thesis christos
TRANSCRIPT
Nitrogen recovery from nitrogen rich wastewaters
and slurries by thermal treatment processes
MSc Thesis: Christos Charisiadis
Supervisor: Dr.-Ing. Dirk Weichgrebe
First Examiner: Prof. Dr.-Ing. K.-H. Rosenwinkel
Second Examiner: Dr.-Ing. D. Weichgrebe
Institute for Sanitary Engineering and Waste Management
Leibniz University, Hannover
October 2015
Contents
1. Introduction 1
2. Background 2
3. State of Knowledge 3
3.1 What is Digestate? 3
3.1.1 Digestate characteristics 4
3.1.2 pH value 4
3.1.3 Nitrogen content 5
3.2 Nitrogen Recovery 6
3.2.1 Drivers for digestate processing for nutrient recovery 6
3.2.2 Legal frameworks 6
3.3 Solid–liquid separation, the first step in digestate processing 7
3.3.1 Moisture Distribution in Sludge 7
3.3.2 Solid–liquid Separation 8
3.3 Processing of the solid fraction 9
3.4 Processing of the liquid fraction 9
3.4 Thermal Processing of the Digestate's Liquid Fraction 11
3.4.1 Thermal Drying Background 11
3.4.2 Direct contact dryers 13
3.5 Indirect contact dryers 17
3.6 Evaporation 19
3.7 Gasification 20
3.8 Incineration 21
3.9 Pyrolysis 24
3.10 Wet Air Oxidation 25
3.10.1 Ammonia stripping and scrubbing 27
3.11 Economics of digestate processing for nutrient recovery 29
3.12 Design of a Multi Criteria Decision Tool 31
3.12.1 Compromise Programming 31
3.12.2 Selection of the best processing method 32
3.13 Schematic Flow of a Digestate Thermal Process 33
3.14 Properties of Aqua Ammonia - Effects of pH and temperature 34
3.15 Membrane Distillation 36
3.15.1 Fundamentals of Membrane Distillation (MD) 36
3.15.2 MD Membranes 37
3.15.3 MD configurations 40
3.16 Performance of MD in Ammonia Recovery 44
3.16.1 Comparison between MD Configurations 44
3.16.2 Performance of (PTFE), (PVDF) and (PP) membranes in ammonia recovery
51
3.17 Handling of the Aqua Ammonia 52
3.17.1 Ammonia Conversion to Urea 52
3.17.2 Ammonia Conversion to Ammonium Carbonate as a means to reduce CO2
emissions 54
3.17.3 Ammonia to Ammonium Sulfate 57
4. Experiment Theory 60
4.1 Fundamentals of evaporation 60
4.2 Effect of Vacuum Pressure in the Evaporation Process 65
4.3 Adding a Hydrophobic Membrane-Contact angle 66
5. Experiment Material and Methods 66
5.1 Experiment Equipment Setup 66
5.2 Initial Operating Conditions 68
5.3 Problems with the Experimental Procedure 69
6. Results 69
6.1 First Phase of the Experiment 69
6.2 Second Phase of the Experiment 72
6.3 Third Phase of the Experiment 75
6.4 Fourth Phase of the Experiment 78
6.5 Experiment Conclusions 82
7. Thesis Conclusions 84
8. References 86
1
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
1. Introduction
In Germany, biogas plants have reached approximately the number of 8,000 with
installed electrical capacity of 3,750MW. To generate this amount of renewable
energy, a mixture of crops and organic waste fractions from agriculture and industry
respectively, is used. Converting these substrates to biogas, depending on the
mixture and the operational and economical parameters, we often have to deal with
huge amounts of digestate. After the conversion of carbon in the substrate to biogas
(methane CH4 and carbon dioxide CO2), ammonia NH3 and phosphate PO4-3 remain in
the digestate. According to EU regulations for environmental protection and in
particular that of groundwater, the use of digestate as fertilizer in arid lands is
limited to the vegetation period and the present nitrogen balance. So the digestate
has to be kept in storage which due to its toxicity, is expensive. Therefore digestate
treatment technologies have been developed for nutrient recovery along with the
reduction of the digestate volume, in order to reduce the required storage expenses.
The first step in reducing the digestate volume is through physical separation
techniques that give a solid and a liquid fraction. In order to process further the
liquid fraction, thermal methods have been developed, each with their product
quality, drawbacks and costs. In this study the author will present each one of these
methods and will try to suggest a multi-criteria model in order to search for the best
choice according to each method's attributes. Finally, this Thesis will present
alternative routes for ammonia recovery, by using membrane distillation and
sequential evaporation with a laboratory experiment and suggest processing
methods for converting the distillate that is produced by the digestate processing
into commercial chemical products.
2
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
2. Background
Anaerobic digestion (AD) converts organic matter (substrate) into 1) biogas
(methane CH4 and carbon dioxide CO2) and 2) digestate, a nutrient rich organic
fraction. Biogas can be used as a source of renewable energy to generate electricity
and heat to power the process, and the excess power is sold if there's the right
infrastructure. The digestate contains the non digested organic fraction, water and
nutrients such as nitrogen and phosphorus. The composition of the digestate
depends directly to the input biomass.
Digestate is mechanically separated into a 1) liquid fraction (water solution) and 2) a
solid fraction (resilient organic matter). The nutrients in the solid fraction offers are
hard to recover, because they are organically bound. On the contrary nitrogen (N),
phosphorus (P), potassium (K), sulphur (S), organics and mineral salts, which are
present in the liquid fraction, are far easier extracted with the right techniques.
Currently the majority of AD facilities transport the digestate to local agricultural
lands, to be used as an organic fertilizer (Fuchs et al., 2010). However the window for
land application is limited to 1) agricultural and crop requirements (Orr, 2011), and
2) large AD plants, need a large nearby agricultural area to provide them with a
secure and suitable market. If the application to the agricultural land is not viable,
due to transport distances, legal or other restrictions, digestate can be used for land
reclamation.
The volume of the digestate is the dominant factor in the expenses for
transportation and storage. The larger the volume, the higher the costs. Thus
industry has tried to reduce the liquid fraction by thermal means. The conventional
methods that are used so far have significant problems that range from excessive
energy consumption to use of dangerous and expensive chemicals. Each has pros
and cons which need to be used in a multi criteria model that will make the decision
process easier and more clear to the user.
So due to the individual faults of the conventional methods, the need has arise in
the last years, for new methods to be created, that are equally or more efficient and
without the drawbacks of the previous ones. In this Thesis, the author will explore
the properties of aqua ammonia and present in theory and through a lab
experiment, membrane distillation and sequential evaporation as alternative
digestate thermal processing methods.
Finally at the end of the processes we're left with a N rich distillate that it can be
exploited to be converted into commercial chemical substances, usually for the
production of fertilizers. The problem with most of the methods used is the
utilization of acids during their process, making it hazardous and expensive. The
3
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 1, Four major steps of the AD Process, (Madsen et al., 2011)
author will try to present methods that not only convert the aqua ammonia without
acids but are environmentally friendly by capturing the industrial CO2 and NOX
effluent gases.
3. State of Knowledge
3.1 What is Digestate?
Digestate is the product of anaerobic digested biodegradable materials. It is normally
liquid, but it can also be a solid, stackable material when it is coming from, e.g. a dry
state AD process. The AD substrate, can be a mixture of different substrates or a
pure mono-substrate. The substrate decomposes without oxygen (anaerobic
conditions), inside the closed digester for several weeks, in which time it is
sequentially decomposed by microorganisms through a complex biochemical
process.
Figure 1 depicts the four major steps of AD: 1) decomposition of organic matter
during hydrolysis, 2) formation of organic acids during acidogenesis, 3) formation of
the main intermediate acetate during acetogenesis, and 4) formation of methane
during methanogenesis from either acetate or carbon dioxide and hydrogen.
Digested substrate is taken out from the tank as digestate and stored. Digestate has
excellent plant fertilizer qualities, based on it being rich in N, P, K, S, various
micronutrients and also organic matter. Digestate is normally applied as fertilizer
without the need for any further processing.
4
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Table 1, Substrate parameters influencing digestate composition, (Fuchs and Drosg, 2010)
Table 2, Process parameters influencing digestate composition, (Fuchs and Drosg, 2010)
3.1.1 Digestate characteristics
The physical and chemical characteristics of the digestate are depending on the 1)
composition of the substrates (illustrated in Table 1 and 2) the operational
parameters of the AD process (illustrated in Table 2). Literature (Holm-Nielsen et al.,
1997; Chantigny et al., 2007; Muller et al., 2008; Tambone et al., 2010; Fouda, 2011)
proves that, when compared with raw animal manures and slurries, digestate
generally has 1) lower total solids (TS) and total organic carbon (C) content, 2) lower
carbon to nitrogen ratio (C:N), 3) and lower viscosity. Although, pH and ammonium
(NH4+) concentration are higher in the digestate compared to raw animal manures
and slurries.
3.1.2 pH value
The pH value of fresh digestate typically is in the range of 7.5 to 8.0 pH. The pH is
mainly depending by the biochemistry of the AD process and the characteristics of
substrates (ARBOR, 2013, WRAP, 2012). For example, the formation of ammonium
carbonate ((NH4)2CO3) as well as the removal of CO2 as a result of the transformation
of CO32− and 2 H3O+ to CO2 and H2O, result in increased pH (BiotecVisions, 2012). The
5
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 2, Examples of the variation of nitrogen in the digestate of biogas plants with different substrate types, (a)TN
concentration in kg/ton fresh matter (FM), (b) ammonium nitrogen as percentage of TN. Horizontally striped
columns indicate digestate from mono-digestion of industrial by-products; and unstriped columns indicate
digestate from typical waste treatment plants (Fuchs and Drosg, 2010)
consumption of volatile fatty acids (VFA) during AD increases the pH. The pH is also
rising with higher concentrations of basic cations like Ca2+ and K+ (ARBOR, 2013), and
decreasing with the precipitation of carbonates such as calcite (CaCO3) and of iron
phosphates (Hjorth et al., 2010).
An increased pH leads to the degradation of foul smelling VFAs, which reduces odour
emissions but on the other hand the degree of ammonia volatilization increases.
Storage of digestate until field application should take place in closed storage tanks
(manure storage tanks with flexible plastic coverage).
3.1.3 Nitrogen content
The AD process degrades organic nitrogen compounds, releasing ammonium NH4-N,
which is immediately bio-available for growing plants. The content of ammonium in
digestate is directly related to the total N content in the substrate. The differences of
nitrogen content in digestates coming from the AD of energy crops compared to
digestate from organic waste and industrial by-products are depicted in Figure 2 (a)
and (b). By looking at the figure, we can tell that nitrogen concentrations in energy
crop AD plants are rather similar, whereas in biogas plants and co-digesting organic
wastes, the nitrogen concentration varies significantly, mainly due to the variations
of N contents. Additionally, processing parameters such as for e.g. the amount of
fresh water and degree of recirculation, also influence the total nitrogen (TN)
content. In mono-digestion of industrial by-products, the influence of nitrogen and
sulphate concentration in the substrate is easy to tell.
6
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
3.2 Nitrogen Recovery
In the last decades we are dealing with pollution problems coming from organic
waste streams and manure management (N eutrophication, nitrate leaching, nitrous
oxide greenhouse gas emissions, and ammonia particulate). The main reason behind
this, is the cycle of large N losses in agricultural systems and subsequent fossil-based
synthetic N replenishment. Around 85% of reactive N (forms other than di-nitrogen
gas, N2) is lost to the environment (waterways, atmosphere, etc.). Meanwhile the
majority (95%) of the remaining 15% that enters the human organism is excreted
and eventually lost to those same waterways and atmosphere (Galloway et al.,
2004). To replace N lost from agricultural systems, industry converts non-reactive N2
to synthetic N fertilizer through energy-intensive and environmentally harsh
processes; e.g. Haber-Bosch process (12 Kwh*kg*N-1 ; Sutton et al., 2009) (1.4-2.6 kg
CO2*kg*N-1 ; Wood and Cowie, 2004). The unsustainable N cycle 1) consumes limited
fossil fuel resources and 2) contributes to environmental pollution. Recovering N
reduces losses of N to the environment while decreasing the demand for synthetic
fertilizers. The methods for nutrient recovery from digestate are developing rapidly
along with the technological advances, improving nutrient management in
agriculture and in waste treatment systems.
3.2.1 Drivers for digestate processing for nutrient recovery
Digestate as fertilizer/ soil conditioner in agriculture, horticulture, forestry etc. can
be directly applied is possible after its removal from the digester tank without any
further processing. However, digestate is rather diluted with respect to nutrients,
which makes the costs of transportation relatively high compared to conventional
fertilizer. Significant costs are also the investments in storage capacity, required by
environmental regulations in many countries, like for e.g. in Denmark, Germany and
France, where not only the nutrient input per hectare is restricted, but also the
period of application is limited to the growing season. However, many crop
cultivators agree that applying digestate as organic fertilizer compared to
conventional fertilizer has synergistic effects.
3.2.2 Legal frameworks
At EU level, the European Nitrate Directive 91/676/EEC restricts the yearly load of N,
applied to agriculture . Livestock production is intensive, usually in areas with limited
land available for manure application. This creates a permanent excess of nutrients,
making such areas highly vulnerable, to surface and ground waters, pollution. The
problem intensifies when animal feed is being imported to such a region, which
makes efficient nutrient management even more crucial. The legal restrictions on
the nutrient input per hectare require the excess nutrients to be recovered,
7
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.3, Water distribution in sludge, (Arun S. Mujumdar, 2015)
exported, and recycled outside the vulnerable areas. Thus digestate processing
technologies have been developed, aiming at volume reduction and nitrogen
removal. More recently, also concerns regarding P excess from manure application
in many areas and high levels of phosphorus found in surface and ground waters
have greatly increased demand for nutrient management and export of excess of
nutrients.
3.3 Solid–liquid separation, the first step in digestate processing.
3.3.1 Moisture Distribution in Sludge
The moisture in the digestate sludge can be as high as 99%. Figure 3 depicts the
moisture distribution in the sludge. This distribution takes the following forms: 1)
free moisture that is not attached to the sludge particles and can be removed by
gravitational settling; 2) interstitial moisture that is trapped within the flocs of solids
or exists in the capillaries of the dewatered cake and can be removed by strong
mechanical forces; 3) surface moisture that is held on the surface of the solid
particles by adsorption and adhesion; and 4) intracellular and chemically bound
moisture. (Mujumdar, 2015)
,
The amount of water that can be removed is dependant on the dewatering process
and the status of the water in the sludge. The water that can be removed by
mechanical dewatering is usually termed as 'free water' and the rest as 'bound
water'. The bound water is the theoretical limit of mechanical dewatering. The free
water includes the free, interstitial, and partially the surface moisture. The bound
water includes the chemically bound moisture and partially the surface moisture.
8
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 4, Distribution of the main components after the solid-liquid fractions seperation (Bauer et al., 2009)
3.3.2 Solid–liquid Separation
Digestate processing technologies technologies are comparable to existing
technologies from manure processing, sewage sludge treatment, and wastewater
treatment. Digestate processing can be 1) partial, mostly for reducing the volume, or
2) complete, dividing the digestate to pure water, a solid bio-fertilizer fraction, and
fertilizer concentrates.
The first step in digestate processing is the separation of the solid from the liquid
phase. The solid fraction can be directly applied as bio-fertilizer in agriculture or it
can be composted/ dried for intermediate storage and transport. To improve the
solid–liquid separation, flocculation or precipitation agents can be added. Typical
ranges for the distribution of the main components of the solid and the liquid
fraction are given in Figure 4.
The major fraction coming out from the first separation step is the liquid fraction.
Depending on the characteristics of the digestate and the efficiency of the
separation itself, its composition widely varied. Frequently, a percentage of the
liquid fraction is recycled to control the dry matter (DM) concentration of the input
substrate (Resch et al. 2008). For the rest of the liquid fraction, there are a variety of
recovery and treatment options. The advantage of solid–liquid separation can be
that reduces the required leftover storage, nevertheless, treatment for further
volume reduction and nutrient recovery can be applied. In most cases, these
9
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
objectives will be achieved only through a sequence of several steps which can be
relatively complex and therefore expensive.
3.3 Processing of the solid fraction
The solid fraction which comes out of the solid–liquid separation has TS
concentrations in the range of 20 – 30 %. It is partially stabilized so that is
appropriately stored and direct applied as bio-fertilizer or soil improver in
agriculture. However, it still contains biodegradable material and consequently
microbial activity can still happen and odour emissions can occur. In order to get a
stable and marketable bio-fertilizer product, the solid fraction needs further
processing, which can be composting, drying or another form of stabilization.
3.4 Processing of the liquid fraction
While the solid-liquid fraction separation uses relatively simple and cheap
technologies, for further processing different methods and technologies are
available, with various degrees of technical maturity, higher energy input, and higher
investment and operating costs. For nutrient recovery, membrane technology, such
as nano and ultrafiltration followed by reverse osmosis, can be used (Fakhru’l-Razi,
1994; Diltz et al., 2007). Membrane filtration produces a nutrient concentrate and
purified process water (Castelblanque and Salimbeni, 1999, Klink et al., 2007). The
liquid digestate can also be purified through aerobic biological wastewater
treatment (Camarero et al., 1996).
However, because of the high nitrogen content and low biological oxygen demand
(BOD), addition of an external carbon source is often necessary in order to achieve
satisfying denitrification. A further possibility for concentrating digestate is
evaporation with excess heat from the biogas plant, combined heat and power (CHP)
unit. In order to reduce the nitrogen content in the digestate, ammonia stripping
(Siegrist et al., 2005), ion exchange (Sanchez et al., 1995) or struvite precipitation
(Uludag- Demirer et al., 2005; Marti et al., 2008) are in use.
Whatever technology is applied, advanced digestate processing in most cases
requires high energy input and chemical reagents, like acid which are a significant
expense. Along with increased investment costs for appropriate machinery, the
treatment costs become considerable. An overview of viable digestate processing
technologies is given in Figure 5.
10
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.5, Overview of viable options for digestate processing (Fuchs and Drosg, 2013)
Fig.6, Overview of the distribution of industrial scale applications for further treatment of the liquid fraction of
digestate in Germany , Austria and Switzerland from 2009 (Fuchs and Drosg, 2013)
As we can see above, a wide range of technologies are currently being used for
digestate processing, depending on the boundary conditions. The most abundant
approach is solid–liquid separation of digestate, where, depending on the
consistency of the digestate, screw presses or centrifuges are mostly used. Solid–
liquid separation can be improved by the addition of precipitating agents.
For further processing of the liquid fraction, membrane purification is the only
process that can achieve a degree of purification that can allow direct discharge. It is
also among the most frequently applied approaches in more complex digestate
processing facilities in Germany, Switzerland, and Austria (Figure 6).
11
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
However, membrane purification is the most expensive technology, with high
potential for optimization in large-scale applications. If excess heat is available,
evaporation is an interesting option, although it gives rise to some controversy.
In Germany digestate processing technologies using heat (e.g. evaporation, drying)
are being used more frequently due to the subsidies for waste heat utilization at
biogas CHPs. Evaporation of the liquid fraction of digestate is a rather robust
technology, however, if the liquid fraction contains considerable amounts of fibrous
material it is necessary to remove this beforehand to avoid clogging in the heat
exchangers. Other technologies that are less commonly applied include ammonia
stripping, ion exchange, solar drying, etc.
3.4 Thermal Processing of the Digestate's Liquid Fraction
3.4.1 Thermal Drying Background (Andreoli et al., 2007)
The thermal drying process is one of the most efficient and flexible ways of
decreasing the moisture content from dewatered organic industrial and domestic
sludges. Thermal drying may be used for different sludge types, either primary or
digested, and a feeding sludge solids content of 15%–30% is recommended.
Under ideal conditions, 2,744 kJ (0.76KW) of energy are needed to evaporate 1 kg of
sludge water , and it is usual to increase this value up to 100% for normal operational
conditions. The total energy demand will depend 1) on the efficiency of the selected
equipment and 2) on the type of the processed sludge. Mainly the energy comes
from external sources, such as fuel oil, natural gas etc. Biogas generated in AD may
provide an alternative energy source.
As the heating power of biogas is 22 MJ/L and burners can usually work at 70%
efficiency, under ideal conditions, 0.17 liters of biogas are required to evaporate 1 kg
of water. Besides this, energy losses (through walls, air, etc.) must also be accounted
for, together with the energy required to increase the sludge temperature to slightly
above 100 ◦C, when the evaporation process starts.
The suppression of the biological stabilization stage significantly reduces capital
costs, and favors the production of pellets with high organic matter content and
heating value, which make the product marketable in agriculture or as fuel.
The main advantages of sludge thermal drying are: 1) significant reduction in
sludge volume; 2) reduction in freight and storage costs of the sludge; 3) generation
of a stabilized product suitable to be easily stocked, handled and transported; 4)
production of a virtually pathogen-free final product; 5) preservation of biosolids
fertilizing properties; 6) no requirements of a special equipment for land application;
12
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.7, Principles of drying processes, drying by convection (left) and drying by contact (right) (Fuchs and Drosg, 2010)
7) sludge is suitable for incineration or landfilling; 8) product may be put into sacks
and distributed by retail dealers.
The main limitations of thermal drying processes are: 1) production of liquid
effluents; 2) release of gases into the atmosphere; 3) risk of foul odours and
disturbing noise
The major types of thermal drying systems are:
• Direct contact dryers (convection): where hot air has direct contact with the
sludge, drawing away moisture, gases and dust (Direct dryers are typically rotary-
drum, flash, moving-belt dryers, or centridryer types.)
• Indirect contact dryers (conduction): where heat is transmitted through heat
exchange plates (Indirect dryers are thin-film, rotary-disc, or rotary-tray dryers.)
Both processes are illustrated in Figure 7,
In convection, we have heat transfer by direct contact of the wet sludge and the hot
gases. The heat of the inlet gas provides the latent heat required for evaporating the
liquid from the sludge. The vaporized liquid is carried by the hot gases. Under
equilibrium conditions of constant-rate drying, mass transfer is proportional to 1) the
area of wetted surface exposed, 2) the difference between the water content of the
drying air and saturation humidity at the wet-bulb temperature of the sludge–air
interface, and 3) other factors, such as velocity and turbulence of drying air
expressed as a mass transfer coefficient (Tchobanoglous et al. 2003). Direct dryers
are the most common type used in thermal drying of sludge. Flash dryers, direct
rotary dryers, and fluidized-bed dryers use this method.
In conduction, we have heat transfer by contact of the wet sludge solids with hot
surfaces. A metal wall separates the sludge and the heating medium (usually, steam
or oil). The vaporized liquid is removed independent of the heating medium. Indirect
dryers for drying municipal sludge include horizontal paddle, hollow-flight or disk
dryers, and vertical indirect dryers. (Turovskiy & Mathai, 2006).
13
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 8, Schematic flow of a Rotary Dryer (Frischman, 2012)
Liquid effluent is usually less than 1% of the total flow and can be recycled to the
plant headworks. Both systems require equipment for enclosure and treatment of
the water vapor and the dust which are released from the dryers to avoid odour and
particle emissions to the atmosphere. Indirect processes produce pellets with up to
85% solids concentration. For solids contents higher than 90% and possible
production of organo-mineral fertilizers, direct drying processes should be preferred.
The drying cost varies among different technologies chosen with a range of US $65-
80/ton DS. Utilizing the waste heat from the burning of sludge is reducing the cost
significantly. For example, flue gas drying preceding incineration can save as much as
60% on the cost compared with direct incineration in large sludge plants. Similarly,
mechanical compression of the vapors generated from indirect dryers can improve
the energy efficiency considerably. Cogeneration-sludge drying units might be an
economically attractive option to consider in large wastewater treatment plants
(Mujumdar, 2015)
Sludge drying is not an isolated issue. It has to be addressed along with other
economical, environmental, and safety concerns. Over the last decade, there has
been a significant increase in investment in environmental protection including for
sludge processing. Innovative drying technologies with higher thermal efficiencies,
lower emissions, less operator involvement, cheaper capital costs, and better final
products are needed by the market.
3.4.2 Direct contact dryers
a. Rotary Drying (Frischman, 2012)
The design of a Rotary Dryed is illustrated in Figure 8,
In a rotary drier digestate fibre is coming in contact with hot gases (convection). The
rotary drier consists of a cylindrical drum which is rotated about its axis. Flights
within the drying drum pick up and cascade the digestate. The drum is mounted on a
14
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 9, Schematic flow of a Belt Dryer (Frischman, 2012)
slight slope from the horizontal to transport the dried product along its length. The
feed to the drier is blended with dried product to give a feed of approximately 65%
dry solids (DS) to improve movement within the drum. Waste gases are passed
through a cyclone to recover solids before further treatment, with large amounts of
ammonia may contained within the exhaust gas stream. Final product DS of up to
95% can be achieved. Screening can be added to give a homogenous product pellet
size.
A rotary drying system in operation in Louisville, Kentucky, has its primary sludge
anaerobically digested first and then blended with thickened waste activated sludge.
The sludge mixture is then dewatered in centrifuges to about 26% solids and fed into
four drying trains, each comprising a rotary drum dryer. Each dryer is sized to
evaporate water from dewatered sludge at a rate of 8500 kg/h. Total installed
evaporative capacity is 34 metric tons/h. The methane generated from the digesters
provides half of the energy required by the dryers. Heat recovered from the dryers is
used to heat the anaerobic digesters. (Turovskiy &Mathai, 2006)
Pros: 1) Reduced volume of digestate for transport and storage; 2) Improved
marketability as a fertilizer/soil conditioner; 3) Effective pathogen kill.
Cons: 1) High energy requirement; 2) High temperature operation; 3) Large capital
investment; 4) Reduced nutrient content of final product; 5) Gas treatment required;
6) Risk of explosive atmosphere within drying plant.
b. Belt Drying (Frischman, 2012)
Belt dryers are used in the water and wood pulp industries. The viability of this
technology is be dependent on the installation and the end use of the dried product.
Figure 9, illustrates the basic concept design,
In a belt dryer digestate is contacted with hot gases (convection). The digestate fibre
is evenly distributed over the drying belt by an extruder. The extruder produces
digestate strands in order to increase surface area and provide a uniform size. The
drier belt passes through a series of successive chambers of increasing temperature.
At the end of the belt digestate is dropped onto a second belt which runs back
15
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 10, Schematic flow of a Solar Dryer (Frischman, 2012)
through the drier, underneath the first belt, to complete the drying process and cool
the product. Dried product is discharged from the drier at the end of the second belt.
Product is discharged at up to 90% DS and at a temperature below 40°C. As the
digestate is not agitated during the drying process the risk of creating an explosive
dust atmosphere within the drier is significantly reduced. This improves the safety
and operability of the process.
Pros: 1) Reduced volume of digestate for transport and storage; 2) Improved
marketability as a fertilizer/soil conditioner; 3) Effective pathogen kill.
Cons: 1) High energy requirement; 2) Large capital investment; 3) Reduced nutrient
content of final product.
c. Solar Drying (Frischman, 2012)
This technology provides a low operational expenditure (Opex) (the day-to-day
management costs) solution for dewatering digestate, however it requires a large
land area. Most operational plants of this type are located in warm climates. Figure
10 depicts the basic design of a solar system,
Solar drying uses a combination of forced ventilation and solar energy to de-water
digestate. Waste heat from a CHP can also be used via underfloor heating. The feed
to the process can be fed with either whole digestate or de-watered fibre. The
digestate is fed into greenhouses where it is distributed across the drying bed.
Digestate is turned and ventilated to increase efficiency and reduce odours. The
greenhouses operate as a batch or semi-continuous process. The final product is a
dried digestate exceeding 50% dry solids.
Pros: 1) Increased concentration. 2) Reduced transport volume; 3) No liquor
treatment required.
Cons: 1) Large surface area required. 2) Cold climates may restrict application (most
current applications are in Spain or Southern France).
16
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 11, Design of a Flash Drying System (Mujumdar, 2015)
d. Flash Dryer (Mujumdar, 2015)
Flash drying is the fast removal of moisture by spraying or injecting the sludge into a
hot gas stream. In a flash drying system (see Figure 11) the wet sludge cake is
blended with previously dried sludge in a mixture to improve pneumatic conveyance.
The blended sludge and hot gases from the furnace at 704°C are mixed ahead of a
cage mill, and flashing of the water vapor begins. Gas velocities on the order of 20 to
30 m/s are used. The cage mill mechanically agitates the sludge–gas mixture, and
drying is virtually complete by the time the sludge leaves the cage mill, with a mean
residence time of a few seconds. The dried sludge is conveyed to a cyclone
pneumatically. The sludge at this stage has moisture content of only 8 to 10%. The
sludge is then separated from the spent drying gases in the cyclone.
Flash dryers have high energy and operation and maintenance costs. Today, other
types of dryers are preferred.
17
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 12, Schematic flow of Horizontal indirect dryer system (Mujumdar, 2015)
3.5 Indirect contact dryers (Mujumdar, 2015)
Indirect dryers produce less gas, so most of their designs are closed loop with heat
recovery and odour removal units. Because indirect dryers depend on heat being
transferred from a heated surface and the dewatered sludge is still relatively wet
(around 25% solid content for activated sludge), the interfacial behavior of the
sludge and the heated surface is an important issue. While there is no air flow to
disperse or disintegrate the wet sludge, mechanical agitation has to be designed to
prevent the heating surface from being fouled, especially in the sticky zone with the
solid content ranging between 55% - 70%.
This technology has been established in the processing of other products. For sludge
processing, usually a horizontal agitated thin film evaporator is selected. Its drying
rate lies between 20 and 160 kg/m2/h. When higher final solid contents are desired,
a rotary paddle or disc dryers may be used either alone or as the second stage
following a thin-film evaporator. Indirect drying with simultaneous sludge drying and
pelletizing emerged in the last decade in Europe as a result of environmental and
energy conservation concerns.
a. Horizontal Indirect Dryer Horizontal indirect dryers for drying municipal
wastewater sludge include the paddle dryer, hollow-flight dryer, and disk dryer.
Figure 12 is a schematic diagram of a horizontal indirect dryer.
The dryer consists of a horizontal jacked vessel with one or two rotating shaft fitted
with paddles, flights, or disks which agitate and transport the sludge through the
dryer. The heat transfer medium (usually, steam) circulates through the jacketed
shell and through the hollow-core shafts and hollow agitators (paddles, flights, or
disks). A weir at the discharge end of the dryer ensures complete submergence of
18
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 13, Vertical indirect dryer by Pelletec (Mujumdar, 2015)
the heat transfer surface in the material being dried. The steam is discharged as
condensate after transferring its available energy to the sludge. Dryers that use hot
water or oil as the heat transfer medium are constructed internally in a manner
different from those required for steam. Dewatered sludge is fed into the vessel
continuously, with or without mixing with any recycled dried product. The transfer of
heat from the heat transfer medium raises the temperature of the sludge and
evaporates the water from the sludge solids surface. The water evaporated is
transported out of the dryer by low-volume sweep gases or exhaust vapors.
If dried product is mixed with dewatered sludge, the moisture of the feed sludge can
be reduced by 40 to 50%. The blending prevents agglomeration and fouling of the
heat transfer surface. Dryers that dry unblended feed sludge should have internal
breaker bars and must provide enough horse power to turn the agitator shafts to
break up the clumps. Horizontal indirect dryers are capable of drying sludge with less
than 10% moisture.
b. Vertical Indirect Dryer A vertical indirect dryer, such as the Pelletech dryer
shown in Figure 13, dries and pelletizes sludge simultaneously.
It is a vertically oriented multistage unit that uses steam or thermal oil in a closed
loop as the heat transfer medium to achieve a dry solids content of 90% or more.
Dewatered sludge cake blended with dried product is fed at the top inlet of the
dryer. The dryer is equipped with several trays heated by the heat transfer medium.
The dryer has a central shaft with attached rotating arms. The rotating arms are
19
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 14, Schematic flow of a Surface Scraped Heat exchanger (Frischman, 2012)
equipped with adjustable scrapers that move and tumble the sludge in thin layers
from one tray to another in a rotating zigzag motion until it exists at the bottom as a
dried pelletized product. The process minimizes the formation of dust and oversized
chunks. The dryer’s exhaust consists of water vapor, air, and some pollutants. After
the water vapor is condensed, only a small amount of gases, mainly moist air,
remains to be treated. These gases are vented from the dryer to an odor control unit
for thermal destruction of odor-causing compounds.
3.6 Evaporation
If we want to concentrate the digestate or increase its dry solids content, we can use
evaporation. Evaporation uses thermal energy (heat) to release the moisture from
the digestate. However, unlike the drying techniques, evaporation preserves the
nutrients and a percentage of the moisture. Evaporation is typically used for liquor
or whole digestate treatment.
The final solids concentration is dependent on the desired product, but
concentrations of up to 20% DS can be achieved. High temperatures will cause
ammonia to be released, this can be solved by decreasing the pH of the digestate,
usually with acid dosing, before the evaporation. This allows the digestate liquor to
be converted into a concentrated fertilizer.
a. Surface Scraped Heat Exchanger (HRS) (Frischman, 2012)
This technology provides a viable method for treating digestate liquor and
producing a balanced fertilizer product. Consideration will need to be given to the
acidic nature of the final product and the affect this may have on agricultural
appliance. Figure 14 illustrates the schematic flow,
Evaporation uses waste heat from the CHP. Surface scraped evaporators are
designed not to be affected by fouling issues by the evaporation of the digestate.
The evaporators use a shell and tube configuration. The interior surface of the heat
exchanger tubes is constantly cleaned by internal scrapers to reduce fouling and
increase heat transfer efficiency. The digestate liquor is dosed with acid prior to
evaporation to prevent ammonia loss within the evaporator. The volume of acid
20
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.15, Schematic flow of Gasification (Frischman, 2012)
dosed is dependent on the digestate and the desired retention time. Within the
evaporator the liquor is concentrated to approximately 20% dry solids. This
concentrate can then be mixed with the (previously) separated digestate fibre to
produce a nutrient rich solid fertilizer. Trials have indicated that the condensate
from the process is suitable for direct discharge to ground water, although further
treatment may be required for certain applications. Alternatively it can be recycled
as process water.
Pros: 1) Reduced transport volume; 2) Potentially no further treatment of
condensate required; 3) Concentrated nutrient rich product; 4) Use of heat eligible
for RHI.
Cons: Acidic product may limit available land bank.
3.7 Gasification (Frischman, 2012)
In the gasification process, the oxygen supply is limited to enable partial combustion
of organic matter within the feed in order to produce a synthesis gas (syngas).
Syngas is a mixture of mainly carbon monoxide and hydrogen, which can be burnt to
produce energy (Perry, 1997). For the process to operate efficiently, the feed
digestate must have a low moisture content and ideally be in a dry pelletized form.
In Fig.15, we have the schematic flow of the process,
Gasification is applied to convert organic matter to a mixture of gases consisting
mostly of carbon monoxide and hydrogen, known as syngas. Reactions take place at
high temperatures with carefully controlled amounts of oxygen, air or steam. The
syngas can be burned in a gas engine to produce heat, and the ash/ char from the
process can be used for road construction, production of concrete, or sent to landfill.
Gasification of traditional fuels such as wood and coal is well established, and the
process has also been used for municipal solid waste. Full scale gasification of dried
sewage sludge has also been shown to be economic.
However the use of gasification for digestate is not well documented. During
digestion, most of the organics have already been released, making the value of the
digestate as a fuel for gasification, relatively low. Digestate must be dried and ideally
21
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Table 3, Calorific power of different sewage sludge (Andreoli et al., 2007)
pelletized before it can be gasified, adding an additional energy demand on the
process.
Pros: 1) Volume reduction; 2) Destruction of pathogens and toxic compounds; 3)
Renewable heat and energy generation.
Cons: 1) High operating cost; 2) Complex operation; 3) Ash disposal to landfill; 4) Loss
of fertilizer potential.
3.8 Incineration (Andreoli et al., 2007)
Incineration provides the greatest volume reduction. The remaining ashes volume is
usually less than 4% of the dewatered feed sludge volume. Incinerators can use
sludge from several treatment plants (Table 3) and are usually designed with
capacities higher than 1 ton/h.
Incineration destroys organic substances and pathogenic organisms through
combustion, using excess oxygen. Incinerators must use sophisticated filter systems
to significantly reduce pollutant emissions. Gas emissions released are regularly
measured to ensure operational efficiency and safety. Incinerator design requires
detailed mass and energy balances. Despite the high organics concentration in
dewatered sludge, combustion is only autogenous when solids concentration is
higher than 35%. If need be, we can use auxiliary fuels, such as boiler fuel with low
sulphur content. The calorific value of sludge crucial in the amount of fuel
consumption.
Products from complete combustion of sludge are water vapour, carbon dioxide,
sulphur dioxide and inert ashes. There are two types of incinerators that are
currently in use for sewage sludge:
• multiple chamber incinerator
• fluidised bed incinerator
A multiple chamber incinerator is divided into three distinct combustion zones. The
higher zone, where final moisture removal occurs, the intermediate zone where
22
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
combustion takes place and the lower or cooling zone. Should supplementary fuel be
required, gas or fuel oil burners are installed in the intermediate chamber. A
fluidized bed incinerator consists of a single-chamber cylindrical vessel with
refractory walls. The organic particles of the dewatered sludge remain in contact
with the fluidized sand bed until complete combustion.
Fluidized bed incinerators are usually preferred over multiple chamber furnaces, due
to lower operational costs and lower gas emission. However, despite the significant
reduction in sludge volume, incineration cannot be considered a final disposal route,
as residual ashes require additional processing. Inadequate ashes disposal could lead
to possible leaching of metals and their absorption by plants. Landfill is the
preferable destination for the ashes.
Incineration is well established technology but investment and operating costs are
high and viable only for large plants or where agricultural application of digestates is
not possible because of the digestate quality or land bank availability.
Pros: 1) Volume reduction; 2) Destruction of pathogens and toxic compounds; 3)
Possible energy recovery.
Cons: 1) High operating cost; 2) Complex operation. 3) Potential environmental
impact of residuals (exhaust air); 4) Loss of fertilizer potential; 5) Public perception.
a. Multiple Chamber Incinerator (Turovskiy &Mathai , 2006)
The flowchart of a system with a multiple hearth furnace is presented in Figure 16.
The furnace shell is a vertical steel cylinder 6 to 8 m in diameter lined internally with
refractory brick or heat-resistant concrete. The furnace is divided vertically into
seven to nine refractory hearths. A vertical rotating shaft passes through the center
of the furnace, to which the horizontal frames of the rake mechanisms, made of
heat-resistant cast iron, are affixed. Each hearth has material transfer openings
located alternatively on the periphery of one hearth and in the center section of the
adjacent hearthThe sludge moves by conveyors into the charging hopper and then
onto the uppermost hearth of the furnace. The sludge is moved by rakes into the
transfer openings, it drops to the next lower hearth, and continues its travel to the
lower hearths. This provides continuous movement of the sludge mass in the
opposite direction to the hot combustion air. The use of rake mechanisms to move
and break up the clumps in the sludge intensifies the drying and combustion
processes.
23
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.16, Flowchart of multiple hearth incinceration (Turovsky &
Mathai, 2012)
Pros: 1) combusting both primary
and secondary sludge, as well as
trash from screens, scum from
settling tanks and oil separators,
dirty grit from grit chambers, and
industrial wastes; 2) they are
characterized by their simplicity of
service and by the reliability and
stability of operation during
significant variations in the quantity
and quality of sludge treated; 3) The
furnaces can be installed in the
open air.
Cons: 1) high capital cost; 2) large
area required; 3)presence of
rotating mechanism in the high-
temperature zone; 4) frequent
failure of the rake devices.
b. Fluidized Bed Incinerator
(Turovskiy &Mathai , 2006)
Fluidized-bed furnaces are adapted
in the industry as a drying and roasting
technology in a wide range of fields. The furnace, a vertical steel cylinder lined
internally with refractory brick or heat-resistant concrete, consists of a cylindrical
furnace chamber, a lower conical section with an impermeable air distribution grate,
and dome-shaped crown. Heat-resistant quartz sand is placed on the grate.
24
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.18, Flowchart of Pyreg (Frischman, 2012)
Fig.17, Flowchart of fluidized bed incineration (Turovskiy
&Mathai , 2006)
The turbulent (fluidized) bed in the furnace is formed when air is blown through the
distribution grate at a rate at which the sand particles move in a turbulent manner
and appear to boil in the flow of gas.
The design of the furnace depends on
the composition of the sludge and
thus its thermal balances for the
combustion process, establishing the
geometric dimensions of the furnace
elements, and the quantities of
auxiliary fuel, air, and exhaust gases.
Figure 17 illustrates the flowchart of
an incineration system with a fluidized
bed furnace.
3.9 Pyrolysis (Frischman, 2012)
Pyrolysis processes heat the digestate
without oxygen, breaking down the
organic content into char and syngas.
For an efficient operation the feed
digestate must have a low moisture
content and ideally be in a dry
pelletized form. Pyrolysis decreases
the digestate mass by 70%, lowering
the transport costs. The char produced
can be used as a soil amendment or as a
partial replacement for peat in growing media production.
a. Pyreg Process
Pyreg (Figure 18) is a proven technology for biomass feedstocks at full scale and
digestate at pilot scale. Its size and modular design make it applicable to a range of
plant sizes. Nevertheless the digestate must be dried before processing. The viability
of Pyreg is dependent on the market for the biochar.
25
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
By 'biochar' we meant the char coming from the pyrolysis of organic matter. In Pyreg
approximately 70% of the feed mass is destroyed within the reactor and the
remainder is a mix of carbon-rich char and ash.
The possibility of installing a turbine to recover electricity from the exhaust gasses,
and make the process entirely self-sufficient, is still tested. The possibility of
increasing the biogas yield and ease of dewatering by mixing biochar with the
digester feedstock is under study as well.
Pros: 1) Carbon capture; 2) Soil amendment - reduced application of compound
fertilizers; 3) Biochar value as a growing media constituent; 4) Renewable heat
generation; 5) Small footprint; 6) Modular units.
Cons: 1) Acceptance of the new technology; 2) Securing a market for biochar; 3)
Establishing a PAS and QP for biochar; 4) Limited process experience; 5) High feed
solids content required.
3.10 Wet Air Oxidation (Andreoli et al., 2007)
Initially wet air oxidation was designed for the paper industry's residues treatment,
but it was adapted for sewage sludge treatment later on. Wet oxidation is
recommended when the sludge is 1) too diluted to be incinerated, and 2) too toxic to
be given to the biological treatment plant. Low-pressure wet air oxidation is used to
decrease the sludge volume and increase its dewaterability for thermal treatment.
Intermediate and high-pressure oxidation are used to decrease sludge volume
through oxidation of volatile organic matter into CO2 and water. Sludge organic
matter, may be considered easily oxidizable (proteins, lipids, sugars and fibers, ,
which are approximately 60% of the total organic matter) or not easily oxidizable.
Wet air oxidation takes advantage of the dissolved or particulate organic matter to
be oxidized at temperatures in the range of 100◦C–374◦C (water critical point). The
temperature of 374◦C stabilizes the water in a liquid form, even at high pressures.
Oxidation is accelerated by the high solubility of oxygen in aqueous solutions at high
temperatures. Wet air oxidation is efficiently applied in destruction of organic
matter of effluents with 1%–20% solids concentration, allowing enough organic
matter to increase the reactor internal temperature through heat generation
without external energy supply. The upper 200 g/L (20%) solids concentration limit
avoids the surplus heat to raise the temperature above the critical value, which
could lead to complete evaporation of the liquid.
Wet air oxidation of organic matter can be described by the following eq.,
CaHbOcNdSeClf + O2 → CO2 + H2O + NH4+ + SO4
2− + Cl− (1)
26
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 19, Conventional wet air oxidation system with a vertical reactor (Andreoli et al., 2007)
Equation (1), is exothermic, so the wet air oxidation process is able to produce
sufficient energy to maintain a self-sustaining process. For this to happen, the
influent COD concentrations should be higher than 10 g/L. The latest developments
in technology and environmental legislations for the final sludge disposal in several
countries, have rekindled the interest for sludge stabilization by wet air oxidation.
Figure 19 shows a vertical reactor wet air oxidation system.
The influent sludge is pumped towards the Wet Air Oxidation (WAO) reactor, passing
through a heat exchanger to raise its temperature. The WAO reactor effluent goes
through a phase splitter, routing the sludge for dewatering, whereas the liquid flows
back through the heat exchanger, where part of the heat is transmitted to the
incoming sludge. The gaseous effluent is released into the atmosphere after being
treated by an electrostatic precipitator and filtered for solid particles and odorous
substances removal. Wet air oxidation may use air or pure oxygen as oxygen supply.
Compressed air as an oxidizing agent is usually found in wastewater treatment
plants.
Pros: 1) able to process sludge too diluted to be incinerated, and too toxic to be
given to the biological treatment plant; 2) less external energy required; 3) solid
produced is sterile, not putrescible, settles readily and may be easily mechanically
dewatered.
Cons: 1) foul odours; 2) corrosion of heat exchangers and reactors; 3) required
power consumption to start-up the oxidation process; 4) high COD in liquid effluent;
27
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 20, Dependence of the volatility of ammonia in water on temperature and pH (Fuchs & Drosg, 2010)
5) high metal content in residual ashes; 6) highly sophisticated, requiring skilled
personnel for operation and maintenance.
3.10.1 Ammonia stripping and scrubbing (Fuchs &Drosg et al., 2015)
In gas stripping volatile substances are extracted from a liquid by gas flow through
the liquid, recovering N (in the form of NH3) from the liquid. The volatility of
ammonia in an aqueous solution is increased by raising the temperature and the pH
(as shown in Figure 20). Excess heat can be used for heating up the digestate and the
pH can be increased by degassing to remove CO2 or by the addition of alkali.
For ammonia stripping of digestate, there are two mainly applied processes: 1) air
stripping and 2) vapour stripping. In air stripping (see Figure 21) heated digestate
enters a stripping column. As a pre-treatment CO2 is removed, this lowers the buffer
capacity. In a subsequent stripping column filled with packing material to increase
surface area available for the ammonia mass transfer, ammonia is transferred from
the liquid digestate to the stripping gas stream. After this, ammonia is recovered
from the gas phase by a sulphuric acid scrubber, where a valuable commercial-grade
ammonium sulphate fertiliser is produced. The cleaned gas can be reused in the
stripping column. For vapour stripping, a much higher temperature is needed to
produce the vapour. The setup can be comparable to Figure 21, only that there is no
need for a final scrubber, as the ammonia can be directly condensed together with
the vapour to produce ammonia water with a concentration of up to 25 – 35 %
ammonia.
28
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.21, Ammonia air stripping including CO2 removal and ammonia recovery by sulphuric acid scrubbers (Fuchs
& Drosg, 2010)
Fig.22, Details of a simplified in-vessel stirring process without stripping columns (Bauermeister et al., 2009)
The usage of packed columns is a big problem for the process, because residual
solids can clog the column, so a prior solid–liquid separation along with a high
maintenance and cleaning effort. Nevertheless, a stripping method performed in
simple stirred tank reactors (see Figure 22) has obtained positive results. A first
large-scale facility using such a type of process principle is already in operation
(Bauermeister et al., 2009).
Pros: 1) Gaseous emissions free from toxins and particulates; 2) Greener image than
incineration; 3) Total oxidation achieved; 4) Renewable heat and energy generation;
5) a standardised, pure nitrogen fertiliser product can be recovered. In addition, such
a fertiliser liquid can be used to enrich other digestate fractions in digestate
processing to a standardised nitrogen concentration, and this can increase their
marketability.
Cons: 1) Relatively high temperature and pressure; 2) The usage of packed columns
is a big problem for the process, because residual solids can clog the column.
29
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
3.11 Economics of digestate processing for nutrient recovery (Fuchs
&Drosg et al., 2015)
Detailed cost analysis of 6 digestate processing scenarios for a model biogas plant
In a study conducted by KTBL (KTBL, 2008), a model biogas plant (50 % manure, 50 %
corn silage) is considered. For the reference scenario (no digestate processing), it is
assumed that about half of the digestate can be applied on agricultural land around
the biogas plant and the other half has to be transported to remote areas. For the
cost analysis both machinery and storage facilities are included. For the digestate
products, a theoretical economic value is assumed according to their nutrient
content (N, P2O5 and K2O).
The following scenarios are investigated:
I. Reference – direct land application
II. Separation (screw press) and separate land application of solid fraction and liquid
phase
III. Separation (screw press) and drying of the solids with a belt dryer
IV. Separation (decanter centrifuge) and purification of the liquid phase by
ultrafiltration and reverse osmosis
V. Separation (decanter centrifuge) and concentration of the liquid phase by
evaporation
VI. Separation (decanter centrifuge) and further treatment of the liquid phase by
nitrogen removal (NH3- stripping and precipitation)
The results of the study can be seen in Figure 23,
30
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 23, Comparison of specific costs for digestate processing at a model biogas plane (KTBL, 2008)
Fig.24, Comparison of cost ranges for specific treatment options versus costs for digestate disposal (Fuchs
&Drosg, 2013)
Figure 23 depicts that the viable implementation of digestate processing is
depending strongly on the specific site. Local conditions cause big differences in the
individual expenses and savings, e.g. for reduced storage facilities or revenues from
the marketing of the processes products, causing large variations of the total costs.
However, typical cost for different digestate treatment can be provided and
compared with the respective costs of digestate disposal. An overview is provided in
Figure 24,
31
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
The transportation and disposal costs come from a study which looked into the
economics of large scale industrial biogas plants (Baernthaler et al., 2008). Costs
include capital and operational costs as well as a theoretical market value for the
products. Most technologies only partially lower the amount of digestate for
disposal, so the costs refer to the amount of digestate saved by each processing.
3.12 Design of a Multi Criteria Decision Tool
The operational costs for to the treatment and disposal of digestate are often
underestimated. Digestate treatment might appear as a minor within the system,
but it has a major impact on the operational costs per year, and thus to the viability
of a biogas installation. So modeling for the evaluation and optimization of the
technology we choose is important.
When we run an evaluation, we have to take different types of data and boundary
conditions into account. We should 1) have reliable and real-case technical data
from several methods of digestate treatment processing; 2) take into account
the legal constraints and applicable values. In this context it is important to not only
to the final disposal of the digestate,, but also on the composition of the input of the
biogas-installation because it has a great impact on the way the digestate has to be
be treated and disposed; 3)have the regional data because, the costs for the
transportation and disposal, vary wildly, even within the same region; 4) get
the input of the biogas installation, like the nutrients (N,P) or DM content because
they will influence the final composition of the digestate.
In our case, in order to simplify the problem, we won't work with any regional data.
Also, due to the fact that the author could not get the technical data needed from
the companies that manufacture the processing technologies, the Thesis will rely on
a comparative study (Frishcman, 2012).
For modeling the decision process, comparative programming is going to be used,
due to its simplicity and speed of process.
3.12.1 Compromise Programming
Compromise programming is a distance based method which determines how much,
different alternatives are from an ideal point (Fig.25) . The smaller the gap, the
higher the alternative is ranked. When the decision factors are chosen and their
importance is weighted, we use Equation 2,
di =[ Σak * (1 -nik)p]1/p (2)
where ak are the weights for the decision factors k (Σak = 1), nik is the performance
value of the criterion k for alternative i and p is a compensation factor between 2
32
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.25, Geometric scheme of distance based methods
and 10. The larger the compensation factor, the more a bad performance influences
the final result.
3.12.2 Selection of the best processing method
The selection of the weights is a pretty complex problem since every country and
every region values differently each and every one aspect. In my effort to design a
general case, I have attributed the values (remember that the total sum of the values
is 1.0): 1) Feed Solids (%) DS: 0,1. The importance to handle DS matter is important
because otherwise a prior separation of the liquid and solid fractions is needed. 2)
Reliability: 0,1. The uneventful operation of the treatment process is a major factor
for the design of the installation. 3) Power Usage: 0,2. The energy consumption is of
paramount important for the financial viability of every project and a chance for
energy savings could make the cut between two choices. 4) Odour Potential: 0,05.
Odours can be a serious problem to the workers and the locals around the
installation. 5) Chemical Usage: 0,15. Chemicals in the industry can get pretty
expensive and influence the yearly operational costs. 6) Noise: 0,05. Same as Sound
7) Hazard (T, P, Chem.): 0,15, same as Sound 8) Carbon footprint: 0,2. The legal
constraints are getting tighter with each year and there are heavy fines for untreated
effluents.
For the selection of the compensation factor, I chose the value 5. For other values
the results may differ wildly.
33
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Table 4, Ranking of the available digestate treatment methods according to their individual score in each of the following categories; 1) Feed Solids,
2) Reliability, 3) Power Usage, 4) Odour, 5) Chemical Usage, 6) Noise, 7) Hazard (T, P, Chem.), 8) Carbon footprint. 5 is the highest score and 1 the
lowest.
Feed Solids (%) DS Reliability Power Usage Odour Potential Chemical Usage Noise Hazard (T, P, Chem.) Carbon footprint Method score Rank
Rotary Drying 2 2 5 5 1 3 2 5 114.92 4
Belt Drying 2 2 5 5 1 3 3 5 115.10 3
J-Vap 1 4 3 5 1 1 2 2 63.31 9
Solar Drying 1 3 5 3 1 1 1 1 100.06 5
Surface Scraped Heat Exchanger (HRS) 1 4 3 3 3 1 1 3 71.31 8
Incineration 5 3 5 5 1 3 2 5 115.29 2
Gasification 4 5 5 5 1 3 2 5 115.38 1
Wet Air Oxidation (WAO) 1 3 3 3 3 3 4 3 76.05 7
Pyreg (slow pyrolysis) 4 3 3 5 5 1 3 1 80.94 6
Fig.26, Flow diagram from the Digestate Treatment System GNS as example of a system with 2 stripping reactors, (Bauermeiter et
al., 2009)
After formulating our algorithm we use a comparative study (Frischman, 2012) to get
Table 4,
So according to the comparative programming method for a compensation factor of
5, gasification is the best choice with pyrolysis being second. As stated before this is
a totally objective result and real-life installation designs should take into account
more and regional data.
3.13 Schematic Flow of a Digestate Thermal Process
Here, is presented, the mass flow schematic of a digestate thermal process. Since a
gasification process was chosen in chapter 3.12, the author will present a case from
a study by Bauermeiter et al. (2009), from a modified stripping process by GNS (gns-
halle.de), where the ammonium nitrogen is removed from the digestate by using
only exhaust heat from the CHP without the use of bases, acids or external stripping
media (Fig.26). The presented data are from the ANAStrip - Plant, BENAS of the
Biogas Plant in Ottersberg from 2007/2008.
34
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
3.14 Properties of Aqua Ammonia - Effects of pH and temperature
As already stated in the previous chapters, each of the current nitrogen recovery
methods suffer from its own drawbacks. Given that the financial budgets and legal
regulations become more and more tight, we need to find alternative digestate
processing methods which can provide us with more efficient procedures. In order to
do that, we need to take a look again at the NH3 properties and its water solution,
aqua ammonia.
Ammonia is a colorless gas with a characteristic odour, highly soluble in water. Its
aqueous solutions are alkaline and have a corrosive effect to metals and tissue. The
pH in an aqueous solution of 0.1 M is 11.2, which is characteristic of a weak base
(pKa = 9.3).
Ammonia is used primarily 1) as a nitrogen source for producing fertilizers; 2) as
refrigerant for producing nitric acid and other chemical reagents such as sulfuric
acid, cyanides, amides, nitrites and intermediaries dyes; 3) as a nitrogen source in
the production of synthetic fiber monomers and plastics; 4) as corrosion inhibitor in
oil refining and other industries such as paper, extractive, food, fur and
pharmaceutical industries. Some of the aforementioned processes produce
wastewater with dissolved toxic gases, ammonia among others, in small
concentrations. For example, in the production of urea fertilizers the wastewater
created has dissolved ammonium in the range of 500 to 2000 ppm.
The removal of this substance before the effluent release, is crucial for two
important reasons:
It is extremely toxic to marine life (concentrations < 0.01 ppm have negative
effects on fish, while 0.1 ppm can be lethal to other species).
It can be bio-oxidized by nitrifying organisms to nitrites and nitrates.
The ammonia in wastewater effluents exists in two forms, as volatile ammonia (NH3)
and ammonium ions (NH4+). Ammonia recovery processes try to maximize as
possible the volatile ammonia component. That depends mainly on two factors, 1)
the temperature (T) and 2) the pH value of the aqueous solution. The T has a direct
effect on the solubility of ammonia in water, which is reducing as the temperature
rises. For example, one volume of water can dissolve 1200 volume of ammonia at 0
◦C (atm. pressure); at 20 ◦C this solubility falls down to 700 vol. of ammonia per 1
volume of water. Nevertheless, raising the temperature cannot release by itself all
the dissolved ammonia in the solution, because much of its quantity dissociates right
away in water to form unstable NH4+ solutions, based on the following chemical
reaction:
35
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Figure 27, Equilibrium Ammonia-Ammonium dependent on pH (Segura, 2012)
NH3 + H2O ↔ NH4+ + OH- (K1 →, K2 ←) (3)
At 25◦C, the equilibrium constants for this chemical reaction are, K1 = 1.8*10−5
towards NH4+ formation and K2 = 5.6*10−10 towards ammonia formation (K1/K2 ≈
3.2×104). Increasing pH drives the equilibrium towards the side of ammonia and
the aqueous solution has a higher ammonia concentration which means more
volatile ammonia molecules instead of ammonium ions, which results in higher
removal efficiencies.
It should be mentioned that the vapor pressure of aqueous ammonia is higher than
that of water. Raising the ammonia concentration in water, would increase
significantly the vapor pressure of the solution. (Bourawi et al., 2007). For a 10 wt%
ammonia solution, the total vapour pressure increases from 12.1 KPa at 20oC to 48.3
KPa at 50oC. In addition, increasing the ammonia concentration in water also
increases the total vapour pressure of the solution. At 20oC, the vapour pressure
increases from 12.1 to 148.8 KPa, when the ammonia concentration is increased
from 10 to 40 wt%. (Xie et al., 2009)
The ammonium equilibrium in water is illustrated in Figure 27. For pH < pKa (9.3), ion
ammonium concentration is higher than ammonia. At pH=9.3 the ionic form as well
as the ammonia molecules were found to 50% in the solution. When pH > pKa,
ammonia is dominant.
So according to Fig.27 many physical and chemical properties of ammonia are a
direct function of its pH value. For example, when the pH of the solution decreases,
the ammonia solubility increases and the ammonia molecules can volatilize freely.
(Segura, 2012)
As mentioned before, the vapor pressure of aqueous ammonia also increases with a
higher pH. To make sure that we have a high ammonia removal and a low water
36
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Figure 28, Influence of temperature and pH on the ratio of ammonia partial pressure to total ammoniacal
nitrogen (TAN) concentration (Zarebska et al., 2014)
content in our distillation product, a moderate temperature combined with a high
pH should be chosen (Fig.28). (Zarebska et al., 2014)
From Fig.28 we can see that the NH3 vapor pressure to total ammonia nitrogen
(TAN) ratio is influenced more from the pH rather than by temperature.
3.15 Membrane Distillation
3.15.1 Fundamentals of Membrane Distillation (MD)
MD is mainly used for desalination purposes but many have gained interest for it as
an advanced treatment of wastewater for water reuse. Researchers have also
studied the viability of MD for brackish water desalination, process water treatment,
and resource concentration for industrial uses. The MD process can use the
advantages of anaerobic processes, while the mesophilic (20-45°C) or thermophilic
(41-122°C) operating conditions that are usually needed to run fermentation
processes (AD) can fulfill the waste heat requirement for the following MD process.
MD has attracted attention for the removal of volatile compounds like ammonia
because of its possibly low energy need. It can potentially recycle and reuse the
industrial wastewater, and can process wastewater streams having a high
temperature but relatively low levels of volatile organic compounds and ammonia. In
MD the driving force for the transport of ammonia across the membrane is the
difference in the partial pressure of ammonia on each side of the membrane. (Xie et
al., 2009). In chapter 6 we explained why by raising the content of volatile ammonia
in its aqueous solution, we increase its vapour pressure, thus making MD an ideal
candidate process for the task of ammonia removal.
37
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Currently, MD is one of the few membrane processes that use a thermal concept.
Energy need is, theoretically, the same as in traditional evaporation processes.
Nevertheless, the operating temperature is much lower than that of a conventional
process because it is not required to heat the solution above its boiling point. The
process can be operated at temperatures typically < 70 °C, and driven by low
temperature difference (20 °C) of the hot and the cold solutions. Thus, low-grade
waste or alternative energy sources (solar and geothermal) can be coupled with MD
systems to drop the cost and raise the efficiency of the separation system.
Consequently, MD might overcome not only the limits of thermal systems drawbacks
but also the ones of the other membrane systems such as reverse osmosis (RO) or
nanofiltration (NF).
MD is not significantly affected by concentration polarization, so high recovery rate
can be achieved, when compared with RO. MD has also all the other properties of
the membrane system (easy scale-up, easy remote control and automation, no
chemicals, low environmental impact, high productivity/size ratio, high productivity/
weight ratio, high simplicity in operation, flexibility, etc.) (E. Drioli et al., 2015)
3.15.2 MD Membranes
MD process is mainly driven by the vapour pressure gradient which is created by a
temperature difference across the membrane. As the latter is not a pure thermal
driving force, MD can be operated at a much lower temperature than conventional
thermal distillation processes. The hydrophobic nature of the membrane prevents
water coming through due to surface tensions, unless a transmembrane pressure
higher than the membrane liquid entry pressure (LEP) is applied. So, liquid/vapour
interfaces are formed at the entrances of each pore. The transfer of water through
the pores, is performed in three steps:
(1) formation of a vapour gap at the hot feed solution–membrane interface;
(2) transport of the vapour phase through the microporous system;
(3) condensation of the vapour at the cold side membrane–permeate solution
interface.
MD membranes are usually made from 1) polytetrafluoroethylene (PTFE), 2)
polyvinylidene fluoride (PVDF), and 2) polypropylene (PP). PTFE has the highest
hydrophobicity, good chemical and thermal stability, and oxidation resistance. PVDF
and PP also show good hydrophobicity and thermal/chemical resistance and can be
easily developed into membranes with versatile pore structures.
38
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Table 5. Specification of hydrophobic microfilters used in the membrane distillation (Hyun-Chul Kim et al., 2015)
Recent studies have researched new membrane materials such as carbon nanotubes,
fluorinated copolymer materials, and surface modified polyethersulfone (PES) that
achieve good mechanical strength and high porosity. Hydro-repellent membranes
allow for the complete rejection of non-volatile solutes (e.g., macromolecules,
colloidal fraction, ionic species and so forth). Typical feed T is in the range of 30-60oC
and lower temperatures than those conventional distillation operate with, are
preferable since the heat loss through thermal conduction is also linear to the
temperature difference across the membrane. (Hyun-Chul Kim et al., 2015)
Membrane distillation efficiency is directly affected by the structure of the
membrane in terms of 1) thickness; 2) porosity; 3) mean pore size; 4) pore
distribution and 5) geometry. So, distillation product of the process is dependent
upon the capability of the membrane to interface two media without dispersing one
phase into another and to combine high volumetric mass transfer with high
resistance to liquid intrusion in the pores. The membranes for membrane contactor
application have to be 1) porous; 2) hydrophobic; 3) with good thermal stability and
4) excellent chemical resistance to feed solutions.
In particular, the characteristics needed are (E. Drioli et al., 2015):
1. High liquid entry pressure (LEP), is the minimum hydrostatic pressure that must
be applied onto the feed solution before it overcomes the hydrophobic forces of the
membrane and penetrates into the membrane pores. LEP is a characteristic of each
membrane and permits to prevent wetting of the membrane pores. High LEP may be
achieved using a membrane material with high hydrophobicity and a small maximum
pore size
LEPW =(B*γL*cosθ)/dmax (4)
B is a geometric factor determined by pore structure with value equal to 1 for
cylindrical pores, γL the liquid surface tension and θ is the liquid/solid contact angle.
39
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
However, as the maximum pore size decreases, the mean pore size of the membrane
decreases and the permeability of the membrane becomes low.
2. High permeability. The flux will “increase” with an increase in the membrane pore
size and porosity, and with a decrease of the membrane thickness and pore
tortuosity. In fact, molar flux through a pore is related to the membrane's average
pore size and other characteristic parameters by:
N≈(<ra>*ε)/(τ*δ) (5)
where ε is the membrane porosity, τ is the membrane tortuosity, δ is the membrane
thickness, ⟨rα⟩ is the average pore size for Knudsen diffusion (when α=1), and ⟨rα⟩ is
the average squared pore size for viscous flux (when α=2).
To obtain a high permeability, the surface layer that drives the membrane transport
must be as thin as possible and its surface porosity as well as pore size must be as
large as possible. In order to do so we see from eq. (5) that we need, in terms of
molar flux, to maximize the membrane porosity and pore size, while minimizing the
transport path length through the membrane, (τ, δ). However, thermal efficiency in
MD increases gradually with increasing the membrane thickness and an optimization
between the two requirements has to be researched.
3. Low fouling problem. Fouling is one of the main problems when using porous
membranes. In the gas–liquid contact processes, since there is no convection flow
through the membrane pores, the contactors are less sensitive to fouling.
Nevertheless, when we have gas and liquid streams with large content of suspended
particles (e.g. industry), we can get clogging of the membrane pores. Ideally pre-
filtration is necessary for an efficient operation.
4. High chemical stability. The life sustainability of the membrane depends on any
reaction between the solvent and membrane material, which could possibly affect
the membrane matrix and surface structure. Liquids with high content of acid gases
are corrosive, which make the membrane material less resistant.
5. High thermal stability. When operating with high temperatures, the membrane
material may be affected by degradation or decomposition. Any change in the
nature of membrane depends on the glass transition temperature Tg for amorphous
polymers or the melting point Tm for crystalline polymers. The factors that increase
the crystallinity (Tg/Tm) of a membrane can improve its chemical and thermal
stability. For processes at high temperatures, fluorinated polymers are used due to
their high hydrophobicity and chemical stability.
40
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.29, Schematic representation of MD configurations (Onsekizoglu, 2012)
3.15.3 MD configurations
MD processes can be categorized into four basic configurations (Fig. 29), depending
on how vapour is condensed in the permeate side. In all four configurations there's
direct contact of one side of the membrane with the feed solution. Direct contact
membrane distillation (DCMD) is the most researched because of its simplicity.
Vacuum membrane distillation (VMD) can be used for high output, while air gap
membrane distillation (AGMD) and sweep gas membrane distillation (SGMD) have
low energy losses and high performance ratio. New configurations with improved
energy efficiency, better permeation flux or smaller foot print have started to be
looked into, such as material gap membrane distillation (MGMD), multi-effect
membrane distillation (MEMD), vacuum-multi-effect membrane distillation (V-
MEMD) and permeate gap membrane distillation (PGMD).
1. In direct contact membrane distillation (DCMD), water with lower temperature
than the liquid feed, is used as condensing medium in the permeate side. In this
configuration, the liquid in both sides of the membrane is in direct contact with the
hydrophobic microporous membrane. DCMD is the most process to set up in a
laboratory. Direct contact of the membrane with the cooling side and poor
41
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Table 6, Advantages, disadvantages and application areas for MD configurations (Kullab, 2011)
conductivity of the polymeric material though, causes heat losses throughout the
membrane. So in DCMD, the thermal efficiency (fraction of heat energy used only for
evaporation), is relatively smaller than the other three configurations.
2. In air gap membrane distillation (AGMD), water vapour is condensed on a cold
surface that has been set apart from the membrane with a stagnant air gap which
reduces the heat losses.
3. In sweeping gas membrane distillation (SGMD), we use a cold inert gas in the
permeate side for sweeping and carrying the vapour molecules to outside the
membrane module where the condensation takes place. Although we have a
relatively low conductive heat loss with a reduced mass transfer resistance, the extra
operational costs of the external condensation system make SGMD the least applied
configuration.
4. In vacuum membrane distillation (VMD), the process is driven by applying
vacuum at the permeate side. The applied vacuum pressure is lower than the
equilibrium vapour pressure. Therefore, condensation takes place outside of the
membrane module.
Each of the MD configurations has its own advantages and disadvantages for a given
application (Table 6),
42
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.30, Different resistances to heat and mass transfer in (a) DCMD, (b) VMD, (c) AGMD, (d) SGMD (Andreoli et al., 2007)
In Fig.30 is illustrated the heat and mass transfer in the 4 configurations,
Recently, MEMSYS (memsys.eu) patented an integration of vacuum with multi-
effects in their module designing for MD. V-MEMD is a modified form of VMD that
integrates the concept of state-of-the-art multi-effect distillation into the VMD. In
the process, the vapors that are produced in each stage are condensed during the
subsequent stages. Vapors are generated in steam raiser working under vacuum by
exchanging the heat provided by external source. The vapors are introduced in the
first stage where these are condensed by exchanging the heat with feed via a foil.
The vapors generated in the first stage are transported through the membrane and
collected on the foil in the second stage. The flow of different streams in a single
stage has been illustrated in Fig. 31.
43
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.31, Schematic illustration of streams in V-MED module (Andreoli et al., 2007)
Fig.32, Frame and stages used by MEMSYS (i) a simple frame, (ii) single stage consisting of welded frames and covering plates, (iii)
multiple stages (Andreoli et al., 2007)
The company claims that this configuration has a high feed to output ratio which is a
important parameter for industrial applications. A condenser is used to condense the
vapors generated in the final stage. The vapor pressure in each stage is less than its
preceding stage. A schematic diagram with the module fabrication is depicted in Fig.
32.
44
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.33, Variation of feed ammonia concentration in HFMC, DCMD and MDCMD (Dan Q et al., 2013)
3.16 Performance of MD in Ammonia Recovery
3.16.1 Comparison between MD Configurations
In a comparative study (Dan Qu et al., 2013), a modified direct contact membrane
distillation (MDCMD) with receiving solution in permeate was developed for
accelerating ammonia extraction from a water solution (Fig. 34). Its efficiency was
then compared to DCMD and to hollow fiber membrane contactor (HFMC). Also
there was researched the effects of feed pH, temperature, flow rate and
concentration on ammonia extraction efficiency and the permeate flux in MDCMD
process.
The developed MDCMD process had the highest ammonia stripping efficiency. The
ammonia removal efficiency of DCMD, HMC and MDCMD was 52%, 88% and 99.5%
within 105 min, respectively, proving MDCMD to have an advantage over the other
two methods and a good option as an alternative process.
In the MDCMD process, feed pH value was the parameter with the highest influence.
Increasing feed pH value was increasing ammonia removal efficiency as well as the
permeate flux, but only up to 12.20, after which it gave no noticeable effect.
Increase of feed temperature and velocity led to an increase of the ammonia
removal efficiency, ammonia mass transfer and the permeate flux. Initial feed
ammonia concentration didn't have a significant effect on ammonia extraction
efficiency (Fig.33).
45
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Table 7, Properties of the PVDF membrane used in the experiment (Dan Q et al., 2013)
Fig.34, Experimental setup for ammonia removal (Dan Q et al., 2013)
Vacuum membrane distillation (VMD) had the highest mass transfer but the lowest
selectivity, while direct contact membrane distillation (DCMD) had the highest
selectivity and moderate mass transfer. The sweeping gas membrane distillation
(SGMD) gave moderate selectivity and the lowest mass transfer. Also, in a DCMD
process for ammonia stripping, water vapor as well as ammonia can both transfer
across the membrane to the permeate side due to the temperature difference,
which may lead to wastewater volume minimization.
The ammonia removal efficiency (R) could be defined as:
R= (1 - Ct/Co) * 100% (6)
The ammonia mass transfer coefficient (Ka) was determined experimentally as
follows:
Ka = Vf/At * ln(Co/Ct) (7)
Therefore, plotting ln(Co/Ct) vs. t yielded a straight line. And the ammonia mass
transfer coefficient, Ka, can be calculated from the slope of the line.
46
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.35, Variation of feed ammonia concentration and average permeate flux at different feed pH (Co=1.5 g/L, Tf=50oC, Tp=28C,
uf=0.5 m/s, up=0.1 m/s),(Dan Q et al., 2013)
The ammonia removal efficiency of DCMD, HFMC and MDCMD was 52%, 88% and
99.5% within 105 min, respectively.
Obviously, compared with the HFMC and DCMD, the MDCMD gave the highest Ka
and R with an acceleration of receiving solution to ammonia removal efficiency. Also,
higher feed temperature leads to a higher NH3 diffusion rate both in the bulk
solution and membrane pores which results in a higher mass transfer coefficient.
Effect of operating parameters in MDCMD process
1.Effect of the feed pH
Figure 35 illustrates the values of feed ammonia concentration and average
permeate flux versus time at different feed pH values. The average permeate flux
increased with increasing pH values till 12.2, and no significant changes of the
permeate flux were observed in the case of pH 12.2 to 13.2. It also can be seen from
Fig.35 that the ammonia concentration in feed was lowered more quickly at higher
pH values. The ammonia removal efficiency reached to 99.5% when the pH was
higher than 12.20. Fig.34 plots the Ka against feed pH values. The Ka increased from
1.89 to 6.29 × 10−5 m/s with increasing pH value from 10.0 to 12.2.However, as the
pH value increased to 13.2, the ammonia transfer coefficient was only raised from
6.29 to 6.74 × 10−5 m/s.
47
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.36, Effect pf feed pH on ammonia mass transfer coefficient (Co=1.5 g/L, Tf=50oC, Tp=28oC, uf=0.5 m/s, up=0.1 m/s),
(Dan Q et al., 2013)
The above results showed that the positive effect of pH on average permeate flux
and Ka was decreased with rising pH and there was no significant increase when pH >
12.20. That was explained with the ammonia dissociate equilibration in water
solutions, which is illustrated in Eq.2.
NH3 + H2O ↔ NH4+ + OH-
Increasing pH value drives the equilibrium move towards the production of NH3, the
only form of ammonia that can be stripped. So we want a higher NH3 concentration
rather than the NH4+, which can give in higher ammonia removal rates. It was found
that the water vapor pressure having volatile NH3 components, was higher than that
of pure water, which could lead to an increased permeate flux. At some point
though, resistance caused by membrane would gradually become the dominant
factor of the mass transfer process with the feed side gaining more flux. This could
be why pH is less influential when the pH > 12.20.
2. Effect of the feed temperature
Fig.37 shows the feed ammonia concentration and the average permeate flux at
different feed temperatures. As we can see, the feed temperature had a significant
effect on the average permeate flux. For example, raising the feed T from 30 to 50 °C
created an increase of the permeate flux of about 250%. As T got higher, there was a
significant increase in the vapor pressure of the feed solution which consequently
increased the transmembrane vapor pressure difference and driving force.
Increasing feed T also favored the ammonia removal. Feed ammonia concentration
decreased more quickly at higher feed T. Ka at different feed T is depicted in Fig.38
and shows the positive effect of increasing feed T. Ka increased from 3.42 to 7.28*
48
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.37, Variation of feed ammonia concentration and average permeate flux at different feed T (Co=1.5 g/L, Tf=50oC,
Tp=28oC, uf=0.5 m/s, up=0.1 m/s), (Dan Q et al., 2013)
Fig.38, Effect of feed T on ammonia mass transfer coefficient (Co=1.5 g/L, Tf=50oC, Tp=28oC, uf=0.5 m/s, up=0.1 m/s),
(Dan Q et al., 2013)
10−5 m/s when the feed temperature increased from 30 to 55 °C. High feed T had a
positive effect in NH3 diffusion both in the bulk solution and membrane pores, which
led to an increased mass transfer coefficient. Also, a higher content of volatile
ammonia was found in the feed solution due to the endothermic nature of
dissociation of ammonium ions.
49
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.39, Variation of feed ammonia concentration and average permeate flux at different feed flow rates (Co=1.5 g/L,
Tf=50oC, Tp=28oC, uf=0.5 m/s, up=0.1 m/s), (Dan Q et al., 2013)
3. Effect of feed flow rate
As depicted in Fig.39, increased flow rates led to a higher diffusion of NH3 from the
feed bulk to the membrane surface and had a positive effect in the mixing condition
of boundary layer which led to a higher ammonia and water vapor mass transfer.
4. Effects of aeration in gas-permeable membrane ammonia recovery
In another study using swine manure with gas-permeable membranes (M.C. García-
Gonzalez et al., 2015), ammonia was successfully separated and recovered operating
with aeration and nitrification inhibition (Fig.40). The aeration reacted with the
natural alkalinity, which released OH- and increased the pH of the bulk solution over
8.5. That change promoted gaseous NH3 release from the manure and an increased
permeation through the submerged membrane. The overall NH4+ recovery obtained
with the aeration approach was 98% (Fig.41). Aeration also substituted for the large
amounts of alkali chemical that were needed to produce the same effect and
reduced the operational costs by 57%.
50
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.40, Schematic diagram of treatment 1, 2 and 4 showing the recovery of NH3 by the gas-permeable membrane manifold as it
was governed by the balance in Eq.2 that depended on manure pH (M.C. García-Gonzalez et al., 2015)
Fig.41, Mass of ammonia recovered in the acid concentrator tank for aerated, not aerated and chemically amended manure
treatments. A second order eq. and R2 are represented. The error bars are the standard deviation of duplicate experiments (M.C.
García-Gonzalez et al., 2015)
51
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.42, Experimental set-up for fermentative wastewater treatment with AMBBR and subsequent distillation using hydrophobic
microfilters. The biogas was analyzed in a gas chromatograph (Hyun-Chul Kim et al., 2015)
3.16.2 Performance of (PTFE), (PVDF) and (PP) membranes in ammonia
recovery
In a study researching MD combined with an anaerobic moving bed biofilm reactor
(AMBBR) for treating municipal wastewater for the treatment of domestic
wastewater (Fig.42), (Hyun-Chul Kim et al., 2015). The viability of using a membrane
separation technique for post processing of anaerobic bio effluent was studied.
Three different hydrophobic 0.2 mm membranes made of polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVDF), and polypropylene (PP) were used in this
study.
From the three different hydrophobic membranes studied, the highest permeate flux
was recorded for the PTFE membrane. A gradual but noteworthy decline of the flux
was seen in the MD treatment with the PTFE membrane for which the permeate flux
decreased up to 84% of the initial value after the 45h distillation. The initial
permeate flux of the PVDF and PP membranes had 81% and 41% respectively. A
longer-term treatment took place, using the MD module with a flat-sheet PVDF
membrane for the reuse of anaerobically treated wastewater. Waste heat leaving
the AMBBR was used as the dominant factor for the mass transfer in the MD
process. The characterization of effluent organic matter (EfOM) using liquid
chromatography - organic carbon detection (LCOCD) verified that almost all of the
52
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
EfOM was rejected by a macro-porous filter placed in the MD module. MD treatment
also lead to the complete rejection of total phosphorus (TP) from the feed
wastewater.
3.17 Handling of the Aqua Ammonia
At the end of processing the sludge/digestate, we have a water solution with high N
content that we still need to extract/use. There are a lot of chemical methods that
produce mainly fertilizing products but the problem is that they use acids during
their process. Acids in the industry are expensive and we would like if possible to
find routes that avoid using them. In this chapter we're concentrating on chemical
conversion of ammonia to Urea, Ammonium Carbonate and Ammonium Sulfate.
3.17.1 Ammonia Conversion to Urea (Fertilizers Europe, 2000)
Urea is made from ammonia and carbon dioxide. The two substances are reacting at
high pressure and temperature, and the urea is formed in a two step reaction,
2NH3 + CO2 ↔1 NH2COONH4 (ammonium carbamate) ↔2 H2O + NH2CONH2 (urea)
(8)
The urea contains unreacted NH3, CO2 and ammonium carbamate. When pressure is
reduced and heat is applied, NH2COONH4 decomposes to NH3 and CO2. The
ammonia and carbon dioxide are recycled. The urea solution is then concentrated to
give 99.6% mass fraction ( wt.%) molten urea, and granulated for use as nitrogen-
rich fertilizer and as a component in the manufacture of resins for timber processing
and in yeast manufacture.
The conversion to ammonium carbamate (Reaction 1) is fast and exothermic and is
almost complete under the industrial reaction conditions. The decomposition to urea
(Reaction 2) is slower, endothermic and) is usually in the order of 50-80%. The
conversion increases with increasing temperature and NH3/CO2 ratio and decreases
with increasing H2O/CO2 ratio.
Modern urea technology installations vary in size from 800 to 2,000 t/d, have very
similar energy requirements and nearly 100% material efficiency with slight
differences in energy balances. The production outputs include, 1) Urea; 2) Process
condensate water which can be used as boiler feed water after treatment; 3) Steam/
turbine condensate which are exported to the battery limits for polishing and re-use
as boiler feed water; 4) Low pressure steam. The LP steam produced in the
carbamate condenser is used for heating purposes in the downstream sections of
the plant. The excess may be sent to the CO2 compressor turbine or CO2 booster or
exported for use in other site activities.
53
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.43, (left) Block diagram of a total recycle CO2 urea stripping process, (right) Block diagram of a total recycle NH3 urea stripping
process, (Fertilizers Europe, 2000)
In urea production we may have to face problems like: 1) Equipment/piping failure
due to corrosion; 2) Explosion hazard due to the formation of an explosive gas
mixture; 3)Toxic hazard due to NH3 release
Corrosion Protection in Urea Plants
The corrosiveness is determined by the temperature, the process components, the
concentration of dissolved oxygen and the presence of contaminants that may
accelerate corrosion. The formation on start-up and maintenance of a passive oxide
layer on stainless steel surfaces is of the utmost importance.
Explosive Gas Mixtures
Explosive gas mixtures may form in the inerts scrubber, the off-gas from which
consists of O2, H2, and N2 and possibly some non-condensed NH3 and CO2. Well
controlled operation is a means of keeping these gas mixtures outside the explosion
hazardous range.
Block flow diagrams for CO2 and NH3 stripping total recycle processes are shown in
Figure 43.
54
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Carbon dioxide stripping process
NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of
approximately 140bar and a temperature of 180-185°C. The molar NH3/CO2 ratio
applied in the reactor is 2.95. The CO2 conversion is about 60% and NH3 is 41%. The
reactor effluent, containing unconverted NH3 and CO2 is subjected to a stripping
operation at essentially reactor pressure, using CO2 as stripping agent. The stripped
off NH3 and CO2 are then partially condensed and recycled to the reactor. The heat
evolving from this condensation is used to produce 4.5bar steam some of which can
be used for heating purposes in the downstream sections of the plant. Surplus
4.5bar steam is sent to the turbine of the CO2 compressor.
The NH3 and CO2 in the stripper effluent are vaporized in a 4bar decomposition stage
and subsequently condensed to form a carbamate solution, which is recycled to the
140bar synthesis section. Further concentration of the urea solution leaving the 4bar
decomposition stage takes place in the evaporation section, where a 99.7% urea
melt is produced.
Ammonia stripping process
NH3 and CO2 are converted to urea via ammonium carbamate at a pressure of
150bar and a temperature of 180°C. A molar ratio of 3.5 is used in the reactor giving
a CO2 conversion of 65%. The reactor effluent enters the stripper where a large part
of the unconverted carbamate is decomposed by the stripping action of the excess
NH3. Residual carbamate and CO2 are recovered downstream of the stripper in two
successive stages operating at 17 and 3.5bar respectively. NH3 and CO2 vapours from
the stripper top are mixed with the recovered carbamate solution from the High
Pressure (HP)/Low Pressure (LP) sections, condensed in the HP carbamate condenser
and fed to the reactor. The heat of condensation is used to produce LP steam. The
urea solution leaving the LP decomposition stage is concentrated in the evaporation
section to a urea melt.
3.17.2 Ammonia Conversion to Ammonium Carbonate as a means to
reduce CO2 emissions (Qiao He et al.,)
Power generation contributes one-third of the CO2 released from fossil fuel
combustion worldwide. There are two typical strategies to reduce CO2 emission
from power plant combustion, 1) concentrate the CO2 in-situ during the fuel
conversion process, which is usually very complex and difficult to control and 2) to
extract the CO2 from the combustion flue gas followed by sequestration process.
For the latter method we have two main industrial applications; 1) Physical solvents
which are favored by high pressures and low concentrations of inert gases; 2)
55
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Chemical solvents which are preferred for cases with low concentrations or amounts
of CO2 in the combustion gases and do not gain significant advantage by operating at
elevated pressure. The chemical solvent methods are currently the most efficient but
the process cost for separation of CO2 is high and the methods in general suffer from
a slow absorption rate, small solvent capacity, and special equipment requirements.
Another alternative which may provide an inexpensive and effective route of
reducing CO2 emissions from power plants is extracting CO2 by an ammonia (NH3)
reagent in a wet scrubber. Unlike the Monoethanolamine (MEA) process, the
aqueous ammonia process does not have absorbent degradation problems, which
are caused by sulfur dioxide and oxygen in the flue gas and does not cause
equipment corrosion. The removal efficiency is approximate 99%. and the maximum
CO2 removal efficiency by NH3 absorbent can reach 99%.
The products in the aqueous ammonia scrubber could include ammonium
bicarbonate, ammonium carbonate, and ammonium carbamate (all in crystalline or
aqueous solution forms), plus reagent CO2, NH3, and NH4OH. Of these by-products,
ammonium bicarbonate has been utilized by certain developing countries as a crop
fertilizer for over 30 years with proven results. A process using coal char to produce
ammonium bicarbonate for fertilizer has been developed and used commercially
over 50 years. As an alternative to use ammonium bicarbonate as fertilizer, the
ammonia (NH3) in ammonium bicarbonate can be regenerated. Ammonium
bicarbonate decomposes at the relatively low temperature of 60oC, compared to a
120oC regeneration temperature for MEA solutions.
Theory of NH3-CO2-H2O reaction
CO2 can be removed by ammonium scrubbing through chemical absorption at
various temperatures and operating conditions. As we can see in Equation (9),
ammonium carbamate (NH2COONH4) is the main product in dry conditions.
However, ammonium carbamate, is very soluble in water. Under moist air, the
hydration product of ammonium carbonate, (NH4)2CO3, is produced at room
temperature, as illustrated in Equation (10),
CO2(g) + 2NH3(g) ↔ NH2COONH4(s) (9)
NH2COONH4(s) + H2O(g) ↔ (NH4)2CO3(s) (10)
NH2COONH4(s) + H2O(g) ↔ NH4HCO3(s) + NH3(g) (11)
The gas-liquid chemical reactions between NH3 and CO2 in the wet scrubber can be
illustrated by the following equations:
CO2(g) + 2NH3(g) ↔CO(NH2)2(s) + H2O(g) (12)
56
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
CO2(g) + 2NH3(l) ↔ NH4+(l) + NH2COO-(l) (13)
2NH3(g) + CO2(g) + H2O(g) ↔ (NH4)2CO3(s) (14)
NH3(g) + CO2(g) + H2O(g) ↔ NH4HCO3(s) (15)
2NH3(l) + CO2(g) + H2O(l) ↔ (NH4)2CO3(s) (16)
NH3(l) + CO2(g) + H2O(l) ↔ NH4HCO3(s) (17)
At room temperature and atmospheric pressure, reactions (12)-(17) would occur.
These reactions are very temperature sensitive. Due to the different concentrations
of reactants and reaction conditions (temperature and pressure), different carbon-
ammonium composites (i.e., different ammonium salts), can be obtained, as
illustrated by Equation (38):
aNH3 + bCO2 + cH2O → NH4HCO3 (C-salt); (NH4)2CO3*H2O (S-salt);
2NH4HCO3*(NH4)2CO3 (P-salt); NH4COONH2 (A-salt) (18)
Equation (11) indicated that ammonium bicarbonate and ammonium carbonate
could be converted to each other at certain condition,
(NH4)2CO3(s) + H2O(l) + CO2(g) ↔ 2NH4HCO3 (s) (19)
All the products are white solids and may be a single salt or mixed salts. For
achieving a maximum of NH3 utilization in the capture of CO2 (minimum inventory
required for CO2 sequestration), reaction (17) is the ideal choice.
In commercial processes, the reaction follows the following two steps: gaseous CO2
dissolves in solution and reacts with aqueous ammonia. Based on previous studies,
the reaction process is diffusion controlled – dissolution of gaseous CO2 in solution.
Therefore, increasing CO2 dissolubility in water (solution) could enhance the reaction
rate.
The schematic diagram of the experimental system for studying on removing CO2 gas
by ammonium scrubbing is shown in Figure 44,
57
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.44, Schematic diagram of the experimental system for studying on removing CO2 gas by ammonium scrubbing, (Qiao He et al.,)
3.17.3 Ammonia to Ammonium Sulfate
Ammonium sulfate (AS), (NH4)2SO4, is a chemical compound that is primarily used
as a nitrogen fertilizer with other minor uses. It occurs in crystals with particle size
that is variable, colored white to beige. It is a two-in-one synthetic fertilizer that
supplies both the essential elements nitrogen and sulfur.
In the last years there has been a decrease in ammonia sulfate production and
utilization. The main reasons are: (1) reduction in the processing of industrial
products in which it is a byproduct (SOH 2006); and (2) preference of other
fertilizers, mostly urea, due to the much higher nitrogen content (46% N) and
therefore a lower cost of transport per unit of nitrogen.
However, ammonium sulfate has distinct advantages: 1) It contains N and S, making
it a valuable fertilizer in soils that are deficient of both; 2) From the major
nitrogenous fertilizers, it has the least tendency to absorb atmospheric water. This
characteristic favours longer storage; 3) Compared to urea, it is more resistant to
ammonia volatilization. In neutral to acid soils, it can be broadcasted or otherwise
applied without soil incorporation. However, on alkaline soils (above pH 7.0) with
high moisture and under high temperature, the possibility of volatilization increases
with soil surface application where it lies exposed for several days. Under this
condition, the applicable rule is soil incorporation like in urea (THORUP, 1984).
About 90% of ammonium sulfate is produced by three different processes: 1) as a
byproduct of caprolactam (CH2)5COHN production, 2) synthetic manufacture from
pure ammonia and concentrated sulfuric acid, and 3) as a coke oven byproduct. The
remaining 10% is produced as a byproduct in nickel manufacture, or as a byproduct
58
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.45, Schematic diagram of an ammonia based WFGD MET process, (MET, 2008)
of methyl methacrylate manufacture or from ammonia scrubbing of tail gas at
sulfuric acid (H2SO4 ) plants. (EPA)
In this chapter we are going to talk about producing ammonia sulfate from Flue Gas
Desulfurization (FGD) which are used for the reduction of sulfur dioxide emissions.
Effective emission controls using wet FGD technologies have been commercially
demonstrated and widely adopted. Utilities burning high-sulfur coals and using an
FGD system also produce solid by-products. The financial burden of FGD
technologies would be less if successful commercial uses, such as ammonium sulfate
fertilizer, were developed for the FGD solid by-products. (M.M. Chou et al., 2005)
Marsulex Environmental Technologies (MET) has developed an effective ammonia
scrubbing technology that removes sulfur dioxide (SO2) from boiler flue gases and
produces high-value ammonium sulfate (AS) fertilizer. The AS scrubbing system uses
proven wet flue gas desulfurization (WFGD) equipment and therefore can achieve
the same high reliability of the more traditional limestone-based WFGD. (MET, 2007)
The AS WFGD ammonia feed system consists of an ammonia storage tank, ammonia
feed pumps and a vaporizer in the event of anhydrous ammonia supply. It employs
centrifuges for dewatering, in lieu of belt or rotary drum filters, based on their ability
to produce a low moisture cake and a small plot area requirement. (Fig. 45)
The patented MET ammonium sulfate scrubbing process in Figure 65 has in general a
higher SO2 removal efficiency, compared to limestone/gypsum scrubbing processes.
The improved SO2 removal efficiency of the AS WFGD relative to limestone WFGD
over a range of fuel % sulfur content for a constant liquid-to-gas ratio (L/G) is
illustrated in Figure 46. This advantage of the ammonia system due to the water
soluble chemistry improves with increasing fuel % sulfur content.
59
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.46, Impact of Fuel %S on SO2 Removal for Ammonia versus Limestone, (MET, 2008)
Process Chemistry
The chemistry of the AS WFGD is similar to the chemistry using a limestone reagent
in that the major steps are absorption and oxidation. The overall reactions which
occur in the MET AS absorber are as follows:
SO2 + 2NH3 + H2O ↔ (NH4)2SO3 (20)
(NH4)2SO3 + ½O2 ↔ (NH4)2SO4 (21)
The actual chemical mechanism is more complex and involves sulfite-bisulfite and
sulfatebisulfate reactions:
SO2 + H2O ↔ H2SO3 (22)
H2SO3 + (NH4)2SO4 ↔ NH4HSO4 + NH4HSO3 (23)
H2SO3 + (NH4)2SO3 ↔ 2NH4HSO3 (24)
SO2 in the flue gas first comes into contact with spray droplets of aqueous slurry. In
reaction (22), SO2 from the flue gas dissolves in the water to form sulfurous acid. In
reactions (23) and (24), the sulfurous acid reacts further with dissolved ammonium
sulfate and sulfite salts in the solution to form intermediate acidic species. The
formation of the sulfurous acid, including the acidic species, lowers the pH of the
slurry.
H2SO3 + NH3 ↔ NH4HSO3 (25)
NH4HSO3 + NH3 ↔ (NH4)2SO3 (26)
NH4HSO4 + NH3 ↔ (NH4)2SO4 (27)
60
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
The ammonia added to the process in reactions (25) to (27) neutralizes the acidic
species to ammonium sulfite and ammonium sulfate. This neutralization serves to
restore the pH to its desired value.
(NH4)2SO3 + 1/2 O2 →(NH4)2SO4 (28)
Oxidation air injected into the absorber tank in reaction (28) oxidizes the remaining
ammonium sulfite to ammonium sulfate.
(NH4)2SO4 (a) + heat of evaporation →(NH4)2SO4 (solid) (29)
The resulting ammonium sulfate solution is saturated and ammonium sulfate
crystals, in reaction (29), precipitate from the solution due to chemical reaction and
water evaporation into the flue gas. The heat of evaporation is supplied by the
residual heat in the flue gas. Nitrogen content of the product is approximately 21%
weight while sulfur content is approximately 24%.weight. When applied to soils, the
sulfur constituent of the ammonium sulfate acts as a catalyst which increases the
rate of nitrogen uptake by various crops.
4. Experiment Theory
Another alternative route from MD in order to take advantage of the increase of
volatile ammonia molecules in aqueous solution with higher temperatures, is
sequential evaporation. For this reason I chose to conduct an experiment in the labs
of the ISAH institute to find out the optimal combination of the evaporation tank
volume and the amount of solution required to recover the highest amounts of total
nitrogen (TN) in the least amount of time. The idea behind this was that the larger
the free surface area of the feed wastewater, the faster it would evaporate (not to
be misunderstood for the evaporation rate per unit area). Then the preferred
solution would be tested with the hydrophobic MEMSYS membrane to study its
effects on the nitrogen content of the distilled solution. However before we go on
with the description of the experiment, we need to understand the mechanism
behind the evaporation of liquids.
4.1 Fundamentals of evaporation
a. Evaporation of Liquids
The molecules of a liquid are kept together by cohesive forces. Each molecule has an
amount of energy but not all molecules have the same amount of energy. If you use
a thermometer to measure the energy of a liquid, the value you get is the average
energy of all the molecules in the system. There are always a few molecules with a
lot of energy and some with almost none. That phenomenon takes place because the
molecules in a liquid can move around, bump into each other, and exchange an
61
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.47, Effect of Molecular Energy and Vapor Pressure on the chance of liquid molecules to evaporate
amount of energy. With trillions of molecules moving around and bumping into each
other, sometimes individual molecules gain enough energy to overcome the
cohesive forces between them. They leave the liquid and get into the gas phase. That
process is called evaporation. When the liquid is given heat and warms up, it has a
higher temperature and the molecules get higher energy. When the energy in
specific molecules reaches a certain level, they have a phase change. Evaporation
has to do with the energy in individual molecules, not with the average energy of a
system (Fig.47). The average energy can be low and the evaporation still goes on.
As we can also see in Fig.47, the rate of evaporation goes up with a decrease in the
gas pressure around a liquid. Molecules want to move from areas of high to low
pressure, basically sucked into the surrounding area to even out the pressure
equilibrium. When the vapor pressure of the system reaches a critical point, the
evaporation rate will slow down.
b. Net Evaporation
As the molecules of the liquid leave the bulk solution, condensation takes place.
Condensation, is the opposite of evaporation. It occurs when saturated air is cooled
below the dew point, the T to which air must be cooled at a constant pressure for it
to become fully saturated with water. A state of saturation exists when these two
process rates are equal, at which point the relative humidity of the air is 100 %. Net
evaporation happens when the rate of evaporation exceeds the rate of
condensation. In order to overcome condensation and vaporize a liquid we need to
provide extra energy in the form of heat. Water evaporates easily when it reaches
the boiling point at 100oC.
Chance of evaporation
Molecular energy
Chance of evaporation
Vapor Pressure
62
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.48, Distribution of a liquid inside an evaporating flask, (heidolph-instruments.com)
c. The effect of the free surface area in evaporation of spherical water surfaces
As we have already we would like to research the ratio of the evaporation tank
volume to the amount of wastewater which it contains to find the combination that
yields the optimal results. We do that because surface area plays a dominant role in
the mass transfer during evaporation. In every liquid evaporation rate per unit area
is the same in every point of the free surface, so bigger areas allow for the liquid to
evaporate faster. However land usage is expensive and we can't invest a huge
amount of capital without having prior investigated the efficiency of the process.
In the case of our experiment we are using evaporating flasks (Fig.48), which
resemble spheres if the liquid stays beyond a certain point and forms a shape called
spherical cup (Fig.49).
The spherical free surface of the liquid resembles that of a droplet. According to a
study (H.Y. Erbil, 2012), in the case of stationary evaporation, the rate of mass loss
by evaporation was given by eq. (30),
− (dm/dt) = −4πR2D * (dC/dR) (30)
where where m is the mass (kg), t is the time (s), R is the radial distance from the
center of the droplet (m), D is the diffusion coefficient (m2 /s) and C is the
concentration of the vapor (kg/m3 ).
63
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
If c∞ is the concentration of the drop at infinite distance from the drop, then we have
the following boundary conditions, C = CS when R = RS and C = C∞ when R = ∞, then
Eq. (7) gives the rate of evaporation as:
− (dm/dt) = -4πRSD * (CS−C∞) (31)
where RS is the radius of the spherical droplet from the center to the surface and cS
is the concentration of vapor at the sphere surface (at RS distance). The vapor
concentration will be equal to 0 at infinite distance (C∞=0) and Eq. (31) becomes:
−ρL * (dV/ dt) = 4πRS D CS (32)
where V is the volume and ρL is the density of the drop liquid.
If we assume that the vapor is to obey the ideal gas law, then it is possible to express
the vapor concentration, C using the measured vapor pressure of the liquid at this
temperature: C = (M* Pv)/( R' * T) (33)
where M is the molecular weight, Pv is the vapor pressure of the evaporating liquid,
R' is the gas constant and T is the absolute temperature in Kelvins. When we
combine eq. 9 and 10, we get the Langmuir equation:
- (dm/dt) = 4*π*M*D*PSV*RS/ (R' * T) (34)
where PSV is the water vapor pressure at the drop surface which is equal to the
saturated water vapor pressure of the medium, (PSv = PO
VS) according to Maxwell
assumption if we neglect the cooling of the drop surface.
Regarding to evaporating of water drops in a humid medium, (CS− C∞) term in Eq.
(30) can be expressed from Eq. (34) as:
(CS− C∞) = M * (PSv - PO
V)/ (R * T) (35)
where POV is the vapor pressure at Ro distance (the actual water vapor pressure of
the medium). The relative humidity (RH) in the medium is defined as:
RH = POV/ PO
VS (36)
By combining Eqs. (31) and (32), and considering PSv = PO
VS by neglecting the cooling
of the drop surface, we obtain:
(CS− C∞) = (M * POVS)*(1-RH) / (R * T) (37)
When Eqs. (30) and (37) are combined, the rate of water drop evaporation in a
humid medium can be expressed as:
−ρL * (dV/ dt) = 4πRS D * (M * POVS)*(1-RH) / (R * T) (38)
64
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.49, Geometry of the spherical cup, (wikipedia.org)
As both seen in Eqs. (31) and (38), the rate of evaporation is proportional to the
radius of the spherical drop. Thus, large drops evaporate faster than small drops.
However, strictly speaking, the evaporation of a drop cannot be a stationary process
since the radius and hence the rate of evaporation is constantly decreasing during
the evaporation.
As far as the spherical cup is concerned, its volume and the curved free surface are
given respectively by equations (39) and (40) respectively,
VSph.Cup = π * h * (3RSC2 + h2)/6 or π * h2 * (3r - h)/3 (41)
A Sph.Cup = 2π * r * h (42)
where RSC is the radius of the base , h is the height of the cap and r is given by:
r2 = (r - h)2 + RSC 2 = r2 -2r*h +h2 + RSC
2 = (RSC 2 + h2) /2h (43)
Substituting this into the area formula gives:
A Sph.Cup = π * (RSC 2 + h2) (44)
By geometry the mass of the spherical cup with respect to the radius of a sphere is:
pL = m/ V => dm = pL * dV = pL *d(π * h * (3 RSC 2 + h2)/6) =pL * π * h * d(3 RSC
2/6 +
h2/6)= pL * π * h * (2* RSC/2 + 0) = pL * π * h * RSC =>
dmSph.Cup = pL * π * h * RSC* dRSC (45)
By applying eq. (21) to (7) we get for the cases under air:
RSC
65
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Table 8, Boiling point of water at various pressures, Data are based on the equation of state recommended by the
International Association for the Properties of Steam in 1984, as presented in Haar, Gallagher, and Kell, NBS-NRC Steam
Tables (Hemisphere Publishing Corp., New York, 1984). The temperature scale is IPTS-68. Note that: 1 mbar = 100 Pa =
0.000986923 atmos = 0.750062 mmHg.
-(dRSC/dt) = 4π * RSC*D*(Cs-Co)/ (pL * π * h * RSC) = 4 * D * (Cs - Co)/ (pL * h)
=>(dRSC2=2RSC*dRSC)=> - dRSC
2/dt = 2 * D * (Cs-Co)/ (pL * h * Ro,SC) =>
Ri,SC2 - Ro,SC
2 = 2 * D * (Cs-Co)/ (pL * h * Ro,SC) (46)
By being able to calculate the reduction of the liquid mass at each point of the
evaporation/distillation process we can adjust the energy we need to boil the
remaining bulk feed, thus making the process more financially efficient.
4.2 Effect of Vacuum Pressure in the Evaporation Process
After trying out different volume combinations, I wanted to apply a vacuum
pressure with the pump. The idea behind this is that by dropping the pressure, the
boiling point of the sludge goes down as well. For water and roughly for its solutions
the boiling point according to pressure is illustrated in Table 8,
In general, pumping costs are less than heating costs and vacuum pressure decreases
the operating time, thus decreasing the running costs as well. What the author
wants to research is, if by applying a vacuum pressure, we also get a beneficial result
in the TN extraction from the HH WWTP sludge.
66
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
4.3 Adding a Hydrophobic Membrane-Contact angle (Onsekizoglu, 2012)
In order to benefit from the MD method, the author added a hydrophobic
membrane in the experiment system. The main factor that defines the
hydrophobicity of a material is its contact angle.
The contact angle is a common measurement of the hydrophobic or hydrophilic
behaviour of a material. It provides information about relative wettability of
membranes. The contact angle is determined as the angle between the surface of
the wetted solid and a line tangent to the curved surface of the drop at the point of
three-phase contact (Figure 50). The value of contact angle is greater than 90° when
there is low affinity between liquid and solid; in case of water, the material is
considered hydrophobic and is less than 90° in the case of high affinity. Wetting
occurs at 0°, when the liquid spreads onto the surface. The wettability of a solid
surface by a liquid decreases as the contact angle increases.
Fig.50, Schematic representation of contact angle, (Onsekizoglu, 2012)
5. Experiment Material and Methods
As we explained analytically in chapter 4.1 the area of the free surface that the
evaporation takes place is a dominant factor on the speed of the process. With that
in mind we tried a number of combinations in the lab between the size of the flask
and the volume of the sample to find the optimum analogy that yields the highest
value of total ammonia extraction.
5.1 Experiment Equipment Setup
The rotary evaporator is a laboratory instrument that we use to evaporate a
solution, condensate the vapours and distill them in the sample flask. In our
experiment we performed sequential evaporation of sludge samples collected after
anaerobic digestion. The evaporator was connected to a pump to create the vacuum
necessary later on in the experiment.
The equipment came from Heidolph Instruments GmbH & Co. The evaporator was a
Hei-VAP model with Glassware set G1 and the pump, a ROTAVAC pumping unit. An
overview of the experiment setup can be seen in Fig.51&52
67
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.51, Overview of a Hei-VAP model (left) with 1) Drive unit with vapor tube and coupling clamp; 2) Evaporator flask; 3) Heating
bath; 4) Base unit duct; 5) Control panel; 6) Receiving flask; 7) Condenser. On the right we have the ROTAVAC pumping unit with
1) Housing cover; 2) Valve; 3) Head cover; 4) Diaphragm clamping disc with connecting screw; 5) Diaphragm; 6) Diaphragm
support disc; 7) Washers; 8) Rod; 9) Housing; 10) Bearing plate, (Heidolph Instruments GmbH & Co.KG)
Fig.52, Photograph of the experiment setup in the ISAH Labs in Appelstr. Hannover with the rotary evaporator Hei-VAP model on
the left and the ROTAVAC pumping unit on the right, 2015
68
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
5.2 Initial Operating Conditions
From the results of a BSc Thesis (Krentz, 2015), the optimal temperature and
pressure combination that gives the highest amount of NH4+ extraction, for the
sequential evaporation of digested sludge, are: 1)Temperature: 110 - 120°C;
2)Pressure: 860 - 1013 mbar; 3)Rotation speed: 10 rpm. Using this parameters, the
total NH4-N content in the condensation was 89.29%. The atmospheric pressure of
Hannover is circa. 1013 mbar so we kept the pump working with it for the first part
of the experiment.
The sludge we used initially came from the Herrenhäuser WWTP that started co-
digestion in 2000. It treats sewage sludge, bio waste, leftovers and agricultural
waste. The organic to sewage ratio is 10:90. The organic part represents 135,000
ton/year.
The procedure of each experiment from was:
1) Open the evaporation equipment and set the operating parameters. Then take
out the sample from the fridge, pour it into the evaporating flask and let it get
heated. The initial low temperature of the sample created a time lag which can
generally be avoided with preheating.
2) For the first and second phase of the experiment we collected 50 ml samples in
sequence from the evaporating medium.
3) After each sample was collected, the pH and the Ks were measured. For the pH we
used an electrode based equipment and for calculating the Ks, we used 10ml of the
sample mixed with 90ml of distilled water. In the latter solution we added four drops
of methyl orange and add 0.1 mol/L HCl, until the color changes from yellow to
orange. The amount of ml we used, stated the Ks value by the following eq.,
Ks (mmol/L) = (V1 *CHCl * 1000)/V2 (47)
where V1 = vol.of the used HCl in ml, CHCl is the volume of the sample in mol/L and
V2 is the volume of the sample in ml. Considering the amounts we use, eq. (47) can
be simplified for this experiment into,
Ks (mmol/L) = V1 (ml) * 0,1 (mol/L) *1000/ 100 (ml) = V1 (mmol/L) (48)
After the pH and Ks calculations, each sample was put in freeze storage.
4) After the needed samples were collected, the remaining sludge was collected,
filtered and measured for pH and Ks and stored with the same procedures as the
previous ones. The samples were then sent to the main ISAH labs were they were
tested with Cuvette tests to measure the NH4-N concentration.
69
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
5.3 Problems with the Experimental Procedure
1) Collecting specific distillation volumes proved to be a strenuous task due to the
absence of volume graduation in the receiving flask. It was difficult to find another
one since we needed a flask with the exact bottleneck proportions and thickness to
withstand a possible vacuum pressure.
2) Cuvette tests are not completely accurate. The tests are appreciated more for
their speed rather than the level of their accuracy. Expect always a 10% deviation
from their given value.
6. Results
6.1 First Phase of the Experiment
For the first phase of the experiment the following Sample Volume (ml)/
Evaporating Flask Volume (ml) were tested, 300/5000, 500/5000, 300/3000,
500/3000 and 300/1000. The main results can be seen in Fig. 53-56
As reported before the pH and Ks values were measured in the experiment lab and
the NH4-N concentration values in the main ISAH lab. From the pH and NH4-N values
we measured the corresponding NH3 values using eq.(2)
NH3 + H2O ↔ NH4+ + OH- (K1 →, K2 ←)
So the concentration of ammonia in each case is,
CNH3 = CNH4-N * COH- / K1 (49), or
CNH3 = CNH4-N *10^(-(14-pH))/(1.8*10^-5) (50)
When both concentrations were measured and calculated, they were multiplied with
their corresponding sample volume and we got the mg of each nitrogen component.
Since the procedure becomes beneficial when we take the highest amount of total
nitrogen (TN) extraction with the least amount of volume, I have named total
nitrogen extraction efficiency the amount,
TNE eff. = (ΣNH4-Ni/ΝΗ4-Nο (‰) + ΣNH3/ΝΗ3ο (‰))/ (ΣVdist/Vsample (‰)) (51)
According to the latter efficiency and the accumulative rate of the NH4-N and NH3
extraction, we chose the best possible combination.
Time in our experiment isn't a crucial factor because the machinery and methods
used in the industry vary wildly from our laboratory equipment but we present the
values in the graphs for future references.
70
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.53, Comparative graph from the combinations of the first phase with the total experiment time (ΣΤi), the removed volume
from the sample (ΣVdist/Vsample (‰)), the total nitrogen extraction efficiency (TNE eff.) and the ammonium extraction efficiency
(NH4 extr. eff.)
Fig.54, Comparative graph of the solubility (Ks) of each combination in the first phase of the experiment
71
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.55, Comparative graph of the pH of each combination in the first phase of the experiment
Fig.56, Comparative graph of the accumulative extraction of NH4-N of each combination in the first phase of the experiment
72
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
As we can see from Fig.53 and 56 the 300/5000 combination yielded the best results
by having the highest TNE eff. (the analogies are the same for the ammonium
extraction efficiency) and the highest NH4-N extraction percentage in comparison to
the rest of the combinations (we used the NH4-N graph because it was delivered
straight from the lab readings instead of the theoretical approach for the NH3
content) . This is more or less expected since the larger free surface of the sludge
allows for more ammonia to be evaporated faster. So at that point we chose to carry
on with the 5000ml flask for the next phase and investigate it further by using the
400/5000, 700/5000 and 1000/5000 combinations.
From Fig. 54 we can see that in each individual case, the solubility is dropping in
accordance with the theory that explains that as the temperature of the aqua
ammonia keeps rising, its solubility drops rapidly.
In literature pH during ammonia extraction, the pH in the distillation samples drops
and also took place in the Krentz BSc thesis (Krentz, 2015) and the MSc thesis
submitted by Yara Montenegro (Montenegro, 2015). In Fig. 55 almost all the cases
reaffirmed this theory with the exception of the 300/5000, 500/5000 which was
initially allocated to the need of recalibration for the pH electrode. Later on in the
chapter, more will be researched about this abnormal behaviour.
The analytical presentation of the data and graphs from the first phase, is presented
in the Appendix, at the end of the Thesis booklet.
6.2 Second Phase of the Experiment
For the second phase of the experiment the following combinations were tested and
compared with the same operating conditions and samples from the same source
(HH WWTP); 300/5000, 500/5000, 400/5000, 700/5000, 1000/5000. The main
results can be seen in Figures 57-60.
73
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.58, Comparative graph from the combinations of the second phase with the total experiment time (ΣΤi), the removed
volume from the sample (ΣVdist/Vsample (‰)), the total nitrogen extraction efficiency (TNE eff.) and the ammonium
extraction efficiency (NH4 extr. eff.)
Fig.57, Comparative graph of the accumulative extraction of NH4-N of each combination in the second phase of the
experiment.
74
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.59, Comparative graph of the solubility (Ks) of each combination in the second phase of the experiment
Fig. 60, Comparative graph of the pH of each combination in the second phase of the experiment
75
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
As we can see from Fig.57 and 58 the 300/5000 combination yielded the best results
by having a high TNE eff. and the by far the highest NH4-N extraction percentage in
comparison to the rest of the combinations. So at that point we chose to carry on
with that combination for the next phase and investigate it further by applying a
vacuum pressure.
From Fig. 59 we can see that in each individual case, the solubility is dropping in
accordance with the theory that explains that as the temperature of the aqua
ammonia keeps rising, its solubility drops rapidly.
In Fig. 60 the general trend was a decreasing pH graph and raising a bit at the end
with the exception of the 1000/5000 which kept rising contrary to our expectations
and at that time it was thought that the pH electrode needed recalibrating again.
The analytical presentation of the data and graphs from the first phase, is presented
in the Appendix, at the end of the Thesis booklet.
6.3 Third Phase of the Experiment
For the third phase of the experiment we wanted to apply a vacuum pressure with
the pump.
The combination we chose from the second phase was 300/5000 but unfortunately
we had to go on with the 3000ml evaporating flask because of an accident in the lab
during which the 5000ml flask broke. We chose to keep the 500/5000 proportions by
going on with the 300/3000 combination and taking 10% of the volume in each
distillation sample (so the distillation samples changed from 50ml to 30ml which are
enough for the Ks and the Cuvette tests). A further accident with the power voltage
and the breaking down of the fridge where the samples were stored was the cause
of the loss of the 300/3000 (700mbar) combination results. Due to the lack of time
we couldn't repeat the experiment but the rest of the combinations used (500mbar,
300mbar, 200mbar and 100mbar) were enough to check the general trends (Fig.61-
64).
76
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.61, Comparative graph from the combinations of the third phase with the total experiment time (ΣΤi), the removed
volume from the sample (ΣVdist/Vsample (‰)), the total nitrogen extraction efficiency (TNE eff.) and the ammonium
extraction efficiency (NH4 extr. eff.)
Fig.62, Comparative graph of the accumulative extraction of NH4-N of each combination in the third phase of the
experiment.
77
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.63, Comparative graph of the solubility (Ks) of each combination in the third phase of the experiment
Fig.64, Comparative graph of the pH of each combination in the third phase of the experiment
78
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
As we can see from Fig.61 and 62 the 300/3000 (500mbar) combination yielded the
best results by having the second highest TNE eff. and the by far the highest NH4-N
extraction percentage in comparison to the rest of the combinations. So at that point
we chose to carry on with that combination for the next phase and investigate it
further by applying a hydrophobic membrane.
From Fig. 63 we can see that in each individual case, the solubility is dropping in
accordance with the theory that explains that as the temperature of the aqua
ammonia keeps rising, its solubility drops rapidly.
In Fig. 64 the general trend was an increasing pH graph and raising with the
exception of 300/3000 (atm. pressure). At this point we asked the main ISAH labs to
re-measure the pH with their pH electrode and the results had the same rising
trend. We can't explain why the pH had this abnormal behaviour contrary to what
we read in the literature but the author's personal opinion is that the fast extraction
of volatile ammonia gas molecules moved the ammonia equilibrium in the slugde, to
the side of ammonia production, thus the continuous rising pH in the distillation
sample.
The analytical presentation of the data and graphs from the first phase, is presented
in the Appendix, at the end of the Thesis booklet
6.4 Fourth Phase of the Experiment
For the fourth phase, the sample source instead of the HH WWTP, was the
Braunschweig Biogas Digestate (BS AD) in Steinhof. The digestate sample was
collected from the outlet digester. After researching the effects of the flask volume,
temperature and pressure in the previous phases, for this part of the experiment, we
decided to try out the 300/3000 (500mbar) combination for the new source sample,
with and without a hydrophobic membrane. The applied membrane was a MEMSYS
patented PTFE membrane with a pore size ~ 0.2 mm and a contact angle of 123o.
Problems with the experimental procedure
During the experiment we faced problems with placing the membrane in the system.
The most obvious place for the membrane was the bottleneck of the evaporating
flask (Fig.65) but this did not let the bottleneck and the inner tube to complete apply
to each other, thus creating a circa. 6mbar pressure leak which immediately became
detectable by the foul odour in the laboratory. In order to correct that we applied a
thermoplastic tape to minimize the leak. The result was a ±0.3 mbar pressure leak
that allowed us to carry on with the experiment.
Another problem was that during the collection of the samples we had to close the
pump and depressurize the system in order to take the distillation flask. During that
79
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.65, Applying the hydrophobic MEMSYS PTFE membrane in the space between the bottleneck of the evaporating flask and the
inner tube of the distillator.
Fig.66, Comparative graph
from the combinations of the
third phase with the total
experiment time (ΣΤi), the
removed volume from the
sample (ΣVdist/Vsample (‰)),
the total nitrogen extraction
efficiency (TNE eff.) and the
ammonium extraction
efficiency (NH4 extr. eff.)
procedure, the released pressure found the weakest spot of the membrane and
pulled it back, spraying the water that had condensated on the outer side of the
membrane, back in the bulk feed. We considered that as soon as the vacuum
pressure was back on, the membrane was pushed in its position and the vapours
were filtered with no leakage.
So the expectations of using a membrane was that it would hold the water
molecules back allowing for the NH3 gas molecule to pass through and have a higher
concentration in the distillation samples. The main results of the experiment can be
described in the Fig. 66-70.
80
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.67, Comparative graph of the accumulative extraction of NH4-N of each combination in the third phase of the
experiment.
Fig.68, Comparative graph of the accumulative extraction of NH4-N of each combination in the third phase of the
experiment.
81
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.69, Comparative graph of the solubility (Ks) of each combination in the third phase of the experiment
Fig.70, Comparative graph of the pH of each combination in the third phase of the experiment
82
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
As we can see from Fig.66-68 the 300/3000 (500mbar) membrane combination
yielded the best results by having the highest TNE eff. and the by far the highest NH3
extraction percentage. Of course the ammonia content was measured by the
theoretical approach that we explained earlier in the chapter but the explanation of
a higher ammonia content explains as well the lower ammonium content in the
samples. Applying the membrane has kept the water back and allowed the ease
mass transfer of the ammonia molecules, which sped up the process of the ammonia
creation in the sludge by the ammonia equilibrium. So definitely using a hydrophobic
membrane has better results and we chose this as our final suggestion.
From Fig.69 we can see that in each individual case, the solubility is dropping in
accordance with the theory that explains that as the temperature of the aqua
ammonia keeps rising, its solubility drops rapidly.
In Fig.70 the general trend was an increasing pH graph. The main ISAH labs were
asked again to re-measure the pH with their pH electrode and the results had the
same rising trend. The author's personal opinion remains the same, that the fast
extraction of volatile ammonia gas molecules moved the ammonia equilibrium in the
slugde, to the side of ammonia production, thus the continuous rising pH in the
distillation sample.
The analytical presentation of the data and graphs from the first phase, is presented
in the Appendix, at the end of the Thesis booklet.
6.5 Experiment Conclusions
This experiment yielded some interested results that need to be discussed:
1) As we expected, by looking the graphs for phases one and two, a larger free
surface for the evaporation of the sludge/digestate offers for better and faster
results for the recovery of ammonia. The problem with using a spherical flask is that
the free surface keeps shrinking due to the loss of mass. From one point with the
loss of mass you can adjust the given heating energy to lower levels, thus making the
process more financially efficient. On the other hand, smaller free surface means a
time lag at the later stages of the evaporation. Of course this problem isn't an issue
by using an orthogonical evaporating tank in the industry. With prior calculations
and the use of weight sensors, the energy needed (depending on the type of the
treated liquid), can be reduced sequencely and offer significant energy savings.
2) By applying vacuum pressure, we dropped the boiling point and the needed
temperature as well. The pH results though, were troubling due to the general trend
of the pH rising instead of dropping and the 500mbar experiment having a 7.85‰
NH4-N extraction rate in comparison to the 5.97‰ of the atmospheric pressure (see
Appendix-Third Phase). So from one side we had an increasing pH and on the other
83
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.71, Comparison of the solubility of ammonia (left) and carbon dioxide (right), (engineeringtoolbox.com)
side an increasing ammonium content. Although we had not dealt with such a case
in the literature and in the previous Thesis (Krentz, 2015 & Montenegro, 2015) we
consulted, the author's personal opinion is that the fast extraction of the volatile
ammonia gas molecule raises their concentration in the distillate sample.
In order to comprehend the phenomenon we need to understand what kind of
substances we have to deal with. From the boiling of the digestate/sludge, we get
water vapours, NH3 gas and CO2 gas. So in the distillate sample we have a mixture of
water, ammonia, carbon dioxide and oxygen. The ammonia and carbon dioxide react
together giving ammonia carbamate, according to the process described in chapter
3.17. The problem is that due to the fact that ammonia has a much higher solubility
in water than carbon dioxide (Fig. 71), there's always an excess of NH3 that is
dissolved into ammonium and OH- anions. Therefore the distillate solution becomes
more and more alkaline.
3) Using the MEMSYS hydrophobic membrane with the biogas digestate instead of
the WWTP sludge increased the recovery rate even further by having a TNE eff. of
95.87‰ to the 47.85‰ (the latter is almost the same percentage of recovery as the
sludge). Of course these numbers are valid if we choose to accept the same
calculations as the ones in the previous steps. If we do, then the use of the
membrane was a significant boost to the procedure by doubling its effect. For the
sake of accuracy though due to the problems we had with placing the membrane
and the difficulties that arose with its slight displacement with turning the pump on
and off in order to collect the distillation sample, the author's personal opinion is
that we should redesign a system in which we could either collect the samples
without affecting the operation of the pump or place the membrane tight enough so
that it's not distorted by the pressure changes.
84
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
4) After all the phases of the experiment the suggested combination would be 300ml
sample, 5000ml flask at 500mbar with membrane. We strongly suggest that the
experiment should be repeated with this combination for the pressure changes
together with the membrane, for the sake of accuracy and to include the 700mbar
pressure that due to the lack of time couldn't be performed by the author.
7. Thesis Conclusions
Nutrient pollution is one of the fundamental environmental problems of our age.
The legal constricts are getting tighter, thus, nutrient recovery processing is more
important than ever.
The existing technologies have their own individual problems ranging from high
energy requirements, high capital investment, need for gas flue treatment,
production of explosive gases, acidic products, loss of nutrient potential,
environmental impact of solid byproducts, low or high DS content. Modeling with
real time regional data becomes a basic need in order to decide in every case, what
method suits most the particular local needs. The same method's results can vary
wildly in different cases.
The products of the nutrient recovery are usually a concentrated solution of aqua
ammonia that needs to be handled. Usually in the industry there are methods using
acids, thus making the process more expensive. There are though alternative
methods that use the aqua ammonia as a way to capture the CO2 and SOX flue gases
from the industry and converting them to ammonia carbonate, urea and ammonium
sulfate. So from one side we get a dangerous effluent reduction and environmental
protection and on the other we get marketable products that are much needed in
the agricultural and chemical industry.
In order to decrease the transportation and storage costs of the concentrated
distillate product of the sludge/digestate processing methods, there's the need for a
method with a higher concentration of the final product in order to reduce the
transportation and storage costs. An alternative route can be membrane distillation,
which has proved its worth already in the desalination field and due to its laboratory
results with nutrient recovery, is investigated more and more. The industrial
application of MD is closer than we think.
An interesting method could also be sequential evaporation which was tested in this
Thesis and proved to have excellent results as the free surface of the feed became
larger along with a vacuum pressure of around 500mbar. Not only did we get better
results than with normal pressure but there are two potential energy saving factors,
1) Lowering the pressure, drops the boiling point of the feed, thus needing less
energy to boil. Pumping costs are lower than heating costs; 2) Adjusting the heating
85
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
according to the reduction of the mass in the evaporation tank, which can be pre-
calculated and supported by weight sensors.
Adding a hydrophobic MEMSYS PTFE membrane to the process increased
dramatically the efficiency of the process, thus making it an ideal coupling choice.
The interesting thing considering the process was the general trend of pH to keep
rising in contrast to previous experiments in the literature or by Leibniz students. The
author's personal opinion is that, due to the fast extraction of volatile ammonia
molecules and the much higher solubility of ammonia in comparison to carbon
dioxide, we have an over-abundance of NH3 in the distillate solution which can't
completely react with CO2, giving us ammonium carbamate and acting as a buffer to
the distillate. The ammonia dissolving into NH4+ cations and OH- anions, gives us a
higher pH. There's a definite need to look further into this abnormal behaviour with
further experimentation and a different experimental setup which will suit better the
application of a membrane in the system.
86
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
8. References
W. Fuchs and B. Drosg (2010). Technologiebewertung von Gärrestbehandlungs- und
Verwertungskonzepten, Eigenverlag der Universitat fόr Bodenkultur Wien; ISBN: 978-
3-900962-86-9
Caroline H. Orr, Angela James, Carlo Leifert, Julia M. Cooper and Stephen P.
Cummings, (2011). Diversity and Activity of Free-Living Nitrogen-Fixing Bacteria and
Total Bacteria in Organic and Conventionally Managed Soils, Appl. Environ.
Microbiol. February 2011 vol. 77 no. 3 911-919
M. Madsen, J.B. Holm-Nielsen and K.H. Esbensen (2011). Monitoring of anaerobic
digestion processes: A review perspective. Renewable and Sustainable Energy
Reviews, 15(6), 3141-3155.
J. Holm-Nielsen, N. Halberg, S. Hutingford and T. Al Seadi (1997). Joint Biogas Plants.
Agricultural advantages-circulation of N, P, and K. Report made for the Danish
Energy Agency. Second Edition, August 1997.
M.H. Chantigny, D. Angers, P. Rochette and G. Belanger, (2007). Gaseous nitrogen
emissions and forage Qual. 2007(36), 1864-1872.
K. Muller, W. Stinner, A. Deuker and G. Leithold, (2008). Effects of different manuring
systems with and without biogas digestion on nitrogen cycle and crop yield in mixed
organic dairy farming systems. Nutr. Cycl. Agroecosys., 82, 209– 232.
F. Tambone, B. Scaglia, G. D'Imporzano, A. Schievano, V. Orzi, S. Salati and F. Adani,
(2010). Assessing amendment and fertilizing properties of digestates from anaerobic
digestion through a comparative study with digested sludge and compost.
Chemosphere 2010 (81), 577–583.
S. Fouda (2011). Nitrogen availability of biogas residues. Ph.D. Thesis, Technische
Universitat Munchen, 2011.
ARBOR ( 2013). Inventory: Techniques for nutrient recovery from digestate. Available
from: http://arbornwe.eu/downloads (www. arbornwe.eu)
WRAP (2012). Enhancement and treatment of digestates from anaerobic digestion.
Available from: http://www.wrap.org.uk/node/13207
BIOTECVISIONS, (2012). January. Biotechnology Journal, 7:n/a.doi:
10.1002/biot.201100482.
Hjorth, K.V. Christensen, M.L. Christensen and S.G. Sommer, (2010). Solid–liquid
separation of animal slurry in theory and practice. A review. Agron. Sustain. Dev.,
2010(30), 153– 180.
87
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Galloway, J., Dentener, F.J., Capone, D.G., Boyer, E.W., Howarth, R.W., Seitzinger,
S.P., Aser, G.P., Cleveland, C., Green, P., Holland, E., Karl, D.M., Michaels, A.F., Porter,
J.H. Townsend, A. and Vorosmarty, C. (2004). Nitrogen cycles: past, present and
future. Biogeochemistry. 70: 153-156.
Sutton, M.A., Reis, S., and Bahl, K.B. (2009). Reactive nitrogen in agroecosystems:
Integration with greenhouse gas interactions. Agriculture, Ecosystems &
Environment, 133(3), 135- 138.
Wood, S., and Cowie, A. (2004). A review of greenhouse gas emission factors for
fertiliser production. In IEA bioenergy task (Vol. 38, p. 20).
Arun S. Mujumdar, (2015). Handbook of Industrial Drying, Fourth Edition.
ISBN 9781466596658 - CAT# K20788
Bauer A., Mayr H., Hopfner-Sixt K. and Amon T., (2009). Detailed monitoring of two
biogas plants and mechanical solid–liquid separation of fermentation residues,
Journal of Biotechnology 142, 56-63.
R. Resch C., Braun R. and Kirchmayr R., (2008). The influence of energy crop
substrates on the mass-flow analysis and the residual methane potential at a rural
anaerobic digestion plant. Water Science and Technology, 57(1), 73-81.
A. Fakhru'l-Razi A.,(1994). Ultrafiltration membrane separation for anaerobic
wastewater treatment, Water Science and Technology, 30(12), 321-327.
Diltz R.A., Marolla T.V. and Henley M.V. (2007). Reverse osmosis processing of
organic model compounds and fermentation broths, Bioresource Technology 98(3),
686-695.
Castelblanque J. and Salimbeni F. (1999). Application of membrane systems for COD
removal and reuse of waste water from anaerobic digestors, Desalination 126(1-3),
293-300.
Camarero L., Diaz J.M. and Romero F., (1996). Final treatments for anaerobically
digested piggery slurry effluents, Biomass and Bioenergy 11(6), 483-489
Sanchez E., Milan Z. Borja R., Weiland P. and Rodriguez X. (1995). Piggery waste
treatment by anaerobic digestion and nutrient removal by ionic exchange, Resources,
Conservation and Recycling, 15(3-4), 235-244.
Uludag-Demirer S., Demirer G.N. and Chen S.(2005). Ammonia removal from
anaerobically digested dairy manure by struvite precipitation. Process Biochemistry
40(12), 3667-3674.
88
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Cleverson Vitorio Andreoli, Marcos von Sperling and Fernando Fernandes, (2007).
Sludge Treatment and Disposal. 2007 IWA Publishing
George Tchobanoglous, Franklin L. Burton, H. David Stensel, (2003), Wastewater
Engineering: Treatment and Reuse 4th Edition, ISBN-13: 978-0070418783
Izrail S. Turovskiy, P. K. Mathai, (2006). Wastewater Sludge Processing, ISBN: 978-0-
471-70054-8
Frischman, (2012). Enhancement and treatment of digestates from anaerobic digestion. www.wrap.org.uk
Bauermeister U., Wild A. and Meier T., (2009). Stickstoffabtrennung mit dem
ANAstrip-Verfahren System GNS (Nitrogen removal by the ANAstrip process system
GNS), Gόlzower Fachgespraeche, Band 30: Gaerrestaufbereitung fur eine pflanzliche
Nutzung – Stand und F&E Bedarf, pp. 78-96.
Zongli Xie, Tuan Duong, Manh Hoang, Cuong Nguyen, Brian Bolto, (2009). Ammonia
removal by sweep gas membrane distillation. Water Research (Impact Factor: 5.53).
02/2009; 43(6):1693-9
A. Zarebska, D. Romero Nieto, K. V. Christensen, L. Fjerbæk Søtoft & B. Norddahl,
(2015). Ammonium Fertilizers Production from Manure: A Critical Review. Critical
Reviews in Environmental Science and Technology, Volume 45, Issue 14, 2015
Enrico Drioli, Aamer Ali, Francesca Macedonio, (2015). Membrane distillation:
Recent developments and perspectives. Water Research (Impact Factor: 5.53).
02/2009; 43(6):1693-9.
Hyun-Chul Kim, Jaewon Shin, Seyeon Won, Jung-Yeol Lee, Sung Kyu Maeng, Kyung
Guen Song, (2015). Membrane distillation combined with an anaerobic moving bed
biofilm reactor for treating municipal wastewater. Water Research (Impact Factor:
5.53). 03/2015; 71. DOI: 10.1016/j.watres.2014.12.048
Pelin Onsekizoglu, (2012). Distillation - Advances from Modeling to Applications.
ISBN 978-953-51-0428-5
Alaa Kullab , (2011). Using Membrane Distillation Experimental and Numerical
Study. Doctoral Thesis; Royal Institute of Technology SE-100 44 STOCKHOLM
Dan Qu, Diyang Sun, HongjieWang, Yanbin Yun, (2013). Experimental study of
ammonia removal from water by modified direct contact membrane distillation.
Desalination; Volume 326, 1 October 2013, Pages 135–140
89
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
M.C. García-Gonzalez, M.B. Vanotti, A.A. Szogi, (2015). Recovery of ammonia from
swine manure using gas-permeable membranes: Effect of aeration. Journal of
Environmental Management Volume 152, 1 April 2015, Pages 19–26
Simon Krentz, (2015). Parametrierung der sequentiellen Eindampfung von
ammoniumhaltigen Wässern am Beispiel von Faulschlamm. Bachelorarbeit Leibniz
Universität Hannover Fakultät für Bauingenieurwesen und Geodäsie, Institut für
Siedlungswasserwirtschaft und Abfalltechnik
Yara Alejandra Montenegro Pinto, (2015). Co-digestion of sewage sludge with the
organic waste and its ammonia recovery potential by thermal evaporation. Master
Thesis, Institute for Sanitary Engineering and Waste Management, Leibniz University,
Hannover
EFMA, European Fertilizer Manufacturers’ Association, (2000). Best Available
Techniques for Pollution Prevention and Control in the European Fertilizer Industry;
Production of Urea
Qiao He, Minggong Chen, Lingyu Meng, Kunlei Liu and Wei-Ping Pan (2015). Study on
Carbon Dioxide Removal from Flue Gas by Absorption of Aqueous Ammonia. Institute
for Combustion Science and Environmental Technology Western Kentucky University
SOH KG. 2006. A review of the global fertilizer use by product. espere.mpch
THORUP RM (ed.). 1984. Ortho Agronomy Handbook: A Practical Guide to Soil
Fertility and Fertilizer Use. San Francisco, CA: Chevron Chemical Company. 454 p.
Mei-In M. Choua, Joshua A. Bruiniusb, Vincent Benigb, Sheng-Fu J. Choub & Ronald
H. Cartyc, (2005). Producing Ammonium Sulfate from Flue Gas Desulfurization By-
Products. Energy Sources; Volume 27, Issue 11
90
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Figures Index
Fig. 1, Four major steps of the AD Process, (Madsen et al., 2011) page 1
Fig. 2, Examples of the variation of nitrogen in the digestate of biogas plants with different
substrate types, (a)TN concentration in kg/ton fresh matter (FM), (b) ammonium nitrogen as
percentage of TN. Horizontally striped columns indicate digestate from mono-digestion of industrial
by-products; and unstriped columns indicate digestate from typical waste treatment plants (Fuchs
and Drosg, 2010) page 5
Fig. 3, Water distribution in sludge, (Arun S. Mujumdar, 2015) page 7
Fig. 4, Distribution of the main components after the solid-liquid fractions seperation (Bauer et al.,
2009) page 8
Fig. 5, Overview of viable options for digestate processing (Fuchs and Drosg, 2013) page 10
Fig. 6, Overview of the distribution of industrial scale applications for further treatment of the
liquid fraction of digestate in Germany , Austria and Switzerland from 2009 (Fuchs and Drosg, 2013)
page 10
Fig. 7, Principles of drying processes, drying by convection (left) and drying by contact (right) (Fuchs
and Drosg, 2010) page 12
Fig. 8, Schematic flow of a Rotary Dryer (Frischman, 2012) page 13
Fig. 9, Schematic flow of a Belt Dryer (Frischman, 2012) page 14
Fig. 10, Schematic flow of a Solar Dryer (Frischman, 2012) page 15
Fig. 11, Design of a Flash Drying System (Mujumdar, 2015) page 16
Fig. 12, Schematic flow of Horizontal indirect dryer system (Mujumdar, 2015) page 17
Fig. 13, Vertical indirect dryer by Pelletec (Mujumdar, 2015) page 18
Fig. 14, Schematic flow of a Surface Scraped Heat exchanger (Frischman, 2012) page 19
Fig. 15, Schematic flow of Gasification (Frischman, 2012) page 20
Fig. 16, Flowchart of multiple hearth incinceration (Turovsky & Mathai, 2012) page 23
Fig. 17, Flowchart of fluidized bed incineration (Turovskiy &Mathai , 2006) page 24
Fig. 18, Flowchart of Pyreg (Frischman, 2012) page 24
Fig. 19, Conventional wet air oxidation system with a vertical reactor (Andreoli et al., 2007)
page 26
Fig. 20, Dependence of the volatility of ammonia in water on temperature and pH (Fuchs & Drosg,
2010) page 27
Fig. 21, Ammonia air stripping including CO2 removal and ammonia recovery by sulphuric acid
scrubbers (Fuchs & Drosg, 2010) page 28
91
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig. 22, Details of a simplified in-vessel stirring process without stripping columns (Bauermeister et
al., 2009) page 28
Fig. 23, Comparison of specific costs for digestate processing at a model biogas plane, (KTBL, 2008)
page 30
Fig. 24, Comparison of cost ranges for specific treatment options versus costs for digestate disposal
(Fuchs &Drosg, 2013) page 30
Fig. 25, Geometric scheme of distance based methods page 32
Fig. 26, Flow diagram from the Digestate Treatment System GNS as example of a system with 2 stripping
reactors, (Bauermeiter et al., 2009) page 33
Fig. 27, Equilibrium Ammonia-Ammonium dependent on pH (Segura, 2012) page 35
Fig. 28, Influence of temperature and pH on the ratio of ammonia partial pressure to total
ammoniacal nitrogen (TAN) concentration (Zarebska et al., 2014) page 36
Fig. 29, Schematic representation of MD configurations (Onsekizoglu, 2012) page 40
Fig.30, Different resistances to heat and mass transfer in (a) DCMD, (b) VMD, (c) AGMD, (d) SGMD (Andreoli et
al., 2007) page 42
Fig.31, Schematic illustration of streams in V-MED module (Andreoli et al., 2007) page 43
Fig.32, Frame and stages used by MEMSYS (i) a simple frame, (ii) single stage consisting of welded frames and
covering plates, (iii) multiple stages (Andreoli et al., 2007) page 43
Fig.33, Variation of feed ammonia concentration in HFMC, DCMD and MDCMD (Dan Q et al., 2013)
page 44
Fig.34, Experimental setup for ammonia removal (Dan Q et al., 2013) page 45
Fig.35, Variation of feed ammonia concentration and average permeate flux at different feed pH (Co=1.5 g/L,
Tf=50oC, Tp=28C, uf=0.5 m/s, up=0.1 m/s), (Dan Q et al., 2013) page 46
Fig.36, Effect pf feed pH on ammonia mass transfer coefficient (Co=1.5 g/L, Tf=50oC, Tp=28oC, uf=0.5 m/s, up=0.1
m/s), (Dan Q et al., 2013) page 47
Fig.37, Variation of feed ammonia concentration and average permeate flux at different feed T (Co=1.5 g/L,
Tf=50oC, Tp=28oC, uf=0.5 m/s, up=0.1 m/s), (Dan Q et al., 2013) page 48
Fig.38, Effect of feed T on ammonia mass transfer coefficient (Co=1.5 g/L, Tf=50oC, Tp=28oC, uf=0.5 m/s, up=0.1
m/s), (Dan Q et al., 2013) page 48
Fig.39, Variation of feed ammonia concentration and average permeate flux at different feed flow rates
(Co=1.5 g/L, Tf=50oC, Tp=28oC, uf=0.5 m/s, up=0.1 m/s), (Dan Q et al., 2013) page 49
Fig.40, Schematic diagram of treatment 1, 2 and 4 showing the recovery of NH3 by the gas-permeable
membrane manifold as it was governed by the balance in Eq.2 that depended on manure pH (M.C. García-
Gonzalez et al., 2015) page 50
Fig.41, Mass of ammonia recovered in the acid concentrator tank for aerated, not aerated and chemically
amended manure treatments. A second order eq. and R2 are represented. The error bars are the standard
deviation of duplicate experiments (M.C. García-Gonzalez et al., 2015) page 50
92
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.42, Experimental set-up for fermentative wastewater treatment with AMBBR and subsequent distillation
using hydrophobic microfilters. The biogas was analyzed in a gas chromatograph (Hyun-Chul Kim et al., 2015)
page 51
Fig.43, (left) Block diagram of a total recycle CO2 urea stripping process, (right) Block diagram of a total recycle
NH3 urea stripping process, (Fertilizers Europe, 2000) page 53
Fig.44, Schematic diagram of the experimental system for studying on removing CO2 gas by ammonium
scrubbing, (Qiao He et al.,) page 57
Fig.45, Schematic diagram of an ammonia based WFGD MET process, (MET, 2008) page 58
Fig.46, Impact of Fuel %S on SO2 Removal for Ammonia versus Limestone, (MET, 2008) page 59
Fig.47, Effect of Molecular Energy and Vapor Pressure on the chance of liquid molecules to evaporate
page 61
Fig.48, Distribution of a liquid inside an evaporating flask, (heidolph-instruments.com) page 62
Fig.49, Geometry of the spherical cup, (wikipedia.org) page 64
Fig.50, Schematic representation of contact angle, (Onsekizoglu, 2012) page 66
Fig.51, Overview of a Hei-VAP model (left) with 1) Drive unit with vapor tube and coupling clamp; 2)
Evaporator flask; 3) Heating bath; 4) Base unit duct; 5) Control panel; 6) Receiving flask; 7) Condenser. On the
right we have the ROTAVAC pumping unit with 1) Housing cover; 2) Valve; 3) Head cover; 4) Diaphragm
clamping disc with connecting screw; 5) Diaphragm; 6) Diaphragm support disc; 7) Washers; 8) Rod; 9)
Housing; 10) Bearing plate, (Heidolph Instruments GmbH & Co.KG) page 67
Fig.52, Photograph of the experiment setup in the ISAH Labs in Appelstr. Hannover with the rotary evaporator
Hei-VAP model on the left and the ROTAVAC pumping unit on the right, 2015 page 67
Fig.53, Comparative graph from the combinations of the first phase with the total experiment time (ΣΤi), the
removed volume from the sample (ΣVdist/Vsample (‰)), the total nitrogen extraction efficiency (TNE eff.) and
the ammonium extraction efficiency (NH4 extr. eff.) page 70
Fig.54, Comparative graph of the solubility (Ks) of each combination in the first phase of the experiment
page 70
Fig.55, Comparative graph of the pH of each combination in the first phase of the experiment page71
Fig.56, Comparative graph of the accumulative extraction of NH4-N of each combination in the first phase of
the experiment page 71
Fig.57, Comparative graph of the accumulative extraction of NH4-N of each combination in the second phase of
the experiment. page 73
Fig.58, Comparative graph from the combinations of the second phase with the total experiment time (ΣΤi),
the removed volume from the sample (ΣVdist/Vsample (‰)), the total nitrogen extraction efficiency (TNE eff.)
and the ammonium extraction efficiency (NH4 extr. eff.) page 73
Fig.59, Comparative graph of the solubility (Ks) of each combination in the second phase of the experiment
page 74
Fig. 60, Comparative graph of the pH of each combination in the second phase of the experiment
page 74
93
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Fig.61, Comparative graph from the combinations of the third phase with the total experiment time (ΣΤi), the
removed volume from the sample (ΣVdist/Vsample (‰)), the total nitrogen extraction efficiency (TNE eff.) and
the ammonium extraction efficiency (NH4 extr. eff.) page 76
Fig.62, Comparative graph of the accumulative extraction of NH4-N of each combination in the third phase of
the experiment. page 76
Fig.63, Comparative graph of the solubility (Ks) of each combination in the third phase of the experiment
page 77
Fig.64, Comparative graph of the pH of each combination in the third phase of the experiment page 77
Fig.65, Applying the hydrophobic MEMSYS PTFE membrane in the space between the bottleneck of the
evaporating flask and the inner tube of the distillator. page 79
Fig.66, Comparative graph from the combinations of the third phase with the total experiment time (ΣΤi), the
removed volume from the sample (ΣVdist/Vsample (‰)), the total nitrogen extraction efficiency (TNE eff.) and
the ammonium extraction efficiency (NH4 extr. eff.) page 79
Fig.67, Comparative graph of the accumulative extraction of NH4-N of each combination in the third phase of
the experiment. page 80
Fig.68, Comparative graph of the accumulative extraction of NH4-N of each combination in the third phase of
the experiment. page 80
Fig.69, Comparative graph of the solubility (Ks) of each combination in the third phase of the experiment
page 81
Fig.70, Comparative graph of the pH of each combination in the third phase of the experiment page 81
Fig.71, Comparison of the solubility of ammonia (left) and carbon dioxide (right), (engineeringtoolbox.com)
page 83
Tables Index
Table 1, Substrate parameters influencing digestate composition, (Fuchs and Drosg, 2010)
page 4
Table 2, Process parameters influencing digestate composition, (Fuchs and Drosg, 2010)
page 4
Table 3, Calorific power of different sewage sludge (Andreoli et al., 2007) page 21
Table 4, Ranking of the available digestate treatment methods according to their individual score in each of the
following categories; 1) Feed Solids, 2) Reliability, 3) Power Usage, 4) Odour, 5) Chemical Usage, 6) Noise, 7)
Hazard (T, P, Chem.), 8) Carbon footprint. 5 is the highest score and 1 the lowest. page 33
Table 5. Specification of hydrophobic microfilters used in the membrane distillation (Hyun-Chul Kim
et al., 2015) page 38
Table 6, Advantages, disadvantages and application areas for MD configurations (Kullab, 2011) page 41
Table 7, Properties of the PVDF membrane used in the experiment (Dan Q et al., 2013) page 45
94
Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015
Table 8, Boiling point of water at various pressures, Data are based on the equation of state
recommended by the International Association for the Properties of Steam in 1984, as presented in
Haar, Gallagher, and Kell, NBS-NRC Steam Tables (Hemisphere Publishing Corp., New York, 1984).
The temperature scale is IPTS-68. Note that: 1 mbar = 100 Pa = 0.000986923 atmos = 0.750062
mmHg. page 65