runoff water (30 - 90% SS; 30 - 60% tot-N and 75 - 95% tot-P), published by CFREC
(2004).
The summary of average load reductions range achieved by each of the tested coagulants
under optimum conditions is presented in Table 7. Optimum conditions here refer to the
results obtained from experiments at 20°C with applied mixing conditions of laboratory
experiments phase 1 (Solid coagulants: 400 rpm for 60s, 70 rpm for 15 min, 30 min
sedimentation; Coagulants in solution form: 300 rpm for 10s, 50 rpm for 25 min, 30 min
sedimentation) and via the application of optimum mixing parameters identified in
laboratory experiments phase 4 for each tested coagulants (Table 6).
79
When comparing the performance of the coagulants tested in solid and in solution form the
overall removal efficiencies achieved were very similar. However, ALG and Ferix-3 when
tested in solution required consistently lower dosages, between 5 and 30%, and presented a
much higher consistency in purification efficiency between experiment replications.
Table 7 – Average load reduction range presented by each of the tested coagulants under
optimum conditions.
Dosage Load reduction range (%)
Coagulant (mg/l) Sample tot-N tot-P PO4-P SS TOC
*AlCl3 50 Nav. 37 79 77 76 42
ALG 70 Nav. 24 - 39 80 - 89 80 - 92 60 - 95 39 – 52
ALF-30 70 Nav. 29 - 35 81 - 85 83 - 90 65 - 83 40 – 51
Ferix-3 60 Nav. 40 - 44 87 - 90 85 - 91 75 - 96 60 – 62
ALG sol 70 Nav. 29 - 33 87 - 88 84 - 89 85 - 88 39 – 42
Ferix-3 sol 60 Nav. 39 - 40 89 - 92 89 - 93 92 - 94 67 – 68
*AlCl3 50 Piip. 18 73 74 74 23
ALG 110 Piip. 15 - 18 68 - 75 76 - 83 59 - 72 20 – 25
ALF-30 80 Piip. 13 - 18 67 - 72 63 - 83 48 - 71 25 – 26
Ferix-3 70 Piip. 33 - 35 87 - 89 87 - 92 59 - 83 56 – 61
*ALG sol 80 Piip. 20 71 75 77 22
*Ferix-3 sol 50 Piip. 33 89 89 88 63 *Average load reductions obtained under test condition of laboratory experiments phase 1. Nav. =
Navettarimpi sample; Piip. = Piipsanneva sample
Coagulation process
A good understanding of the coagulation mechanisms occurring when a coagulant agent is
added to the water is crucial for the optimization of the chemical purification process. In
other words, it is important to understand the ways in which the coagulant reacts or
interacts with the water and the different substances in solution. This understanding
provides the knowledge that field conditions can favour or can be altered to favour the
performance of a particular coagulant and the extraction of the most undesirable
substances. It is nevertheless important to highlight that the identification of the exact
coagulation mechanisms occurring is a difficult task. The physicochemical characteristics
of the water (pH, temperature, etc.), the nature and characteristics of other substances in
80
solution (e.g. NOM and alkalinity), the type and dosage of the coagulant as well as the
mixing effect applied upon coagulant addition are all factors which directly influence the
reactions occurring during the coagulation process. (Omoike & Vanloon, 1999; Lin & Lee,
2007, p. 377; Jiang & Wang, 2009)
Based on the experiments performed and the results obtained during this study is not
possible to affirm the exact reactions occurring when the metal salts of iron and aluminium
were added to the water. It is nevertheless possible based on the coagulation diagrams of
aluminium and iron coagulants (Figures 3 and 4) proposed by Amirtharajah and Mills
(1982, cited in Bratby, 2006, p. 85) and Johnson and Amirtharajah (1983, cited in Bratby,
2006, p. 86) to generally evaluate the coagulation mechanisms occurring under the
conditions observed during our experiments. The required coagulants dosages and the
observed pH of the water after coagulant addition (Table 4) were applied to the above
mentioned diagrams to identify the most likely prevailing coagulation mechanisms. It is
important to emphasize that the results been analyzed here were obtained via application of
optimum mixing effect upon and after the coagulants addition. This has assured
instantaneous spread of the coagulants throughout the liquid mass, enabling the occurrence
of all coagulation mechanisms and effective agglomeration of the flocs in the subsequent
flocculation process.
For the purification tests with Navettarimpi sample the aluminium based coagulants
required dosages between 50 and 100 mg/l for optimum purification efficiency. The
observed pH of the resulting solutions stood between 4.5 and 5. The iron based coagulants
also require dosages between 50 and 100 mg/l resulting in pH values between 3.5 and 4. By
applying this information to the diagrams presented in Figures 3 and 4 it is possible to
conclude that although all four coagulation mechanisms (double layer compression, charge
neutralization, entrapment in a precipitate and intra-particle bridging) occurred the
mechanism which prevailed for the aluminium based coagulants was that of charge
neutralization with cationic metal hydroxides species and amorphous Al(OH)3. The
coagulation mechanism followed by iron based coagulants appeared to have fallen into a
zone of direct charge neutralization via trivalent metal cations and possible re-stabilization
of colloidal charges due to the high dosage of coagulant.
81
The observed behavioural patterns of turbidity removal with increasing coagulant dosage
(Figures 14, 15, 16 and 17) may be directly linked to the coagulation mechanisms occurring
when the individual chemicals were added to the water. Also according to the diagrams
presented in Figures 3 and 4, metal salts of iron and aluminium possess a coagulation
mechanism zone where re-stabilization of the colloids in solution occurs. This is due to the
effective neutralization of the surface charges of colloidal particles. Through the adsorption
of positively charged hydrolysis products the previously negatively charged colloidal
particles can acquire strong enough positive charges which cause them to once again repel
each other and remain in suspension.
During our previous evaluation regarding the coagulation mechanisms occurring for the
conditions applied during our experiments, only the iron based coagulants appeared to
present the re-stabilization as an occurring mechanism. This statement can be further
confirmed by the observed patterns of turbidity removal with increasing dosages of
aluminium and iron based coagulants. As it is shown in Figures 14, 15, 16 and 17 only for
iron and mixed based coagulants (Ferix-3 and ALF-30) presented a decrease and
subsequent increase in turbidity values with increasing dosages of coagulants. It is safe to
assume that destabilization and subsequent re-stabilization of the colloidal suspension has
occurred when increasing dosages of iron based coagulants were applied.
The occurrence of re-stabilization of colloidal particles and the subsequent increase in
turbidity of the treated samples have a direct impact on the field application of iron based
coagulants. Over dosages of any metal salt coagulant should be prevented not only for
financial reasons but also because of the increase in the discharge of metal residuals.
However, while working with aluminium coagulants under or over dosage conditions do
not render the purification process completely inadequate. For iron based coagulants
dosages lower or higher than optimum can cause an immediate increase in the SS
concentration of the water sample. In summary, for the purification of runoff waters
presenting the water quality characteristics of Navettarimpi and Piipsanneva samples, iron
based coagulants required much stricter control of applied dosages if high purification
efficiencies are to be achieved.
82
Removal of analyzed substances
The best removal of particles from solution is known to be achieved when conditions
favouring the sweep coagulation mechanism (high coagulant dosages and neutral pH
values) are applied. (Gregory and Duan, 2001; Bratby, 2006, p. 85) Throughout our study
the identified optimum dosages of the tested coagulants were high enough to enable the
sweep coagulation mechanism. However, the pH of the resulting solutions (Table 4)
according to Lindquist (2003, p. 20) were outside the optimum sweep mechanism
requirements (5.5 ≤ pH ≤ 6.5 for Al and 5.5 ≤ pH ≤ 8 for Fe). Note that the sweep
coagulation mechanism still occurs outside the optimum pH range, it is only less effective.
The influence of pH on the removal of SS was made clear by the results observed when
Piipsanneva sample was purified. The removal of SS from Piipsanneva sample at field pH
was around 20% lower than when it was tested after its pH had been raised ( Table 4). The
pH of the purified water sample was only shifted half unit towards a more basic condition;
nevertheless it was shift closer to the optimum sweep mechanism requirements.
The SS removal efficiencies achieved by Ferix-3 and ALG in solution were higher when
compared to their solid counterparts (Table 4). This may be attributed to the fact that the
pre-dissolution of the coagulant allows the formation of more suitable hydroxide species
favouring the sweep coagulation mechanism and the subsequent flocculation process.
Regarding the removal of organic matter via chemical purification, the obtained results
agree with reports by Pernitsky and Edzwald (2006) and a series of other researcher
(Gregory & Duan, 2001; Matilainen, et al., 2005; Libecki & Dziejowski, 2007; Jiang &
Wang, 2009; etc.) which affirmed that the pH of the solution has a direct effect on the
required coagulant dosages and the NOM removal efficiencies to be achieved. The
aforementioned authors also affirmed that due to a stronger formed humic-iron bound than
humic-aluminium bound, the ionic metal hydroxides species of iron react more strongly
with humic acids than those of aluminium, resulting in a higher NOM removal efficiency
for iron salts.
83
Our results show that overall Ferix-3 removed about 20% more organic matter (TOC) than
the aluminium based coagulants (Table 7). The pH of the resulting solution was found to
have a direct impact on NOM removal (Table 4). It is important to note that due to a greater
affinity of ferric ions for OH-, iron and aluminium based coagulants have different pH
ranges for best removal of NOM; 5 ≤ pH ≤ 5.5 for Al and 3.7 ≤ pH ≤ 4.2 for Fe. (Jiang &
Wang, 2009; Bratby, 2006, p. 95) Throughout our experiments, the pH of the purified water
when Ferix-3 was applied fell just inside iron‟s optimum pH range for NOM removal,
resulting in TOC removal efficiencies of over 60%. The pH of the purified water samples
when aluminium based coagulants were applied were consistently lower than the pH for
best removal of NOM by aluminium salts, fact that may explain the observed lower
removal efficiencies of TOC by aluminium than by iron salts.
According to Bratby (2006, p. 98) coagulation via addition of metal salt coagulants
promptly removes from solution the higher molecular weight fraction of NOM. The lower
molecular weight fraction of NOM mostly composed of fulvic acids is recalcitrant to the
removal by metal salt coagulants and is often the fraction present in treated samples. The
percentages of organic matter removal achieved during our studies (up to 60%) are in
agreement with the removal achieved by normal coagulation process via metal salts.
Enhance coagulation method where the pH and other water characteristics (e.g. alkalinity)
are closely monitored and higher coagulant dosages are applied is known to remove up to
90% of the TOC from solution. (Bratby, 2006, pp. 97-100)
It was emphasized in the results section of this work that the optimum dosages of
aluminium based coagulants for the purification of Piipsanneva sample may had been
overestimated. According to statements already made in this discussion section it is clear
that, the physicochemical characteristics presented by Piipsanneva sample, mainly its pH,
did not favour the removal of particulates and organic matter by aluminium coagulants. The
high applied dosages of aluminium coagulants resulted on even lower pH levels moving the
solutions physicochemical characteristics further away from the previously mentioned
optimum condition for the removal of SS and NOM by aluminium salts. Furthermore, the
removal efficiencies obtained via the addition of the limit dosages of aluminium salts were
overall only 10 to 15% lower than those obtained at optimum dosages. However, the
84
optimum dosages were around 50 to 300% higher than the applied limit dosages. It is of the
author‟s opinion that the obtained 10 to 15% increase in purification efficiencies does not
justify the application of 300% higher dosages. In this line, the identified optimum dosages
of aluminium based coagulants for the purification of Piipsanneva water sample were
somewhat overestimated.
The removal of nitrogen via chemical purification with metal salts is very limited. Nitrogen
is usually removed in water and wastewater treatment facilities via bio-chemical
nitrification-denitrification processes. (Sincero & Sincero, 2003, p. 659) The nitrogen
compounds removed via chemical purification are those linked to organic matter which
according to Kløve (2001) account for 25 to 50% of the nitrogen load discharged from peat
harvesting sites. A closer look at Table 7 and the removal efficiencies achieved for tot-N
and TOC by all coagulants confirms that the removal of tot-N increased or decreased
according to the removal of TOC from solution.
According to Yang et al. (2010) the coagulation mechanisms of metal salt coagulants
involved in phosphorous removal at the acidic pH values observed during our studies are:
precipitation via amorphous hydroxides and mostly adsorption to cationic hydroxides
species of iron and aluminium. Under acidic conditions the most abundant phosphate specie
is H2PO4- (Figure 6) which is readily adsorbed onto the strong positive charged metal
hydroxides species in solution (Figures 9 and 10). Lindquist (2003, p. 127) stated that the
best removal of phosphate via addition of aluminium and iron salts occurs at acidic pH
conditions (Figures 7 and 8).
The observed removal of phosphorous by all tested coagulants was reasonably high (Table
4). Some distinction was nevertheless noticed between tested water samples. For
Navettarimpi sample, the high purification efficiencies achieved may be attributed to the
fact that the pH of the purified water was just inside the recommended values for
phosphates removal by iron (< 4) and aluminium (ca 5) salts. (Sincero & Sincero, 2003, p.
642) As well as to the high removal of SS solids from solution, resulting in the extraction
of the particulate phosphorous fraction present in the water. When Piipsanneva sample was
concerned, the removal of PO4-P by aluminium based coagulants was slightly lower than
85
for iron based coagulants. This may be once again due to the fact that pH of the purified
water was lower than optimum conditions for aluminium coagulants but still favoured
phosphates removal from solution.
The hypotheses that the lower pH presented by Piipsanneva sample was responsible for the
overall lower removal efficiencies achieved by all but especially by aluminium based
coagulants was confirmed. The clear influence of pH on the mechanisms involved in the
removal of tot-P, PO4-P, SS and TOC from solution (Gregory & Duan, 2001; Pernitsky &
Edzwald, 2006; Jiang & Wang, 2009; Yang et al. 2010) have already been extensively
discussed. It has become clear that the lower pH presented by Piipsanneva sample (5.8) had
a detrimental effect on the removal mechanisms of most concerned substances. This fact is
further confirmed by the removal efficiencies obtained for the purification of Piipsanneva
raised pH (6.85) sample (Figure 18) which are considerably higher at lower coagulant
dosages than those observed for the sample at lower field pH.
It is important to emphasize that the identified optimum dosages of all tested coagulants for
the purification of Navettarimpi and Piipsanneva samples are somewhat water quality
specific. Although other process parameters also influence the required coagulant dosages,
the physicochemical characteristics of the runoff water mostly dictate the dosages to be
applied. It has been evident throughout this discussion section that the pH of the water
sample greatly influences the purification process as a whole and the required coagulant
dosages. In the case of solid coagulants the pH of the solution affects not only the molar
fraction of formed hydroxides species but also the dissolution of the applied coagulant.
The concentration of NOM and SS in the water samples tested during our studies were very
similar. Nevertheless it is important to note that the concentration of and type of NOM in
the runoff water samples is known to have a large influence on the coagulant dosages
needed during chemical purification. It is believed that for high coloured waters with low
SS concentration there is stoichiometric relation between NOM concentration and
coagulant dosage requirements. (Gregory & Duan, 2001; Jiang & Wang, 2009) On the
other hand the concentration of SS in the raw water samples has a definite but not direct
influence on the required coagulant dosages. The dosage needed for destabilization of a
86
suspension containing certain SS concentration may not increase when the SS
concentration of the sample is increased. According to Bratby (2006, p. 79) this may be
explained by the fact that under specific conditions increases in SS concentration result in
an increase in contact opportunity between coagulant species and colloidal particles,
improving colloid destabilization and the sweep coagulation mechanism.
Peat harvesting runoff water quality characteristics are known to vary not only with
location but also with variations in runoff and peak discharge occurrences, it is important to
understand that the identified optimum coagulant dosages are to some extent sample
specific. The identified dosages can and should be used as guide line values for water
samples presenting similar characteristics. Furthermore, the identified dosages should also
be used as starting point for water samples whose quality characteristics greatly differ from
the samples tested during this study.
Coagulation process parameters; mixing, temperature and the settling characteristics
of the formed flocs
The variations in purification efficiencies observed throughout the laboratory experiments
phase 4 when the influence of the applied mixing parameters was evaluated are a clear
statement of the importance of the optimization of these parameters to the chemical
purification process. The mixing effect applied during the coagulation process affects, not
only the occurring coagulation mechanisms, but also in the case of solid coagulants, the
dissolution of the applied metal salts. The mixing effect applied during the flocculation
process has a direct impact on the characteristics of the final formed flocs and consequently
has a direct impact on the removal efficiencies achieved.
The mixing parameters applied during the laboratory experiments phase 1 were chosen to
comply with literature related guidelines and have produced satisfactory purification levels.
However, the optimization of these mixing parameters carried out in laboratory
experiments phase 4 (Table 6) have greatly increased the removal efficiencies achieved by
all tested coagulants. This highlights the fact that the optimization of the mixing parameters
87
has the potential of reducing the maintenance costs of chemical purification by increasing
removal efficiencies without increasing coagulant dosages.
The majority of published studies reporting the influence of mixing on coagulation and
flocculation processes are based on results obtained via addition of coagulants in liquid
form (e.g. Rossini e al., 1999; Yan e al., 2009). Due to the clear distinction presented
between the optimized mixing parameters of coagulants applied in solid and in solution
form (Table 8), it is safe to conclude that the dissolution of the coagulant should also be
taken into account when evaluating mixing requirements of the chemical purification
process.
While dosing the coagulants in solid form, the time consumed for the dissolution of the
added solid coagulants, especially Ferix-3, called our attention. Due to its large sized
granules, Ferix-3 required much longer dissolution time than the other tested coagulants. It
was possible to see vestiges of the solid coagulant up to 5 minutes into flocculation time.
All solid coagulants required high mixing speeds or high velocity gradients during the fast
mixing stage (Table 8). Fact that may be explained by the higher turbulence required for the
dissolution of the coagulant. The slow dissolution of the applied solid coagulants may have
direct implications on the purification levels achieved during field applications. In
processes where not enough turbulence and prolonged mixing effect is provided upon
chemical addition, the applied coagulant may settle in layers to the bottom of the
sedimentation pond. Because part of the added coagulant fails to react lower than expected
purification levels may then be observed.
The estimated velocity gradient values and mixing times identified as the optimum mixing
parameters for each of the tested coagulants are presented in Table 8. Note that the velocity
gradients applied during the coagulation and flocculation stages and the mixing time
applied during flocculation stage are mostly in accordance with the recommended values
(G = 500 to 1000 s-1
for 60 to 120 s and G = 20 to 70 s-1
for 10 to 30 min.) by Lin and Lee
(2007, pp. 379 - 380). However, the mixing times applied during the coagulation stage are
mostly in accordance with the views presented by Bratby (2006, p. 220). Bratby in contrary
88
to most available literature affirmed that shorter mixing times of less than 10 s should be
applied during the coagulation stage of chemical purification.
Table 8 – Estimated optimum (coagulant specific) velocity gradient „G‟ values for the
coagulation and flocculation stages of chemical purification.
Coagulant Dosage
(mg/l) Sample
G (s-1
)
coagulation
phase
Time
(s)
G (s-1
)
flocculation
phase
Time
(min)
ALG 70 Nav. 756 10 55 25
ALF-30 70 Nav. 756 10 55 25
Ferix-3 60 Nav. 756 10 55 25
ALG sol 70 Nav. 95 10 34 25
Ferix-3 sol 60 Nav. 491 10 34 25
ALG 80 Piip. 756 60 80 15
ALF-30 80 Piip. 756 60 80 15
Ferix-3 70 Piip. 756 60 80 25
ALG sol 80 Piip. 491 10 34 25
Ferix-3 sol 50 Piip. 491 10 34 25 Nav. = Navettarimpi sample; Piip. = Piipsanneva sample
The individual influence of mixing parameters on the coagulation and flocculation
processes can be evaluated based on the purification efficiencies achieved under different
applied mixing times and intensity (Tables C, D and E – Appendix 1). Our findings
regarding the influence of mixing on the coagulation process appear to agree with the views
of Rossini e al. (1999) and Yan et al. (2009). The aforementioned authors affirmed that if
suitable mixing intensity is provided, the duration of the applied mixing has little influence
on the removal efficiencies of dissolved substances such as organic matter. While the
removal of SS from solution decreases slightly with increasing mixing times. On the other
hand, when suitable mixing times are applied the removal of SS from solution increases
with increasing mixing intensity while no significant influence is observed in the removal
of dissolved substances.
As expected the mixing parameter applied during the flocculation process had a direct
influence on the removal of SS from solution. The observations made during our
experiments regarding the characteristics of the formed flocs mostly agreed with widely
89
reported characteristics in the literature (e.g. AWWA, 1990, p. 350; Sincero & Sincero,
2003, p. 315). Low velocity gradients of less than 20 s-1
and long retention times of 25 min.
produced large but light flocs which did not settle well. Short retention times of 5 minutes
and higher mixing intensity produced very small flocs, especially in the case of solid
coagulants, which also remained in suspension.
It is important to note that the characteristics of the final formed flocs are also dependent on
the concentration of SS in solution and the characteristics of the primary flocs formed
during the coagulation stage. As it has already been discussed, different coagulants will
react through different coagulation mechanisms under different process conditions,
resulting in coagulated species with different properties which will then agglomerate and
form flocs with different characteristics.
This becomes evident when analysing the results obtained for the settling characteristics of
all tested coagulants. Different coagulants produced flocs with different characteristics
which settle at different rates and resulted in supernatant water with different quality.
Clearly the process condition observed during our study favoured the purification via
addition of iron salts and as expected, Ferix-3 presented the best settling characteristics
among the tested coagulants. Our observations made during settling experiments
highlighted the fact that the flocs formed via addition of Ferix-3 appeared more compact
and well formed than flocs formed via addition of aluminium based coagulants. And
although it is possible to see slight difference between the settling characteristics of the
coagulants tested in solid and in solution form, the greatest distinctions are nevertheless
observed between the settling characteristics of different coagulants.
The settling characteristics presented by the flocs of the applied coagulant are very
important design parameters in the implementation of chemical purification process. It is
extremely important for example, that the surface load applied to the sedimentation pond is
at all times lower than the terminal settling velocities of the formed flocs. If the surface
load or overflow rate applied is higher than the terminal settling velocities of the flocs, they
will be then carried with the out flowing water and discharged into the receiving water
body. The settling curves for the flocs formed via addition of solid Ferix-3 and ALG to
90
Navettarimpi sample are presented in Figure 28. It is clear that under the conditions
observed during our experiments, samples treated with Ferix-3 would support much higher
overflow rates in the sedimentation basin than samples treated with ALG. For the removal
of 60% of the SS concentration from solution, surface loads equal or lower than 0.045 cm/s
could be applied for samples treated with Ferix-3. While samples treated with ALG would
require surface loads equal or lower than 0.02 cm/s. This has a direct impact on the
dimensions of the sedimentation basin to be constructed and consequently on the costs
involved and feasibility of the treatment method implementation.
Figure 28 – Settling curves of Ferix-3 and ALG when applied in solid form to
Navettarimpi water sample.
It cannot be forgotten that the physicochemical characteristics of the formed flocs are also
dependent on the temperature of the water to be purified. Furthermore the temperature of
the water also influences the actual settling of the flocs since it has a direct effect on water
properties such as density and viscosity. The dynamic viscosity of the water at 5°C, for
example, is around 50% higher than at 20°C (AWWA, 1990, p. 421). This increase in the
water viscosity alone according to Stoke‟s law (equation 9) would result in a substantial
decrease in the flocs settling velocity with decreasing temperatures.
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0,00 0,05 0,10 0,15
Yi =
Ti/
T0
vi (cm/s)
Ferix-3 ALG
91
According to Kang, et al. (1995) and Xiao, et al. (2009) low temperatures affect the
chemical purification process in several ways. Under acidic conditions and temperatures
around 5°C the equilibrium between metal hydroxides species is reported to be dislocated
and more of stronger positively charged fraction are present than at 20°C (Figures 7 and 9).
This would favour the occurrence of charge neutralization and adsorption reactions
enhancing the removal of dissolved substances. This statement is somewhat supported by
our experiments where the removal of all phosphorus fractions, organic matter and nitrogen
suffered little or no worsening effect due to the low applied temperature. Kang and Xiao
also affirmed that the hydrolysis reactions are slower at low temperatures. Slow reaction
should lead to late precipitate formation and cause a negative effect on purification
efficiencies especially in SS removal. This effect is reported to be reduced by application of
pre-hydrolyzed coagulants. The results obtained throughout our experiments at low
temperature are consistent with these affirmations. All solid coagulants had their SS
removal efficiency greatly reduced, with iron and mix based coagulants been the most
affected. And although the removal of SS by the coagulants in solution form (pre-
hydrolyzed) was also reduced at low temperature, the negative effect was much lower when
compared to the coagulants in solid form.
Nevertheless, the majority of the recent studies (Northcott, et al., 2005; Xiao, et al., 2008,
2009) do agree that the biggest influence of low temperatures in the chemical purification
process is on flocs aggregation rate and consequently on the characteristics of the final
formed floc. The flocs formed at low temperature are reported to be smaller and less
compact having direct impact on the removal of SS from solution. Observation made
during our laboratory experiments at low temperature are in agreement with this previous
statement. Smaller and apparently less dense flocs were observed for all tested coagulants.
These mostly unwanted flocs characteristics coupled with an increase in water viscosity
may account for the slower settling rates obtained at 5°C (Table 5). Furthermore, a higher
fraction of the formed flocs were observed to remain in suspension after the sedimentation
time had elapsed, resulting in the lower removal of SS obtained for all tested coagulants.
Another important factor regarding the chemical purification treatment method is the
concentration of metal residuals remaining in the water after the purification process. Table
92
F (Appendix 1) contains the measured concentration of iron and aluminium present in
Navettarimpi water sample prior and after chemical purification. When aluminium and mix
based coagulants were applied, over 90% of the iron background concentration was
removed from solution. However, the aluminium concentration in the purified water sample
was significantly increased to values between 2400 and 3000 µg/l which are much higher
than the recommended 100 µg/l (Butcher, 2001) concentration for aquatic systems. When
Ferix-3 was applied, the background aluminium concentration was reduced in around 60%
and a 30% reduction in the background concentration of iron was also observed.
Nevertheless the iron concentration in the treated sample, over 2500 µg/l, is still high and
clearly above the 1000 µg/l (Vuori, 1995) recommended for aquatic ecosystems.
The high metal residual concentration observed in the purified water samples were mostly a
negative effect of the acidic nature of the water samples being treated. The solubility of the
amorphous metal hydroxides species formed when metal salt coagulants are added to water
(Figure 10) is particularly high at pH levels observed during our experiments. The
solubility also increases with decreasing temperatures under acidic pH conditions. Fact
which is supported by the even higher metal residual concentrations found in the samples
from the experiments carried out at 5 °C.
The concentration of metal residuals in the purified samples could be significantly reduced
if the pH of the water was to be neutralized prior or after the chemical purification process.
Raising the pH of the purified water to levels of least solubility of aluminium (≈ 6) and iron
(≈ 8) hydroxide species (Figure 11) would assure higher formation of metal precipitates and
lower metal residual concentrations. Nevertheless the negative environmental impacts
related to aluminium and iron concentration in aquatic systems must be taken into account
while evaluating the viability of chemical treatment method. The sensitivity of the
receiving water body and factors such as dilution and accumulation of the discharging
metal concentrations must be carefully analyzed.
93
8 Conclusion and future aspects
Based on the presented results all tested solid coagulants obtained, under optimum
chemical purification conditions, high load reduction levels. The purification efficiencies
achieved regarding the removal of tot-N (15 – 44%), tot-P (67 – 90%), PO4-P (63 – 93%),
SS (48 – 96%) and TOC (20 – 62%) from the tested peat harvesting runoff water samples
were mostly inside the expected reduction levels for the chemical purification treatment
method. Nevertheless as expected, the results obtained throughout the four phases of
laboratory experiments highlighted the fact that chemical purification of peat harvesting
runoff water is highly dependent on a series of process parameters such as: coagulant type
and dosage, pH, temperature, applied mixing and physicochemical characteristics of the
water.
Overall, Ferix-3 was the best performing coagulant. It required, under optimized mixing
conditions, around 15% lower dosages (60 to 70 mg/l) than ALG and ALF-30 (70 to 80
mg/l) for the purification of Navettarimpi and Piipsanneva water samples. Furthermore,
even at lower dosages, it achieved slightly higher removal efficiencies. Ferix-3 also
produced the best settling characteristics among the tested coagulants. It presented the
fastest sedimentation rates and best final clarification of the supernatant water. However,
solid Ferix-3 presented a very narrow optimum dosage range and appears to have suffered
to a higher extend the effects of low temperature. The application of dosages lower or
higher than optimum, caused an immediate increase in the SS concentration of the water
samples being purified at 20 °C. Ferix-3 thus requires stricter dosage control than the other
tested coagulants. When tested at 5°C the dissolution of the large granules of Ferix-3 was
delayed and, to a greater extent than to the other tested coagulants, it produced flocs of
deteriorated quality significantly affecting the SS removal from solution.
When comparing the performance of the coagulants tested in solid and in solution form the
removal efficiencies achieved were very similar. However, ALG and Ferix-3 when tested in
solution required lower dosages, between 5 and 30%, and presented a much higher
consistency in purification efficiency between experiment replications.
94
The pH of the water sample being purified had a direct impact on the coagulants
performance. The acidic nature (pH ≈ 4.5 for Al and pH ≈ 4 for Fe) of the produced
solutions (water sample plus added coagulants) in the purification of Navettarimpi sample
favoured the removal of phosphorous (> 85%) and organic matter (40 – 68%) by all tested
coagulants. And although the removal of suspended solids from solution was not favoured
by the observed pH, the removal of particles from suspension was not impaired and high
removal efficiencies were observed (> 80%). The lower pH presented by Piipsanneva
sample resulted in final solutions of even stronger acidic nature (pH ≈ 4 for Al and pH ≈ 3.5
for Fe) which had a deteriorating effect on the performance of all tested coagulants but
specially on the aluminium (ALG) and mixed based (ALF-30) chemicals. ALG and ALF-
30 had the removal efficiencies of all concerning substances reduced between 5 and 20%
when compared to the removal efficiencies achieved for the purification of Navettarimpi
sample. While Ferrix-3 presented only a reduction of about 20% on the removal of organic
matter form solution.
Overall, reducing the purification process temperature from 20 to 5°C had a negative
influence on removal efficiencies, or purification levels, achieved by all added coagulants.
The magnitude of this influence depended on the applied coagulant and its physical form,
the substances being removed and to a less extent the water sample being purified. While
the removal of dissolved substances such as nitrogen, phosphorous and organic matter was
only slightly influenced (ca 10%) by the lowering of temperature, the removal of suspended
solids from solution was significantly reduced at 5°C (ca 40% reduction for Al and 60% for
Fe). The flocs formed by the addition of all tested coagulants to water samples at 5°C
appeared not as compact and well formed as the flocs formed at 20°C.
The variations in purification efficiency observed throughout the mixing parameters
optimization highlighted the fact that the applied mixing time and intensity on both,
coagulation and flocculation stages of the chemical purification process, have a measurable
influence on the removal efficiencies to be achieved. Furthermore the obtained results
indicated that the influence of the mixing parameters applied during the flocculation
process was somewhat greater than the influence of the mixing parameters applied during
the coagulation process.
95
The optimum mixing parameters for the coagulation and flocculation processes identified
during the laboratory experiments greatly increased the removal efficiencies achieved by all
tested coagulants. For the coagulation stage high mixing speeds of 400 rpm (G ≈ 760 s-1
)
and mixing times from 10 to 60 s provided the best removal efficiencies for all solid
coagulants. The coagulants tested in solution form required lower mixing speeds and
shorter mixing times for the coagulation stage compared to their solid counterparts. ALG
and Ferix-3 in solution required 10 seconds of fast mixing time and mixing speeds from
100 to 300 rpm (100 ≤ G ≤ 500 s-1
). For the flocculation stage of the purification process,
mixing times from 15 to 25 minutes and mixing speeds from 70 to 90 rpm (55 ≤ G ≤ 80 s-1
)
produced the best removal efficiencies for all solid coagulants. While for the coagulants in
solution form the longest applied slow mixing time of 25 minutes combined with a middle
range mixing speed of 50 rpm (G ≈ 35 s-1
) resulted in the best removal efficiencies.
Based on the obtained results it is clear that chemical purification via addition of solid
metal salt coagulants is able to achieve high load reduction levels. Nevertheless, further
research is required to enable the application of the knowledge obtained under laboratory
conditions to be translated into successful field application of this treatment method. The
now standing treatment facilities do not provide the strict control of process parameters
such as coagulant dosage and applied mixing effect required for the achievement of high
purification efficiency. Pilot and field test experiments are necessary in order for innovative
solutions to be created which will enable the application of the laboratory identified
optimum conditions to an already existing and to new field treatment structures. These
developments will assure that high load reduction and low metal residual levels can also be
achieved under field conditions where coagulant dosages can be minimized and the
treatment maintenance costs reduced.
Chemical purification has the potential of becoming a viable treatment method to be
applied on peat production sites of all sizes. It can replace other treatment methods which
require significant surface area or be used in conjunction with already used methods which
do not present satisfactory purification levels. It is however necessary to emphasize that
although the costs of implementation and maintenance of chemical purification via solid
coagulants are lower than for liquid coagulants, the costs linked to this treatment method
96
are still substantial. The environmental impacts related to metal residual discharge is
another important factor to be evaluated when considering chemical purification via
addition of metal salt coagulants. The sensitiveness of the receiving water bodies should be
carefully assessed prior to the method implementation and the optimization of the chemical
purification process kept as priority to assure low metal residual discharge and to reduce as
much as possible the related environmental impacts.
97
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Appendix 1
Table A. Methodology of the outsourced performed analyzes.
Analyze Method Unit
TOC SFS-EN 1484:1997 mg/l
tot-N SFS-EN ISO 11905-1:1998 µg/l
tot-P In-house method 51b, FIA method µg/l
PO4-P In-house method 55b, FIA method µg/l
SS SFS-EN 872:2005 mg/l
Al K222A (ISO 11885:2007), ICP-OES µg/l
Fe K222A (ISO 11885:2007), ICP-OES µg/l
pH SFS 3021:1979
Cond. SFS-EN 27888:1994 mS/m
Table B – Methodology equipment used for laboratory performed analyzes.
Analyze Method Equipment Unit
Turbidity EN 27027:1994 Hatch Ratio/XR Turbimeter NTU
Colour ISO 7887:1994 Lovibond Nessleriser Daylight 2000 mg/l Pt
pH SFS – EN 13037:1994 WTW Universal meter Multiline P4
Sensor: WTW Electrode Sentix 81
Temperature Equipment instructions WTW Universal meter Multiline P4
Sensor: WTW Electrode Sentix 81 °C
Conductivity SFS –EN 27888:1993 WTW Universal meter Multiline P4
Sensor: WTW TetraCon 325 mS/cm
For Tables C, D and E which follow the highlighted values are described as: Green shaded
cells represent the varying parameter. Bold values represent the best removal efficiency and
the underlined values represent the worst removal efficiency observed for the indicated
substance.
Table C – Optimization of flocculation (slow) and coagulation (fast) mixing parameters for
all tested solid coagulants in the purification of Navettarimpi water sample.
Dosa
ge
Slow
mixing
speed
Slow
mixing
time
Fast
mixing
speed
Fast
mixing
time Average removal efficiency (%)
Coagulant (mg/l) (rpm) (min) (rpm) (s) tot-N tot-P PO4-P SS TOC
ALG 70 20 15 400 60 21 11 42 61 33
ALG 70 50 15 400 60 21 80 91 75 33
ALG 70 70 15 400 60 24 80 80 60 39
ALG 70 90 15 400 60 9 80 78 76 30
ALG 70 70 5 400 60 24 81 84 61 31
ALG 70 70 10 400 60 22 84 83 75 34
ALG 70 70 25 400 60 25 82 84 77 34
ALG 70 70 25 100 60 28 82 83 89 38
ALG 70 70 25 200 60 28 80 83 89 38
ALG 70 70 25 300 60 33 87 92 90 45
ALG 70 70 25 400 10 39 89 91 95 52
ALG 70 70 25 400 120 33 87 83 55 52
ALF-30 70 20 15 400 60 23 77 76 18 32
ALF-30 70 50 15 400 60 29 79 80 53 38
ALF-30 70 70 15 400 60 29 81 83 65 40
ALF-30 70 90 15 400 60 26 81 85 61 40
ALF-30 70 70 5 400 60 26 75 77 51 41
ALF-30 70 70 10 400 60 27 79 80 56 39
ALF-30 70 70 25 400 60 30 83 82 77 45
ALF-30 70 70 25 100 60 27 83 85 79 46
ALF-30 70 70 25 200 60 30 82 78 83 44
ALF-30 70 70 25 300 60 29 83 90 79 49
ALF-30 70 70 25 400 10 35 85 87 82 51
ALF-30 70 70 25 400 120 32 85 83 66 51
Ferix-3 60 20 15 400 60 21 79 78 41 44
Ferix-3 60 50 15 400 60 27 88 87 75 52
Ferix-3 60 70 15 400 60 40 87 85 75 60
Ferix-3 60 90 15 400 60 27 86 87 81 56
Ferix-3 60 70 5 400 60 21 72 63 -27 33
Ferix-3 60 70 10 400 60 28 85 83 60 52
Ferix-3 60 70 25 400 60 33 89 89 83 56
Ferix-3 60 70 25 100 60 33 87 87 86 62
Ferix-3 60 70 25 200 60 33 89 87 84 59
Ferix-3 60 70 25 300 60 39 89 91 88 62
Ferix-3 60 70 25 400 10 44 90 87 88 62
Ferix-3 60 70 25 400 120 39 85 83 96 62
Table D - Optimization of flocculation (slow) and coagulation (fast) mixing parameters for
coagulants in solution during the purification of Navettarimpi water sample.
Dosage
Slow
mixing
speed
Slow
mixing
time
Fast
mixing
time
Fast
mixing
speed Average removal efficiency (%)
Coagulant (mg/l) (rpm) (min) (s) (rpm) tot-N tot-P PO4-P SS TOC
ALG 70 20 25 60 400 18 80 84 41 21
ALG 70 50 25 60 400 24 80 84 66 25
ALG 70 90 25 60 400 18 76 84 67 25
ALG 70 50 5 60 400 18 76 72 29 21
ALG 70 50 10 60 400 24 78 84 46 21
ALG 70 50 25 60 400 24 80 84 66 25
ALG 70 50 25 10 400 24 83 84 74 29
ALG 70 50 25 120 400 24 80 84 88 25
ALG 70 50 25 10 100 33 88 89 85 39
ALG 70 50 25 10 200 28 88 86 85 39
ALG 70 50 25 10 300 29 87 84 85 42
Ferix-3 60 20 15 60 400 35 89 88 56 58
Ferix-3 60 50 15 60 400 35 87 84 82 60
Ferix-3 60 90 15 60 400 35 85 92 77 61
Ferix-3 60 50 5 60 400 35 89 88 54 58
Ferix-3 60 50 10 60 400 35 89 92 70 59
Ferix-3 60 50 25 60 400 35 87 84 82 60
Ferix-3 60 50 25 10 400 35 91 92 86 62
Ferix-3 60 50 25 120 400 35 87 88 85 58
Ferix-3 60 50 25 10 100 39 89 93 92 67
Ferix-3 60 50 25 10 200 39 75 93 94 68
Ferix-3 60 50 25 10 300 40 92 89 94 68
Table E – Optimization of flocculation (slow) and coagulation (fast) mixing parameters for
all tested solid coagulants in the purification of Piipsanneva water sample
Dosage
Slow
mixing
speed
Slow
mixing
time
Fast
mixing
speed
Fast
mixing
time Average removal efficiency (%)
Coagulant (mg/l) (rpm) (min) (rpm) (s) tot-N tot-P PO4-P SS TOC
ALG 80 20 15 400 60 15 49 57 16 13
ALG 80 50 15 400 60 15 61 67 65 17
ALG 80 70 15 400 60 15 73 65 72 25
ALG 80 90 15 400 60 15 75 83 68 20
ALG 80 90/50/20 15 400 60 ---- 68 72 45 21
ALG 80 90/70/40 15 400 60 ---- 71 72 64 22
ALG 80 70 5 400 60 13 63 67 34 15
ALG 80 70 10 400 60 15 65 67 43 17
ALG 80 70 25 400 60 18 72 73 69 19
ALF-30 80 20 15 400 60 15 52 45 -29 11
ALF-30 80 50 15 400 60 15 64 65 26 20
ALF-30 80 70 15 400 60 18 67 63 48 25
ALF-30 80 90 15 400 60 13 72 83 54 26
ALF-30 80 70 5 400 60 15 56 54 8 17
ALF-30 80 70 10 400 60 15 66 60 43 20
ALF-30 80 70 25 400 60 15 71 71 71 24
Ferix-3 70 20 15 400 60 30 82 79 26 54
Ferix-3 70 50 15 400 60 30 86 83 49 58
Ferix-3 70 70 15 400 60 33 87 87 59 56
Ferix-3 70 90 15 400 60 35 86 92 77 60
Ferix-3 70 90/50/20 15 400 60 ---- 86 88 50 57
Ferix-3 70 90/70/40 15 400 60 ---- 88 92 69 61
Ferix-3 70 70 5 400 60 28 74 79 16 48
Ferix-3 70 70 10 400 60 30 88 90 58 56
Ferix-3 70 70 25 400 60 30 89 92 83 61
Table F – Metal residual concentration of Navettarimpi water sample prior and after
chemical purification.
Dosage
Average
incoming Al
concentration
Average
added Al
residual
Average
incoming Fe
concentration
Average
added Fe
residual Temperature Coagulant (mg/l) (µg/l) (µg/l) (µg/l) (µg/l) °C
AlCl3 50 420 2780 3800 -3543 20
ALG 70 520 2680 3700 -3540 20
ALF-30 70 435 2435 3850 -3165 20
Ferix-3 60 520 -420 3700 -1100 20
ALG sol 70 450 3450 3800 -3605 20
Ferix-3 sol 60 450 -297 3800 -1233 20
ALG 80 490 5560 3800 -3310 5
ALF-30 70 490 3910 3800 -2150 5
Ferix-3 70 435 -290 3900 2350 5
ALG sol 70 380 3870 400 -3880 5
Ferix-3 sol 60 380 -265 400 -1100 5 Values of added residual concentration are related to experiments where the optimized mixing
parameters of each coagulant were applied. The negative sign represents that the presented
concentration was not added but extracted from the incoming concentration.