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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

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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

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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.

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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

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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.

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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

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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.

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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,

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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.

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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

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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.

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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).

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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;

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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).

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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

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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

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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).

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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.

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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

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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

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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.

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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.

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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.

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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)

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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.

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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:

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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

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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.

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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.

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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.

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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.

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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

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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),

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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.

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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.

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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).

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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.

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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.

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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*

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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.

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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%.

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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)

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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

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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.

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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.

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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)

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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)

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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,

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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

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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.

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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)

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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

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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

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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 ).

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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)

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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

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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.

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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

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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

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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.

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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.

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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

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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

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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.

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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.

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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

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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).

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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.

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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

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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

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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.

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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.

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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

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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

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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.

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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

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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.

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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015

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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

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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

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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

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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

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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

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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

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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015