Ammonia Recovery using membrane distillation

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.

2
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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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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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 CO3
2−
and 2 H3O+
to CO2 and H2O, result in increased pH (BiotecVisions, 2012). The

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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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3.2 Nitrogen Recovery
In the last decades we are dealing with pollution problems coming from organic
waste streams and manure management (N eutrophication, nitrate leaching, nitrous
oxide greenhouse gas emissions, and ammonia particulate). The main reason behind
this, is the cycle of large N losses in agricultural systems and subsequent fossil-based
synthetic N replenishment. Around 85% of reactive N (forms other than di-nitrogen
gas, N2) is lost to the environment (waterways, atmosphere, etc.). Meanwhile the
majority (95%) of the remaining 15% that enters the human organism is excreted
and eventually lost to those same waterways and atmosphere (Galloway et al.,
2004). To replace N lost from agricultural systems, industry converts non-reactive N2
to synthetic N fertilizer through energy-intensive and environmentally harsh
processes; e.g. Haber-Bosch process (12 Kwh*kg*N-1
; Sutton et al., 2009) (1.4-2.6 kg
CO2*kg*N-1
; Wood and Cowie, 2004). The unsustainable N cycle 1) consumes limited
fossil fuel resources and 2) contributes to environmental pollution. Recovering N
reduces losses of N to the environment while decreasing the demand for synthetic
fertilizers. The methods for nutrient recovery from digestate are developing rapidly
along with the technological advances, improving nutrient management in
agriculture and in waste treatment systems.
3.2.1 Drivers for digestate processing for nutrient recovery
Digestate as fertilizer/ soil conditioner in agriculture, horticulture, forestry etc. can
be directly applied is possible after its removal from the digester tank without any
further processing. However, digestate is rather diluted with respect to nutrients,
which makes the costs of transportation relatively high compared to conventional
fertilizer. Significant costs are also the investments in storage capacity, required by
environmental regulations in many countries, like for e.g. in Denmark, Germany and
France, where not only the nutrient input per hectare is restricted, but also the
period of application is limited to the growing season. However, many crop
cultivators agree that applying digestate as organic fertilizer compared to
conventional fertilizer has synergistic effects.
3.2.2 Legal frameworks
At EU level, the European Nitrate Directive 91/676/EEC restricts the yearly load of N,
applied to agriculture . Livestock production is intensive, usually in areas with limited
land available for manure application. This creates a permanent excess of nutrients,
making such areas highly vulnerable, to surface and ground waters, pollution. The
problem intensifies when animal feed is being imported to such a region, which
makes efficient nutrient management even more crucial. The legal restrictions on
the nutrient input per hectare require the excess nutrients to be recovered,

7
Fig.3, Water distribution in sludge, (Arun S. Mujumdar, 2015)
exported, and recycled outside the vulnerable areas. Thus digestate processing
technologies have been developed, aiming at volume reduction and nitrogen
removal. More recently, also concerns regarding P excess from manure application
in many areas and high levels of phosphorus found in surface and ground waters
have greatly increased demand for nutrient management and export of excess of
nutrients.
3.3 Solid–liquid separation, the first step in digestate processing.
3.3.1 Moisture Distribution in Sludge
The moisture in the digestate sludge can be as high as 99%. Figure 3 depicts the
moisture distribution in the sludge. This distribution takes the following forms: 1)
free moisture that is not attached to the sludge particles and can be removed by
gravitational settling; 2) interstitial moisture that is trapped within the flocs of solids
or exists in the capillaries of the dewatered cake and can be removed by strong
mechanical forces; 3) surface moisture that is held on the surface of the solid
particles by adsorption and adhesion; and 4) intracellular and chemically bound
moisture. (Mujumdar, 2015)
,
The amount of water that can be removed is dependant on the dewatering process
and the status of the water in the sludge. The water that can be removed by
mechanical dewatering is usually termed as 'free water' and the rest as 'bound
water'. The bound water is the theoretical limit of mechanical dewatering. The free
water includes the free, interstitial, and partially the surface moisture. The bound
water includes the chemically bound moisture and partially the surface moisture.

8
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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objectives will be achieved only through a sequence of several steps which can be
relatively complex and therefore expensive.
3.3 Processing of the solid fraction
The solid fraction which comes out of the solid–liquid separation has TS
concentrations in the range of 20 – 30 %. It is partially stabilized so that is
appropriately stored and direct applied as bio-fertilizer or soil improver in
agriculture. However, it still contains biodegradable material and consequently
microbial activity can still happen and odour emissions can occur. In order to get a
stable and marketable bio-fertilizer product, the solid fraction needs further
processing, which can be composting, drying or another form of stabilization.
3.4 Processing of the liquid fraction
While the solid-liquid fraction separation uses relatively simple and cheap
technologies, for further processing different methods and technologies are
available, with various degrees of technical maturity, higher energy input, and higher
investment and operating costs. For nutrient recovery, membrane technology, such
as nano and ultrafiltration followed by reverse osmosis, can be used (Fakhru’l-Razi,
1994; Diltz et al., 2007). Membrane filtration produces a nutrient concentrate and
purified process water (Castelblanque and Salimbeni, 1999, Klink et al., 2007). The
liquid digestate can also be purified through aerobic biological wastewater
treatment (Camarero et al., 1996).
However, because of the high nitrogen content and low biological oxygen demand
(BOD), addition of an external carbon source is often necessary in order to achieve
satisfying denitrification. A further possibility for concentrating digestate is
evaporation with excess heat from the biogas plant, combined heat and power (CHP)
unit. In order to reduce the nitrogen content in the digestate, ammonia stripping
(Siegrist et al., 2005), ion exchange (Sanchez et al., 1995) or struvite precipitation
(Uludag- Demirer et al., 2005; Marti et al., 2008) are in use.
Whatever technology is applied, advanced digestate processing in most cases
requires high energy input and chemical reagents, like acid which are a significant
expense. Along with increased investment costs for appropriate machinery, the
treatment costs become considerable. An overview of viable digestate processing
technologies is given in Figure 5.

10
Fig.5, Overview of viable options for digestate processing (Fuchs and Drosg, 2013)
Fig.6, Overview of the distribution of industrial scale applications for further treatment of the liquid fraction of
digestate in Germany , Austria and Switzerland from 2009 (Fuchs and Drosg, 2013)
As we can see above, a wide range of technologies are currently being used for
digestate processing, depending on the boundary conditions. The most abundant
approach is solid–liquid separation of digestate, where, depending on the
consistency of the digestate, screw presses or centrifuges are mostly used. Solid–
liquid separation can be improved by the addition of precipitating agents.
For further processing of the liquid fraction, membrane purification is the only
process that can achieve a degree of purification that can allow direct discharge. It is
also among the most frequently applied approaches in more complex digestate
processing facilities in Germany, Switzerland, and Austria (Figure 6).

11
However, membrane purification is the most expensive technology, with high
potential for optimization in large-scale applications. If excess heat is available,
evaporation is an interesting option, although it gives rise to some controversy.
In Germany digestate processing technologies using heat (e.g. evaporation, drying)
are being used more frequently due to the subsidies for waste heat utilization at
biogas CHPs. Evaporation of the liquid fraction of digestate is a rather robust
technology, however, if the liquid fraction contains considerable amounts of fibrous
material it is necessary to remove this beforehand to avoid clogging in the heat
exchangers. Other technologies that are less commonly applied include ammonia
stripping, ion exchange, solar drying, etc.
3.4 Thermal Processing of the Digestate's Liquid Fraction
3.4.1 Thermal Drying Background (Andreoli et al., 2007)
The thermal drying process is one of the most efficient and flexible ways of
decreasing the moisture content from dewatered organic industrial and domestic
sludges. Thermal drying may be used for different sludge types, either primary or
digested, and a feeding sludge solids content of 15%–30% is recommended.
Under ideal conditions, 2,744 kJ (0.76KW) of energy are needed to evaporate 1 kg of
sludge water , and it is usual to increase this value up to 100% for normal operational
conditions. The total energy demand will depend 1) on the efficiency of the selected
equipment and 2) on the type of the processed sludge. Mainly the energy comes
from external sources, such as fuel oil, natural gas etc. Biogas generated in AD may
provide an alternative energy source.
As the heating power of biogas is 22 MJ/L and burners can usually work at 70%
efficiency, under ideal conditions, 0.17 liters of biogas are required to evaporate 1 kg
of water. Besides this, energy losses (through walls, air, etc.) must also be accounted
for, together with the energy required to increase the sludge temperature to slightly
above 100 ◦C, when the evaporation process starts.
The suppression of the biological stabilization stage significantly reduces capital
costs, and favors the production of pellets with high organic matter content and
heating value, which make the product marketable in agriculture or as fuel.
The main advantages of sludge thermal drying are: 1) significant reduction in
sludge volume; 2) reduction in freight and storage costs of the sludge; 3) generation
of a stabilized product suitable to be easily stocked, handled and transported; 4)
production of a virtually pathogen-free final product; 5) preservation of biosolids
fertilizing properties; 6) no requirements of a special equipment for land application;

12
Fig.7, Principles of drying processes, drying by convection (left) and drying by contact (right) (Fuchs and Drosg, 2010)
7) sludge is suitable for incineration or landfilling; 8) product may be put into sacks
and distributed by retail dealers.
The main limitations of thermal drying processes are: 1) production of liquid
effluents; 2) release of gases into the atmosphere; 3) risk of foul odours and
disturbing noise
The major types of thermal drying systems are:
• Direct contact dryers (convection): where hot air has direct contact with the
sludge, drawing away moisture, gases and dust (Direct dryers are typically rotary-
drum, flash, moving-belt dryers, or centridryer types.)
• Indirect contact dryers (conduction): where heat is transmitted through heat
exchange plates (Indirect dryers are thin-film, rotary-disc, or rotary-tray dryers.)
Both processes are illustrated in Figure 7,
In convection, we have heat transfer by direct contact of the wet sludge and the hot
gases. The heat of the inlet gas provides the latent heat required for evaporating the
liquid from the sludge. The vaporized liquid is carried by the hot gases. Under
equilibrium conditions of constant-rate drying, mass transfer is proportional to 1) the
area of wetted surface exposed, 2) the difference between the water content of the
drying air and saturation humidity at the wet-bulb temperature of the sludge–air
interface, and 3) other factors, such as velocity and turbulence of drying air
expressed as a mass transfer coefficient (Tchobanoglous et al. 2003). Direct dryers
are the most common type used in thermal drying of sludge. Flash dryers, direct
rotary dryers, and fluidized-bed dryers use this method.
In conduction, we have heat transfer by contact of the wet sludge solids with hot
surfaces. A metal wall separates the sludge and the heating medium (usually, steam
or oil). The vaporized liquid is removed independent of the heating medium. Indirect
dryers for drying municipal sludge include horizontal paddle, hollow-flight or disk
dryers, and vertical indirect dryers. (Turovskiy & Mathai, 2006).

13
Fig. 8, Schematic flow of a Rotary Dryer (Frischman, 2012)
Liquid effluent is usually less than 1% of the total flow and can be recycled to the
plant headworks. Both systems require equipment for enclosure and treatment of
the water vapor and the dust which are released from the dryers to avoid odour and
particle emissions to the atmosphere. Indirect processes produce pellets with up to
85% solids concentration. For solids contents higher than 90% and possible
production of organo-mineral fertilizers, direct drying processes should be preferred.
The drying cost varies among different technologies chosen with a range of US $65-
80/ton DS. Utilizing the waste heat from the burning of sludge is reducing the cost
significantly. For example, flue gas drying preceding incineration can save as much as
60% on the cost compared with direct incineration in large sludge plants. Similarly,
mechanical compression of the vapors generated from indirect dryers can improve
the energy efficiency considerably. Cogeneration-sludge drying units might be an
economically attractive option to consider in large wastewater treatment plants
(Mujumdar, 2015)
Sludge drying is not an isolated issue. It has to be addressed along with other
economical, environmental, and safety concerns. Over the last decade, there has
been a significant increase in investment in environmental protection including for
sludge processing. Innovative drying technologies with higher thermal efficiencies,
lower emissions, less operator involvement, cheaper capital costs, and better final
products are needed by the market.
3.4.2 Direct contact dryers
a. Rotary Drying (Frischman, 2012)
The design of a Rotary Dryed is illustrated in Figure 8,
In a rotary drier digestate fibre is coming in contact with hot gases (convection). The
rotary drier consists of a cylindrical drum which is rotated about its axis. Flights
within the drying drum pick up and cascade the digestate. The drum is mounted on a

14
Fig. 9, Schematic flow of a Belt Dryer (Frischman, 2012)
slight slope from the horizontal to transport the dried product along its length. The
feed to the drier is blended with dried product to give a feed of approximately 65%
dry solids (DS) to improve movement within the drum. Waste gases are passed
through a cyclone to recover solids before further treatment, with large amounts of
ammonia may contained within the exhaust gas stream. Final product DS of up to
95% can be achieved. Screening can be added to give a homogenous product pellet
size.
A rotary drying system in operation in Louisville, Kentucky, has its primary sludge
anaerobically digested first and then blended with thickened waste activated sludge.
The sludge mixture is then dewatered in centrifuges to about 26% solids and fed into
four drying trains, each comprising a rotary drum dryer. Each dryer is sized to
evaporate water from dewatered sludge at a rate of 8500 kg/h. Total installed
evaporative capacity is 34 metric tons/h. The methane generated from the digesters
provides half of the energy required by the dryers. Heat recovered from the dryers is
used to heat the anaerobic digesters. (Turovskiy &Mathai, 2006)
Pros: 1) Reduced volume of digestate for transport and storage; 2) Improved
marketability as a fertilizer/soil conditioner; 3) Effective pathogen kill.
Cons: 1) High energy requirement; 2) High temperature operation; 3) Large capital
investment; 4) Reduced nutrient content of final product; 5) Gas treatment required;
6) Risk of explosive atmosphere within drying plant.
b. Belt Drying (Frischman, 2012)
Belt dryers are used in the water and wood pulp industries. The viability of this
technology is be dependent on the installation and the end use of the dried product.
Figure 9, illustrates the basic concept design,
In a belt dryer digestate is contacted with hot gases (convection). The digestate fibre
is evenly distributed over the drying belt by an extruder. The extruder produces
digestate strands in order to increase surface area and provide a uniform size. The
drier belt passes through a series of successive chambers of increasing temperature.
At the end of the belt digestate is dropped onto a second belt which runs back

15
Fig. 10, Schematic flow of a Solar Dryer (Frischman, 2012)
through the drier, underneath the first belt, to complete the drying process and cool
the product. Dried product is discharged from the drier at the end of the second belt.
Product is discharged at up to 90% DS and at a temperature below 40°C. As the
digestate is not agitated during the drying process the risk of creating an explosive
dust atmosphere within the drier is significantly reduced. This improves the safety
and operability of the process.
Pros: 1) Reduced volume of digestate for transport and storage; 2) Improved
marketability as a fertilizer/soil conditioner; 3) Effective pathogen kill.
Cons: 1) High energy requirement; 2) Large capital investment; 3) Reduced nutrient
content of final product.
c. Solar Drying (Frischman, 2012)
This technology provides a low operational expenditure (Opex) (the day-to-day
management costs) solution for dewatering digestate, however it requires a large
land area. Most operational plants of this type are located in warm climates. Figure
10 depicts the basic design of a solar system,
Solar drying uses a combination of forced ventilation and solar energy to de-water
digestate. Waste heat from a CHP can also be used via underfloor heating. The feed
to the process can be fed with either whole digestate or de-watered fibre. The
digestate is fed into greenhouses where it is distributed across the drying bed.
Digestate is turned and ventilated to increase efficiency and reduce odours. The
greenhouses operate as a batch or semi-continuous process. The final product is a
dried digestate exceeding 50% dry solids.
Pros: 1) Increased concentration. 2) Reduced transport volume; 3) No liquor
treatment required.
Cons: 1) Large surface area required. 2) Cold climates may restrict application (most
current applications are in Spain or Southern France).

16
Fig. 11, Design of a Flash Drying System (Mujumdar, 2015)
d. Flash Dryer (Mujumdar, 2015)
Flash drying is the fast removal of moisture by spraying or injecting the sludge into a
hot gas stream. In a flash drying system (see Figure 11) the wet sludge cake is
blended with previously dried sludge in a mixture to improve pneumatic conveyance.
The blended sludge and hot gases from the furnace at 704°C are mixed ahead of a
cage mill, and flashing of the water vapor begins. Gas velocities on the order of 20 to
30 m/s are used. The cage mill mechanically agitates the sludge–gas mixture, and
drying is virtually complete by the time the sludge leaves the cage mill, with a mean
residence time of a few seconds. The dried sludge is conveyed to a cyclone
pneumatically. The sludge at this stage has moisture content of only 8 to 10%. The
sludge is then separated from the spent drying gases in the cyclone.
Flash dryers have high energy and operation and maintenance costs. Today, other
types of dryers are preferred.

17
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
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
Fig. 14, Schematic flow of a Surface Scraped Heat exchanger (Frischman, 2012)
equipped with adjustable scrapers that move and tumble the sludge in thin layers
from one tray to another in a rotating zigzag motion until it exists at the bottom as a
dried pelletized product. The process minimizes the formation of dust and oversized
chunks. The dryer’s exhaust consists of water vapor, air, and some pollutants. After
the water vapor is condensed, only a small amount of gases, mainly moist air,
remains to be treated. These gases are vented from the dryer to an odor control unit
for thermal destruction of odor-causing compounds.
3.6 Evaporation
If we want to concentrate the digestate or increase its dry solids content, we can use
evaporation. Evaporation uses thermal energy (heat) to release the moisture from
the digestate. However, unlike the drying techniques, evaporation preserves the
nutrients and a percentage of the moisture. Evaporation is typically used for liquor
or whole digestate treatment.
The final solids concentration is dependent on the desired product, but
concentrations of up to 20% DS can be achieved. High temperatures will cause
ammonia to be released, this can be solved by decreasing the pH of the digestate,
usually with acid dosing, before the evaporation. This allows the digestate liquor to
be converted into a concentrated fertilizer.
a. Surface Scraped Heat Exchanger (HRS) (Frischman, 2012)
This technology provides a viable method for treating digestate liquor and
producing a balanced fertilizer product. Consideration will need to be given to the
acidic nature of the final product and the affect this may have on agricultural
appliance. Figure 14 illustrates the schematic flow,
Evaporation uses waste heat from the CHP. Surface scraped evaporators are
designed not to be affected by fouling issues by the evaporation of the digestate.
The evaporators use a shell and tube configuration. The interior surface of the heat
exchanger tubes is constantly cleaned by internal scrapers to reduce fouling and
increase heat transfer efficiency. The digestate liquor is dosed with acid prior to
evaporation to prevent ammonia loss within the evaporator. The volume of acid

20
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
Table 3, Calorific power of different sewage sludge (Andreoli et al., 2007)
pelletized before it can be gasified, adding an additional energy demand on the
process.
Pros: 1) Volume reduction; 2) Destruction of pathogens and toxic compounds; 3)
Renewable heat and energy generation.
Cons: 1) High operating cost; 2) Complex operation; 3) Ash disposal to landfill; 4) Loss
of fertilizer potential.
3.8 Incineration (Andreoli et al., 2007)
Incineration provides the greatest volume reduction. The remaining ashes volume is
usually less than 4% of the dewatered feed sludge volume. Incinerators can use
sludge from several treatment plants (Table 3) and are usually designed with
capacities higher than 1 ton/h.
Incineration destroys organic substances and pathogenic organisms through
combustion, using excess oxygen. Incinerators must use sophisticated filter systems
to significantly reduce pollutant emissions. Gas emissions released are regularly
measured to ensure operational efficiency and safety. Incinerator design requires
detailed mass and energy balances. Despite the high organics concentration in
dewatered sludge, combustion is only autogenous when solids concentration is
higher than 35%. If need be, we can use auxiliary fuels, such as boiler fuel with low
sulphur content. The calorific value of sludge crucial in the amount of fuel
consumption.
Products from complete combustion of sludge are water vapour, carbon dioxide,
sulphur dioxide and inert ashes. There are two types of incinerators that are
currently in use for sewage sludge:
• multiple chamber incinerator
• fluidised bed incinerator
A multiple chamber incinerator is divided into three distinct combustion zones. The
higher zone, where final moisture removal occurs, the intermediate zone where

22
combustion takes place and the lower or cooling zone. Should supplementary fuel be
required, gas or fuel oil burners are installed in the intermediate chamber. A
fluidized bed incinerator consists of a single-chamber cylindrical vessel with
refractory walls. The organic particles of the dewatered sludge remain in contact
with the fluidized sand bed until complete combustion.
Fluidized bed incinerators are usually preferred over multiple chamber furnaces, due
to lower operational costs and lower gas emission. However, despite the significant
reduction in sludge volume, incineration cannot be considered a final disposal route,
as residual ashes require additional processing. Inadequate ashes disposal could lead
to possible leaching of metals and their absorption by plants. Landfill is the
preferable destination for the ashes.
Incineration is well established technology but investment and operating costs are
high and viable only for large plants or where agricultural application of digestates is
not possible because of the digestate quality or land bank availability.
Pros: 1) Volume reduction; 2) Destruction of pathogens and toxic compounds; 3)
Possible energy recovery.
Cons: 1) High operating cost; 2) Complex operation. 3) Potential environmental
impact of residuals (exhaust air); 4) Loss of fertilizer potential; 5) Public perception.
a. Multiple Chamber Incinerator (Turovskiy &Mathai , 2006)
The flowchart of a system with a multiple hearth furnace is presented in Figure 16.
The furnace shell is a vertical steel cylinder 6 to 8 m in diameter lined internally with
refractory brick or heat-resistant concrete. The furnace is divided vertically into
seven to nine refractory hearths. A vertical rotating shaft passes through the center
of the furnace, to which the horizontal frames of the rake mechanisms, made of
heat-resistant cast iron, are affixed. Each hearth has material transfer openings
located alternatively on the periphery of one hearth and in the center section of the
adjacent hearthThe sludge moves by conveyors into the charging hopper and then
onto the uppermost hearth of the furnace. The sludge is moved by rakes into the
transfer openings, it drops to the next lower hearth, and continues its travel to the
lower hearths. This provides continuous movement of the sludge mass in the
opposite direction to the hot combustion air. The use of rake mechanisms to move
and break up the clumps in the sludge intensifies the drying and combustion
processes.

23
Fig.16, Flowchart of multiple hearth incinceration (Turovsky &
Mathai, 2012)
Pros: 1) combusting both primary
and secondary sludge, as well as
trash from screens, scum from
settling tanks and oil separators,
dirty grit from grit chambers, and
industrial wastes; 2) they are
characterized by their simplicity of
service and by the reliability and
stability of operation during
significant variations in the quantity
and quality of sludge treated; 3) The
furnaces can be installed in the
open air.
Cons: 1) high capital cost; 2) large
area required; 3)presence of
rotating mechanism in the high-
temperature zone; 4) frequent
failure of the rake devices.
b. Fluidized Bed Incinerator
(Turovskiy &Mathai , 2006)
Fluidized-bed furnaces are adapted
in the industry as a drying and roasting
technology in a wide range of fields. The furnace, a vertical steel cylinder lined
internally with refractory brick or heat-resistant concrete, consists of a cylindrical
furnace chamber, a lower conical section with an impermeable air distribution grate,
and dome-shaped crown. Heat-resistant quartz sand is placed on the grate.

24
Fig.18, Flowchart of Pyreg (Frischman, 2012)
Fig.17, Flowchart of fluidized bed incineration (Turovskiy
&Mathai , 2006)
The turbulent (fluidized) bed in the furnace is formed when air is blown through the
distribution grate at a rate at which the sand particles move in a turbulent manner
and appear to boil in the flow of gas.
The design of the furnace depends on
the composition of the sludge and
thus its thermal balances for the
combustion process, establishing the
geometric dimensions of the furnace
elements, and the quantities of
auxiliary fuel, air, and exhaust gases.
Figure 17 illustrates the flowchart of
an incineration system with a fluidized
bed furnace.
3.9 Pyrolysis (Frischman, 2012)
Pyrolysis processes heat the digestate
without oxygen, breaking down the
organic content into char and syngas.
For an efficient operation the feed
digestate must have a low moisture
content and ideally be in a dry
pelletized form. Pyrolysis decreases
the digestate mass by 70%, lowering
the transport costs. The char produced
can be used as a soil amendment or as a
partial replacement for peat in growing media production.
a. Pyreg Process
Pyreg (Figure 18) is a proven technology for biomass feedstocks at full scale and
digestate at pilot scale. Its size and modular design make it applicable to a range of
plant sizes. Nevertheless the digestate must be dried before processing. The viability
of Pyreg is dependent on the market for the biochar.

25
By 'biochar' we meant the char coming from the pyrolysis of organic matter. In Pyreg
approximately 70% of the feed mass is destroyed within the reactor and the
remainder is a mix of carbon-rich char and ash.
The possibility of installing a turbine to recover electricity from the exhaust gasses,
and make the process entirely self-sufficient, is still tested. The possibility of
increasing the biogas yield and ease of dewatering by mixing biochar with the
digester feedstock is under study as well.
Pros: 1) Carbon capture; 2) Soil amendment - reduced application of compound
fertilizers; 3) Biochar value as a growing media constituent; 4) Renewable heat
generation; 5) Small footprint; 6) Modular units.
Cons: 1) Acceptance of the new technology; 2) Securing a market for biochar; 3)
Establishing a PAS and QP for biochar; 4) Limited process experience; 5) High feed
solids content required.
3.10 Wet Air Oxidation (Andreoli et al., 2007)
Initially wet air oxidation was designed for the paper industry's residues treatment,
but it was adapted for sewage sludge treatment later on. Wet oxidation is
recommended when the sludge is 1) too diluted to be incinerated, and 2) too toxic to
be given to the biological treatment plant. Low-pressure wet air oxidation is used to
decrease the sludge volume and increase its dewaterability for thermal treatment.
Intermediate and high-pressure oxidation are used to decrease sludge volume
through oxidation of volatile organic matter into CO2 and water. Sludge organic
matter, may be considered easily oxidizable (proteins, lipids, sugars and fibers, ,
which are approximately 60% of the total organic matter) or not easily oxidizable.
Wet air oxidation takes advantage of the dissolved or particulate organic matter to
be oxidized at temperatures in the range of 100◦C–374◦C (water critical point). The
temperature of 374◦C stabilizes the water in a liquid form, even at high pressures.
Oxidation is accelerated by the high solubility of oxygen in aqueous solutions at high
temperatures. Wet air oxidation is efficiently applied in destruction of organic
matter of effluents with 1%–20% solids concentration, allowing enough organic
matter to increase the reactor internal temperature through heat generation
without external energy supply. The upper 200 g/L (20%) solids concentration limit
avoids the surplus heat to raise the temperature above the critical value, which
could lead to complete evaporation of the liquid.
Wet air oxidation of organic matter can be described by the following eq.,
CaHbOcNdSeClf + O2 → CO2 + H2O + NH4
+
+ SO4
2−
+ Cl−
(1)

26
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
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
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
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
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
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
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
Table 4, Ranking of the available digestate treatment methods according to their individual score in each of the following categories; 1) Feed Solids,
2) Reliability, 3) Power Usage, 4) Odour, 5) Chemical Usage, 6) Noise, 7) Hazard (T, P, Chem.), 8) Carbon footprint. 5 is the highest score and 1 the
lowest.
Feed Solids (%) DS Reliability Power Usage Odour Potential Chemical Usage Noise Hazard (T, P, Chem.) Carbon footprint Method score Rank
Rotary Drying 2 2 5 5 1 3 2 5 114.92 4
Belt Drying 2 2 5 5 1 3 3 5 115.10 3
J-Vap 1 4 3 5 1 1 2 2 63.31 9
Solar Drying 1 3 5 3 1 1 1 1 100.06 5
Surface Scraped Heat Exchanger (HRS) 1 4 3 3 3 1 1 3 71.31 8
Incineration 5 3 5 5 1 3 2 5 115.29 2
Gasification 4 5 5 5 1 3 2 5 115.38 1
Wet Air Oxidation (WAO) 1 3 3 3 3 3 4 3 76.05 7
Pyreg (slow pyrolysis) 4 3 3 5 5 1 3 1 80.94 6
Fig.26, Flow diagram from the Digestate Treatment System GNS as example of a system with 2 stripping reactors, (Bauermeiter et
al., 2009)
After formulating our algorithm we use a comparative study (Frischman, 2012) to get
Table 4,
So according to the comparative programming method for a compensation factor of
5, gasification is the best choice with pyrolysis being second. As stated before this is
a totally objective result and real-life installation designs should take into account
more and regional data.
3.13 Schematic Flow of a Digestate Thermal Process
Here, is presented, the mass flow schematic of a digestate thermal process. Since a
gasification process was chosen in chapter 3.12, the author will present a case from
a study by Bauermeiter et al. (2009), from a modified stripping process by GNS (gns-
halle.de), where the ammonium nitrogen is removed from the digestate by using
only exhaust heat from the CHP without the use of bases, acids or external stripping
media (Fig.26). The presented data are from the ANAStrip - Plant, BENAS of the
Biogas Plant in Ottersberg from 2007/2008.

34
3.14 Properties of Aqua Ammonia - Effects of pH and temperature
As already stated in the previous chapters, each of the current nitrogen recovery
methods suffer from its own drawbacks. Given that the financial budgets and legal
regulations become more and more tight, we need to find alternative digestate
processing methods which can provide us with more efficient procedures. In order to
do that, we need to take a look again at the NH3 properties and its water solution,
aqua ammonia.
Ammonia is a colorless gas with a characteristic odour, highly soluble in water. Its
aqueous solutions are alkaline and have a corrosive effect to metals and tissue. The
pH in an aqueous solution of 0.1 M is 11.2, which is characteristic of a weak base
(pKa = 9.3).
Ammonia is used primarily 1) as a nitrogen source for producing fertilizers; 2) as
refrigerant for producing nitric acid and other chemical reagents such as sulfuric
acid, cyanides, amides, nitrites and intermediaries dyes; 3) as a nitrogen source in
the production of synthetic fiber monomers and plastics; 4) as corrosion inhibitor in
oil refining and other industries such as paper, extractive, food, fur and
pharmaceutical industries. Some of the aforementioned processes produce
wastewater with dissolved toxic gases, ammonia among others, in small
concentrations. For example, in the production of urea fertilizers the wastewater
created has dissolved ammonium in the range of 500 to 2000 ppm.
The removal of this substance before the effluent release, is crucial for two
important reasons:
 It is extremely toxic to marine life (concentrations < 0.01 ppm have negative
effects on fish, while 0.1 ppm can be lethal to other species).
 It can be bio-oxidized by nitrifying organisms to nitrites and nitrates.
The ammonia in wastewater effluents exists in two forms, as volatile ammonia (NH3)
and ammonium ions (NH4
+
). Ammonia recovery processes try to maximize as
possible the volatile ammonia component. That depends mainly on two factors, 1)
the temperature (T) and 2) the pH value of the aqueous solution. The T has a direct
effect on the solubility of ammonia in water, which is reducing as the temperature
rises. For example, one volume of water can dissolve 1200 volume of ammonia at 0
◦C (atm. pressure); at 20 ◦C this solubility falls down to 700 vol. of ammonia per 1
volume of water. Nevertheless, raising the temperature cannot release by itself all
the dissolved ammonia in the solution, because much of its quantity dissociates right
away in water to form unstable NH4
+
solutions, based on the following chemical
reaction:

35
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 20o
C to 48.3
KPa at 50o
C. In addition, increasing the ammonia concentration in water also
increases the total vapour pressure of the solution. At 20o
C, the vapour pressure
increases from 12.1 to 148.8 KPa, when the ammonia concentration is increased
from 10 to 40 wt%. (Xie et al., 2009)
The ammonium equilibrium in water is illustrated in Figure 27. For pH < pKa (9.3), ion
ammonium concentration is higher than ammonia. At pH=9.3 the ionic form as well
as the ammonia molecules were found to 50% in the solution. When pH > pKa,
ammonia is dominant.
So according to Fig.27 many physical and chemical properties of ammonia are a
direct function of its pH value. For example, when the pH of the solution decreases,
the ammonia solubility increases and the ammonia molecules can volatilize freely.
(Segura, 2012)
As mentioned before, the vapor pressure of aqueous ammonia also increases with a
higher pH. To make sure that we have a high ammonia removal and a low water

36
Figure 28, Influence of temperature and pH on the ratio of ammonia partial pressure to total ammoniacal
nitrogen (TAN) concentration (Zarebska et al., 2014)
content in our distillation product, a moderate temperature combined with a high
pH should be chosen (Fig.28). (Zarebska et al., 2014)
From Fig.28 we can see that the NH3 vapor pressure to total ammonia nitrogen
(TAN) ratio is influenced more from the pH rather than by temperature.
3.15 Membrane Distillation
3.15.1 Fundamentals of Membrane Distillation (MD)
MD is mainly used for desalination purposes but many have gained interest for it as
an advanced treatment of wastewater for water reuse. Researchers have also
studied the viability of MD for brackish water desalination, process water treatment,
and resource concentration for industrial uses. The MD process can use the
advantages of anaerobic processes, while the mesophilic (20-45°C) or thermophilic
(41-122°C) operating conditions that are usually needed to run fermentation
processes (AD) can fulfill the waste heat requirement for the following MD process.
MD has attracted attention for the removal of volatile compounds like ammonia
because of its possibly low energy need. It can potentially recycle and reuse the
industrial wastewater, and can process wastewater streams having a high
temperature but relatively low levels of volatile organic compounds and ammonia. In
MD the driving force for the transport of ammonia across the membrane is the
difference in the partial pressure of ammonia on each side of the membrane. (Xie et
al., 2009). In chapter 6 we explained why by raising the content of volatile ammonia
in its aqueous solution, we increase its vapour pressure, thus making MD an ideal
candidate process for the task of ammonia removal.

37
Currently, MD is one of the few membrane processes that use a thermal concept.
Energy need is, theoretically, the same as in traditional evaporation processes.
Nevertheless, the operating temperature is much lower than that of a conventional
process because it is not required to heat the solution above its boiling point. The
process can be operated at temperatures typically < 70 °C, and driven by low
temperature difference (20 °C) of the hot and the cold solutions. Thus, low-grade
waste or alternative energy sources (solar and geothermal) can be coupled with MD
systems to drop the cost and raise the efficiency of the separation system.
Consequently, MD might overcome not only the limits of thermal systems drawbacks
but also the ones of the other membrane systems such as reverse osmosis (RO) or
nanofiltration (NF).
MD is not significantly affected by concentration polarization, so high recovery rate
can be achieved, when compared with RO. MD has also all the other properties of
the membrane system (easy scale-up, easy remote control and automation, no
chemicals, low environmental impact, high productivity/size ratio, high productivity/
weight ratio, high simplicity in operation, flexibility, etc.) (E. Drioli et al., 2015)
3.15.2 MD Membranes
MD process is mainly driven by the vapour pressure gradient which is created by a
temperature difference across the membrane. As the latter is not a pure thermal
driving force, MD can be operated at a much lower temperature than conventional
thermal distillation processes. The hydrophobic nature of the membrane prevents
water coming through due to surface tensions, unless a transmembrane pressure
higher than the membrane liquid entry pressure (LEP) is applied. So, liquid/vapour
interfaces are formed at the entrances of each pore. The transfer of water through
the pores, is performed in three steps:
(1) formation of a vapour gap at the hot feed solution–membrane interface;
(2) transport of the vapour phase through the microporous system;
(3) condensation of the vapour at the cold side membrane–permeate solution
interface.
MD membranes are usually made from 1) polytetrafluoroethylene (PTFE), 2)
polyvinylidene fluoride (PVDF), and 2) polypropylene (PP). PTFE has the highest
hydrophobicity, good chemical and thermal stability, and oxidation resistance. PVDF
and PP also show good hydrophobicity and thermal/chemical resistance and can be
easily developed into membranes with versatile pore structures.

38
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 30-60o
C
and lower temperatures than those conventional distillation operate with, are
preferable since the heat loss through thermal conduction is also linear to the
temperature difference across the membrane. (Hyun-Chul Kim et al., 2015)
Membrane distillation efficiency is directly affected by the structure of the
membrane in terms of 1) thickness; 2) porosity; 3) mean pore size; 4) pore
distribution and 5) geometry. So, distillation product of the process is dependent
upon the capability of the membrane to interface two media without dispersing one
phase into another and to combine high volumetric mass transfer with high
resistance to liquid intrusion in the pores. The membranes for membrane contactor
application have to be 1) porous; 2) hydrophobic; 3) with good thermal stability and
4) excellent chemical resistance to feed solutions.
In particular, the characteristics needed are (E. Drioli et al., 2015):
1. High liquid entry pressure (LEP), is the minimum hydrostatic pressure that must
be applied onto the feed solution before it overcomes the hydrophobic forces of the
membrane and penetrates into the membrane pores. LEP is a characteristic of each
membrane and permits to prevent wetting of the membrane pores. High LEP may be
achieved using a membrane material with high hydrophobicity and a small maximum
pore size
LEPW =(B*γL*cosθ)/dmax (4)
B is a geometric factor determined by pore structure with value equal to 1 for
cylindrical pores, γL the liquid surface tension and θ is the liquid/solid contact angle.

39
However, as the maximum pore size decreases, the mean pore size of the membrane
decreases and the permeability of the membrane becomes low.
2. High permeability. The flux will “increase” with an increase in the membrane pore
size and porosity, and with a decrease of the membrane thickness and pore
tortuosity. In fact, molar flux through a pore is related to the membrane's average
pore size and other characteristic parameters by:
N≈(<ra
>*ε)/(τ*δ) (5)
where ε is the membrane porosity, τ is the membrane tortuosity, δ is the membrane
thickness, 〈rα
〉 is the average pore size for Knudsen diffusion (when α=1), and 〈rα
〉 is
the average squared pore size for viscous flux (when α=2).
To obtain a high permeability, the surface layer that drives the membrane transport
must be as thin as possible and its surface porosity as well as pore size must be as
large as possible. In order to do so we see from eq. (5) that we need, in terms of
molar flux, to maximize the membrane porosity and pore size, while minimizing the
transport path length through the membrane, (τ, δ). However, thermal efficiency in
MD increases gradually with increasing the membrane thickness and an optimization
between the two requirements has to be researched.
3. Low fouling problem. Fouling is one of the main problems when using porous
membranes. In the gas–liquid contact processes, since there is no convection flow
through the membrane pores, the contactors are less sensitive to fouling.
Nevertheless, when we have gas and liquid streams with large content of suspended
particles (e.g. industry), we can get clogging of the membrane pores. Ideally pre-
filtration is necessary for an efficient operation.
4. High chemical stability. The life sustainability of the membrane depends on any
reaction between the solvent and membrane material, which could possibly affect
the membrane matrix and surface structure. Liquids with high content of acid gases
are corrosive, which make the membrane material less resistant.
5. High thermal stability. When operating with high temperatures, the membrane
material may be affected by degradation or decomposition. Any change in the
nature of membrane depends on the glass transition temperature Tg for amorphous
polymers or the melting point Tm for crystalline polymers. The factors that increase
the crystallinity (Tg/Tm) of a membrane can improve its chemical and thermal
stability. For processes at high temperatures, fluorinated polymers are used due to
their high hydrophobicity and chemical stability.

40
Fig.29, Schematic representation of MD configurations (Onsekizoglu, 2012)
3.15.3 MD configurations
MD processes can be categorized into four basic configurations (Fig. 29), depending
on how vapour is condensed in the permeate side. In all four configurations there's
direct contact of one side of the membrane with the feed solution. Direct contact
membrane distillation (DCMD) is the most researched because of its simplicity.
Vacuum membrane distillation (VMD) can be used for high output, while air gap
membrane distillation (AGMD) and sweep gas membrane distillation (SGMD) have
low energy losses and high performance ratio. New configurations with improved
energy efficiency, better permeation flux or smaller foot print have started to be
looked into, such as material gap membrane distillation (MGMD), multi-effect
membrane distillation (MEMD), vacuum-multi-effect membrane distillation (V-
MEMD) and permeate gap membrane distillation (PGMD).
1. In direct contact membrane distillation (DCMD), water with lower temperature
than the liquid feed, is used as condensing medium in the permeate side. In this
configuration, the liquid in both sides of the membrane is in direct contact with the
hydrophobic microporous membrane. DCMD is the most process to set up in a
laboratory. Direct contact of the membrane with the cooling side and poor

41
Table 6, Advantages, disadvantages and application areas for MD configurations (Kullab, 2011)
conductivity of the polymeric material though, causes heat losses throughout the
membrane. So in DCMD, the thermal efficiency (fraction of heat energy used only for
evaporation), is relatively smaller than the other three configurations.
2. In air gap membrane distillation (AGMD), water vapour is condensed on a cold
surface that has been set apart from the membrane with a stagnant air gap which
reduces the heat losses.
3. In sweeping gas membrane distillation (SGMD), we use a cold inert gas in the
permeate side for sweeping and carrying the vapour molecules to outside the
membrane module where the condensation takes place. Although we have a
relatively low conductive heat loss with a reduced mass transfer resistance, the extra
operational costs of the external condensation system make SGMD the least applied
configuration.
4. In vacuum membrane distillation (VMD), the process is driven by applying
vacuum at the permeate side. The applied vacuum pressure is lower than the
equilibrium vapour pressure. Therefore, condensation takes place outside of the
membrane module.
Each of the MD configurations has its own advantages and disadvantages for a given
application (Table 6),

42
Fig.30, Different resistances to heat and mass transfer in (a) DCMD, (b) VMD, (c) AGMD, (d) SGMD (Andreoli et al., 2007)
In Fig.30 is illustrated the heat and mass transfer in the 4 configurations,
Recently, MEMSYS (memsys.eu) patented an integration of vacuum with multi-
effects in their module designing for MD. V-MEMD is a modified form of VMD that
integrates the concept of state-of-the-art multi-effect distillation into the VMD. In
the process, the vapors that are produced in each stage are condensed during the
subsequent stages. Vapors are generated in steam raiser working under vacuum by
exchanging the heat provided by external source. The vapors are introduced in the
first stage where these are condensed by exchanging the heat with feed via a foil.
The vapors generated in the first stage are transported through the membrane and
collected on the foil in the second stage. The flow of different streams in a single
stage has been illustrated in Fig. 31.

43
Fig.31, Schematic illustration of streams in V-MED module (Andreoli et al., 2007)
Fig.32, Frame and stages used by MEMSYS (i) a simple frame, (ii) single stage consisting of welded frames and covering plates, (iii)
multiple stages (Andreoli et al., 2007)
The company claims that this configuration has a high feed to output ratio which is a
important parameter for industrial applications. A condenser is used to condense the
vapors generated in the final stage. The vapor pressure in each stage is less than its
preceding stage. A schematic diagram with the module fabrication is depicted in Fig.
32.

44
Fig.33, Variation of feed ammonia concentration in HFMC, DCMD and MDCMD (Dan Q et al., 2013)
3.16 Performance of MD in Ammonia Recovery
3.16.1 Comparison between MD Configurations
In a comparative study (Dan Qu et al., 2013), a modified direct contact membrane
distillation (MDCMD) with receiving solution in permeate was developed for
accelerating ammonia extraction from a water solution (Fig. 34). Its efficiency was
then compared to DCMD and to hollow fiber membrane contactor (HFMC). Also
there was researched the effects of feed pH, temperature, flow rate and
concentration on ammonia extraction efficiency and the permeate flux in MDCMD
process.
The developed MDCMD process had the highest ammonia stripping efficiency. The
ammonia removal efficiency of DCMD, HMC and MDCMD was 52%, 88% and 99.5%
within 105 min, respectively, proving MDCMD to have an advantage over the other
two methods and a good option as an alternative process.
In the MDCMD process, feed pH value was the parameter with the highest influence.
Increasing feed pH value was increasing ammonia removal efficiency as well as the
permeate flux, but only up to 12.20, after which it gave no noticeable effect.
Increase of feed temperature and velocity led to an increase of the ammonia
removal efficiency, ammonia mass transfer and the permeate flux. Initial feed
ammonia concentration didn't have a significant effect on ammonia extraction
efficiency (Fig.33).

45
Table 7, Properties of the PVDF membrane used in the experiment (Dan Q et al., 2013)
Fig.34, Experimental setup for ammonia removal (Dan Q et al., 2013)
Vacuum membrane distillation (VMD) had the highest mass transfer but the lowest
selectivity, while direct contact membrane distillation (DCMD) had the highest
selectivity and moderate mass transfer. The sweeping gas membrane distillation
(SGMD) gave moderate selectivity and the lowest mass transfer. Also, in a DCMD
process for ammonia stripping, water vapor as well as ammonia can both transfer
across the membrane to the permeate side due to the temperature difference,
which may lead to wastewater volume minimization.
The ammonia removal efficiency (R) could be defined as:
R= (1 - Ct/Co) * 100% (6)
The ammonia mass transfer coefficient (Ka) was determined experimentally as
follows:
Ka = Vf/At * ln(Co/Ct) (7)
Therefore, plotting ln(Co/Ct) vs. t yielded a straight line. And the ammonia mass
transfer coefficient, Ka, can be calculated from the slope of the line.

46
Fig.35, Variation of feed ammonia concentration and average permeate flux at different feed pH (Co=1.5 g/L, Tf=50oC, Tp=28C,
uf=0.5 m/s, up=0.1 m/s),(Dan Q et al., 2013)
The ammonia removal efficiency of DCMD, HFMC and MDCMD was 52%, 88% and
99.5% within 105 min, respectively.
Obviously, compared with the HFMC and DCMD, the MDCMD gave the highest Ka
and R with an acceleration of receiving solution to ammonia removal efficiency. Also,
higher feed temperature leads to a higher NH3 diffusion rate both in the bulk
solution and membrane pores which results in a higher mass transfer coefficient.
Effect of operating parameters in MDCMD process
1.Effect of the feed pH
Figure 35 illustrates the values of feed ammonia concentration and average
permeate flux versus time at different feed pH values. The average permeate flux
increased with increasing pH values till 12.2, and no significant changes of the
permeate flux were observed in the case of pH 12.2 to 13.2. It also can be seen from
Fig.35 that the ammonia concentration in feed was lowered more quickly at higher
pH values. The ammonia removal efficiency reached to 99.5% when the pH was
higher than 12.20. Fig.34 plots the Ka against feed pH values. The Ka increased from
1.89 to 6.29 × 10−5
m/s with increasing pH value from 10.0 to 12.2.However, as the
pH value increased to 13.2, the ammonia transfer coefficient was only raised from
6.29 to 6.74 × 10−5
m/s.

47
Fig.36, Effect pf feed pH on ammonia mass transfer coefficient (Co=1.5 g/L, Tf=50
o
C, Tp=28
o
C, uf=0.5 m/s, up=0.1 m/s),
(Dan Q et al., 2013)
The above results showed that the positive effect of pH on average permeate flux
and Ka was decreased with rising pH and there was no significant increase when pH >
12.20. That was explained with the ammonia dissociate equilibration in water
solutions, which is illustrated in Eq.2.
NH3 + H2O ↔ NH4
+
+ OH-
Increasing pH value drives the equilibrium move towards the production of NH3, the
only form of ammonia that can be stripped. So we want a higher NH3 concentration
rather than the NH4
+
, which can give in higher ammonia removal rates. It was found
that the water vapor pressure having volatile NH3 components, was higher than that
of pure water, which could lead to an increased permeate flux. At some point
though, resistance caused by membrane would gradually become the dominant
factor of the mass transfer process with the feed side gaining more flux. This could
be why pH is less influential when the pH > 12.20.
2. Effect of the feed temperature
Fig.37 shows the feed ammonia concentration and the average permeate flux at
different feed temperatures. As we can see, the feed temperature had a significant
effect on the average permeate flux. For example, raising the feed T from 30 to 50 °C
created an increase of the permeate flux of about 250%. As T got higher, there was a
significant increase in the vapor pressure of the feed solution which consequently
increased the transmembrane vapor pressure difference and driving force.
Increasing feed T also favored the ammonia removal. Feed ammonia concentration
decreased more quickly at higher feed T. Ka at different feed T is depicted in Fig.38
and shows the positive effect of increasing feed T. Ka increased from 3.42 to 7.28*

48
Fig.37, Variation of feed ammonia concentration and average permeate flux at different feed T (Co=1.5 g/L, Tf=50
o
C,
Tp=28
o
C, 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=50
o
C, Tp=28
o
C, uf=0.5 m/s, up=0.1 m/s),
(Dan Q et al., 2013)
10−5
m/s when the feed temperature increased from 30 to 55 °C. High feed T had a
positive effect in NH3 diffusion both in the bulk solution and membrane pores, which
led to an increased mass transfer coefficient. Also, a higher content of volatile
ammonia was found in the feed solution due to the endothermic nature of
dissociation of ammonium ions.

49
Fig.39, Variation of feed ammonia concentration and average permeate flux at different feed flow rates (Co=1.5 g/L,
Tf=50
o
C, Tp=28
o
C, uf=0.5 m/s, up=0.1 m/s), (Dan Q et al., 2013)
3. Effect of feed flow rate
As depicted in Fig.39, increased flow rates led to a higher diffusion of NH3 from the
feed bulk to the membrane surface and had a positive effect in the mixing condition
of boundary layer which led to a higher ammonia and water vapor mass transfer.
4. Effects of aeration in gas-permeable membrane ammonia recovery
In another study using swine manure with gas-permeable membranes (M.C. García-
Gonzalez et al., 2015), ammonia was successfully separated and recovered operating
with aeration and nitrification inhibition (Fig.40). The aeration reacted with the
natural alkalinity, which released OH-
and increased the pH of the bulk solution over
8.5. That change promoted gaseous NH3 release from the manure and an increased
permeation through the submerged membrane. The overall NH4
+
recovery obtained
with the aeration approach was 98% (Fig.41). Aeration also substituted for the large
amounts of alkali chemical that were needed to produce the same effect and
reduced the operational costs by 57%.

50
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 R
2
are represented. The error bars are the standard deviation of duplicate experiments (M.C.
García-Gonzalez et al., 2015)

51
Fig.42, Experimental set-up for fermentative wastewater treatment with AMBBR and subsequent distillation using hydrophobic
microfilters. The biogas was analyzed in a gas chromatograph (Hyun-Chul Kim et al., 2015)
3.16.2 Performance of (PTFE), (PVDF) and (PP) membranes in ammonia
recovery
In a study researching MD combined with an anaerobic moving bed biofilm reactor
(AMBBR) for treating municipal wastewater for the treatment of domestic
wastewater (Fig.42), (Hyun-Chul Kim et al., 2015). The viability of using a membrane
separation technique for post processing of anaerobic bio effluent was studied.
Three different hydrophobic 0.2 mm membranes made of polytetrafluoroethylene
(PTFE), polyvinylidene fluoride (PVDF), and polypropylene (PP) were used in this
study.
From the three different hydrophobic membranes studied, the highest permeate flux
was recorded for the PTFE membrane. A gradual but noteworthy decline of the flux
was seen in the MD treatment with the PTFE membrane for which the permeate flux
decreased up to 84% of the initial value after the 45h distillation. The initial
permeate flux of the PVDF and PP membranes had 81% and 41% respectively. A
longer-term treatment took place, using the MD module with a flat-sheet PVDF
membrane for the reuse of anaerobically treated wastewater. Waste heat leaving
the AMBBR was used as the dominant factor for the mass transfer in the MD
process. The characterization of effluent organic matter (EfOM) using liquid
chromatography - organic carbon detection (LCOCD) verified that almost all of the

52
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.

Ammonia Recovery using membrane distillation

Ammonia Recovery using membrane distillation

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