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Nitrogen recovery from nitrogen rich wastewaters
and slurries by thermal treatment processes
MSc Thesis: Christos Charisia...
Contents
1. Introduction 1
2. Background 2
3. State of Knowledge 3
3.1 What is Digestate? 3
3.1.1 Digestate characteristic...
3.11 Economics of digestate processing for nutrient recovery 29
3.12 Design of a Multi Criteria Decision Tool 31
3.12.1 Co...
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...
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
processes, Christos Charisiadis 2015
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment
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Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
Ammonia Recovery using membrane distillation
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Ammonia Recovery using membrane distillation

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A short description of thermal technologies for the recovery of ammonia from N-rich wastewaters and expirementing with membrane distillation for getting better results.

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Ammonia Recovery using membrane distillation

  1. 1. 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
  2. 2. 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. 3. 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
  4. 4. 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
  5. 5. 1 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 1. Introduction In Germany, biogas plants have reached approximately the number of 8,000 with installed electrical capacity of 3,750MW. To generate this amount of renewable energy, a mixture of crops and organic waste fractions from agriculture and industry respectively, is used. Converting these substrates to biogas, depending on the mixture and the operational and economical parameters, we often have to deal with huge amounts of digestate. After the conversion of carbon in the substrate to biogas (methane CH4 and carbon dioxide CO2), ammonia NH3 and phosphate PO4 -3 remain in the digestate. According to EU regulations for environmental protection and in particular that of groundwater, the use of digestate as fertilizer in arid lands is limited to the vegetation period and the present nitrogen balance. So the digestate has to be kept in storage which due to its toxicity, is expensive. Therefore digestate treatment technologies have been developed for nutrient recovery along with the reduction of the digestate volume, in order to reduce the required storage expenses. The first step in reducing the digestate volume is through physical separation techniques that give a solid and a liquid fraction. In order to process further the liquid fraction, thermal methods have been developed, each with their product quality, drawbacks and costs. In this study the author will present each one of these methods and will try to suggest a multi-criteria model in order to search for the best choice according to each method's attributes. Finally, this Thesis will present alternative routes for ammonia recovery, by using membrane distillation and sequential evaporation with a laboratory experiment and suggest processing methods for converting the distillate that is produced by the digestate processing into commercial chemical products.
  6. 6. 2 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 2. Background Anaerobic digestion (AD) converts organic matter (substrate) into 1) biogas (methane CH4 and carbon dioxide CO2) and 2) digestate, a nutrient rich organic fraction. Biogas can be used as a source of renewable energy to generate electricity and heat to power the process, and the excess power is sold if there's the right infrastructure. The digestate contains the non digested organic fraction, water and nutrients such as nitrogen and phosphorus. The composition of the digestate depends directly to the input biomass. Digestate is mechanically separated into a 1) liquid fraction (water solution) and 2) a solid fraction (resilient organic matter). The nutrients in the solid fraction offers are hard to recover, because they are organically bound. On the contrary nitrogen (N), phosphorus (P), potassium (K), sulphur (S), organics and mineral salts, which are present in the liquid fraction, are far easier extracted with the right techniques. Currently the majority of AD facilities transport the digestate to local agricultural lands, to be used as an organic fertilizer (Fuchs et al., 2010). However the window for land application is limited to 1) agricultural and crop requirements (Orr, 2011), and 2) large AD plants, need a large nearby agricultural area to provide them with a secure and suitable market. If the application to the agricultural land is not viable, due to transport distances, legal or other restrictions, digestate can be used for land reclamation. The volume of the digestate is the dominant factor in the expenses for transportation and storage. The larger the volume, the higher the costs. Thus industry has tried to reduce the liquid fraction by thermal means. The conventional methods that are used so far have significant problems that range from excessive energy consumption to use of dangerous and expensive chemicals. Each has pros and cons which need to be used in a multi criteria model that will make the decision process easier and more clear to the user. So due to the individual faults of the conventional methods, the need has arise in the last years, for new methods to be created, that are equally or more efficient and without the drawbacks of the previous ones. In this Thesis, the author will explore the properties of aqua ammonia and present in theory and through a lab experiment, membrane distillation and sequential evaporation as alternative digestate thermal processing methods. Finally at the end of the processes we're left with a N rich distillate that it can be exploited to be converted into commercial chemical substances, usually for the production of fertilizers. The problem with most of the methods used is the utilization of acids during their process, making it hazardous and expensive. The
  7. 7. 3 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 1, Four major steps of the AD Process, (Madsen et al., 2011) author will try to present methods that not only convert the aqua ammonia without acids but are environmentally friendly by capturing the industrial CO2 and NOX effluent gases. 3. State of Knowledge 3.1 What is Digestate? Digestate is the product of anaerobic digested biodegradable materials. It is normally liquid, but it can also be a solid, stackable material when it is coming from, e.g. a dry state AD process. The AD substrate, can be a mixture of different substrates or a pure mono-substrate. The substrate decomposes without oxygen (anaerobic conditions), inside the closed digester for several weeks, in which time it is sequentially decomposed by microorganisms through a complex biochemical process. Figure 1 depicts the four major steps of AD: 1) decomposition of organic matter during hydrolysis, 2) formation of organic acids during acidogenesis, 3) formation of the main intermediate acetate during acetogenesis, and 4) formation of methane during methanogenesis from either acetate or carbon dioxide and hydrogen. Digested substrate is taken out from the tank as digestate and stored. Digestate has excellent plant fertilizer qualities, based on it being rich in N, P, K, S, various micronutrients and also organic matter. Digestate is normally applied as fertilizer without the need for any further processing.
  8. 8. 4 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Table 1, Substrate parameters influencing digestate composition, (Fuchs and Drosg, 2010) Table 2, Process parameters influencing digestate composition, (Fuchs and Drosg, 2010) 3.1.1 Digestate characteristics The physical and chemical characteristics of the digestate are depending on the 1) composition of the substrates (illustrated in Table 1 and 2) the operational parameters of the AD process (illustrated in Table 2). Literature (Holm-Nielsen et al., 1997; Chantigny et al., 2007; Muller et al., 2008; Tambone et al., 2010; Fouda, 2011) proves that, when compared with raw animal manures and slurries, digestate generally has 1) lower total solids (TS) and total organic carbon (C) content, 2) lower carbon to nitrogen ratio (C:N), 3) and lower viscosity. Although, pH and ammonium (NH4 + ) concentration are higher in the digestate compared to raw animal manures and slurries. 3.1.2 pH value The pH value of fresh digestate typically is in the range of 7.5 to 8.0 pH. The pH is mainly depending by the biochemistry of the AD process and the characteristics of substrates (ARBOR, 2013, WRAP, 2012). For example, the formation of ammonium carbonate ((NH4)2CO3) as well as the removal of CO2 as a result of the transformation of CO3 2− and 2 H3O+ to CO2 and H2O, result in increased pH (BiotecVisions, 2012). The
  9. 9. 5 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 2, Examples of the variation of nitrogen in the digestate of biogas plants with different substrate types, (a)TN concentration in kg/ton fresh matter (FM), (b) ammonium nitrogen as percentage of TN. Horizontally striped columns indicate digestate from mono-digestion of industrial by-products; and unstriped columns indicate digestate from typical waste treatment plants (Fuchs and Drosg, 2010) consumption of volatile fatty acids (VFA) during AD increases the pH. The pH is also rising with higher concentrations of basic cations like Ca2+ and K+ (ARBOR, 2013), and decreasing with the precipitation of carbonates such as calcite (CaCO3) and of iron phosphates (Hjorth et al., 2010). An increased pH leads to the degradation of foul smelling VFAs, which reduces odour emissions but on the other hand the degree of ammonia volatilization increases. Storage of digestate until field application should take place in closed storage tanks (manure storage tanks with flexible plastic coverage). 3.1.3 Nitrogen content The AD process degrades organic nitrogen compounds, releasing ammonium NH4-N, which is immediately bio-available for growing plants. The content of ammonium in digestate is directly related to the total N content in the substrate. The differences of nitrogen content in digestates coming from the AD of energy crops compared to digestate from organic waste and industrial by-products are depicted in Figure 2 (a) and (b). By looking at the figure, we can tell that nitrogen concentrations in energy crop AD plants are rather similar, whereas in biogas plants and co-digesting organic wastes, the nitrogen concentration varies significantly, mainly due to the variations of N contents. Additionally, processing parameters such as for e.g. the amount of fresh water and degree of recirculation, also influence the total nitrogen (TN) content. In mono-digestion of industrial by-products, the influence of nitrogen and sulphate concentration in the substrate is easy to tell.
  10. 10. 6 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 3.2 Nitrogen Recovery In the last decades we are dealing with pollution problems coming from organic waste streams and manure management (N eutrophication, nitrate leaching, nitrous oxide greenhouse gas emissions, and ammonia particulate). The main reason behind this, is the cycle of large N losses in agricultural systems and subsequent fossil-based synthetic N replenishment. Around 85% of reactive N (forms other than di-nitrogen gas, N2) is lost to the environment (waterways, atmosphere, etc.). Meanwhile the majority (95%) of the remaining 15% that enters the human organism is excreted and eventually lost to those same waterways and atmosphere (Galloway et al., 2004). To replace N lost from agricultural systems, industry converts non-reactive N2 to synthetic N fertilizer through energy-intensive and environmentally harsh processes; e.g. Haber-Bosch process (12 Kwh*kg*N-1 ; Sutton et al., 2009) (1.4-2.6 kg CO2*kg*N-1 ; Wood and Cowie, 2004). The unsustainable N cycle 1) consumes limited fossil fuel resources and 2) contributes to environmental pollution. Recovering N reduces losses of N to the environment while decreasing the demand for synthetic fertilizers. The methods for nutrient recovery from digestate are developing rapidly along with the technological advances, improving nutrient management in agriculture and in waste treatment systems. 3.2.1 Drivers for digestate processing for nutrient recovery Digestate as fertilizer/ soil conditioner in agriculture, horticulture, forestry etc. can be directly applied is possible after its removal from the digester tank without any further processing. However, digestate is rather diluted with respect to nutrients, which makes the costs of transportation relatively high compared to conventional fertilizer. Significant costs are also the investments in storage capacity, required by environmental regulations in many countries, like for e.g. in Denmark, Germany and France, where not only the nutrient input per hectare is restricted, but also the period of application is limited to the growing season. However, many crop cultivators agree that applying digestate as organic fertilizer compared to conventional fertilizer has synergistic effects. 3.2.2 Legal frameworks At EU level, the European Nitrate Directive 91/676/EEC restricts the yearly load of N, applied to agriculture . Livestock production is intensive, usually in areas with limited land available for manure application. This creates a permanent excess of nutrients, making such areas highly vulnerable, to surface and ground waters, pollution. The problem intensifies when animal feed is being imported to such a region, which makes efficient nutrient management even more crucial. The legal restrictions on the nutrient input per hectare require the excess nutrients to be recovered,
  11. 11. 7 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.3, Water distribution in sludge, (Arun S. Mujumdar, 2015) exported, and recycled outside the vulnerable areas. Thus digestate processing technologies have been developed, aiming at volume reduction and nitrogen removal. More recently, also concerns regarding P excess from manure application in many areas and high levels of phosphorus found in surface and ground waters have greatly increased demand for nutrient management and export of excess of nutrients. 3.3 Solid–liquid separation, the first step in digestate processing. 3.3.1 Moisture Distribution in Sludge The moisture in the digestate sludge can be as high as 99%. Figure 3 depicts the moisture distribution in the sludge. This distribution takes the following forms: 1) free moisture that is not attached to the sludge particles and can be removed by gravitational settling; 2) interstitial moisture that is trapped within the flocs of solids or exists in the capillaries of the dewatered cake and can be removed by strong mechanical forces; 3) surface moisture that is held on the surface of the solid particles by adsorption and adhesion; and 4) intracellular and chemically bound moisture. (Mujumdar, 2015) , The amount of water that can be removed is dependant on the dewatering process and the status of the water in the sludge. The water that can be removed by mechanical dewatering is usually termed as 'free water' and the rest as 'bound water'. The bound water is the theoretical limit of mechanical dewatering. The free water includes the free, interstitial, and partially the surface moisture. The bound water includes the chemically bound moisture and partially the surface moisture.
  12. 12. 8 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 4, Distribution of the main components after the solid-liquid fractions seperation (Bauer et al., 2009) 3.3.2 Solid–liquid Separation Digestate processing technologies technologies are comparable to existing technologies from manure processing, sewage sludge treatment, and wastewater treatment. Digestate processing can be 1) partial, mostly for reducing the volume, or 2) complete, dividing the digestate to pure water, a solid bio-fertilizer fraction, and fertilizer concentrates. The first step in digestate processing is the separation of the solid from the liquid phase. The solid fraction can be directly applied as bio-fertilizer in agriculture or it can be composted/ dried for intermediate storage and transport. To improve the solid–liquid separation, flocculation or precipitation agents can be added. Typical ranges for the distribution of the main components of the solid and the liquid fraction are given in Figure 4. The major fraction coming out from the first separation step is the liquid fraction. Depending on the characteristics of the digestate and the efficiency of the separation itself, its composition widely varied. Frequently, a percentage of the liquid fraction is recycled to control the dry matter (DM) concentration of the input substrate (Resch et al. 2008). For the rest of the liquid fraction, there are a variety of recovery and treatment options. The advantage of solid–liquid separation can be that reduces the required leftover storage, nevertheless, treatment for further volume reduction and nutrient recovery can be applied. In most cases, these
  13. 13. 9 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 objectives will be achieved only through a sequence of several steps which can be relatively complex and therefore expensive. 3.3 Processing of the solid fraction The solid fraction which comes out of the solid–liquid separation has TS concentrations in the range of 20 – 30 %. It is partially stabilized so that is appropriately stored and direct applied as bio-fertilizer or soil improver in agriculture. However, it still contains biodegradable material and consequently microbial activity can still happen and odour emissions can occur. In order to get a stable and marketable bio-fertilizer product, the solid fraction needs further processing, which can be composting, drying or another form of stabilization. 3.4 Processing of the liquid fraction While the solid-liquid fraction separation uses relatively simple and cheap technologies, for further processing different methods and technologies are available, with various degrees of technical maturity, higher energy input, and higher investment and operating costs. For nutrient recovery, membrane technology, such as nano and ultrafiltration followed by reverse osmosis, can be used (Fakhru’l-Razi, 1994; Diltz et al., 2007). Membrane filtration produces a nutrient concentrate and purified process water (Castelblanque and Salimbeni, 1999, Klink et al., 2007). The liquid digestate can also be purified through aerobic biological wastewater treatment (Camarero et al., 1996). However, because of the high nitrogen content and low biological oxygen demand (BOD), addition of an external carbon source is often necessary in order to achieve satisfying denitrification. A further possibility for concentrating digestate is evaporation with excess heat from the biogas plant, combined heat and power (CHP) unit. In order to reduce the nitrogen content in the digestate, ammonia stripping (Siegrist et al., 2005), ion exchange (Sanchez et al., 1995) or struvite precipitation (Uludag- Demirer et al., 2005; Marti et al., 2008) are in use. Whatever technology is applied, advanced digestate processing in most cases requires high energy input and chemical reagents, like acid which are a significant expense. Along with increased investment costs for appropriate machinery, the treatment costs become considerable. An overview of viable digestate processing technologies is given in Figure 5.
  14. 14. 10 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.5, Overview of viable options for digestate processing (Fuchs and Drosg, 2013) Fig.6, Overview of the distribution of industrial scale applications for further treatment of the liquid fraction of digestate in Germany , Austria and Switzerland from 2009 (Fuchs and Drosg, 2013) As we can see above, a wide range of technologies are currently being used for digestate processing, depending on the boundary conditions. The most abundant approach is solid–liquid separation of digestate, where, depending on the consistency of the digestate, screw presses or centrifuges are mostly used. Solid– liquid separation can be improved by the addition of precipitating agents. For further processing of the liquid fraction, membrane purification is the only process that can achieve a degree of purification that can allow direct discharge. It is also among the most frequently applied approaches in more complex digestate processing facilities in Germany, Switzerland, and Austria (Figure 6).
  15. 15. 11 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 However, membrane purification is the most expensive technology, with high potential for optimization in large-scale applications. If excess heat is available, evaporation is an interesting option, although it gives rise to some controversy. In Germany digestate processing technologies using heat (e.g. evaporation, drying) are being used more frequently due to the subsidies for waste heat utilization at biogas CHPs. Evaporation of the liquid fraction of digestate is a rather robust technology, however, if the liquid fraction contains considerable amounts of fibrous material it is necessary to remove this beforehand to avoid clogging in the heat exchangers. Other technologies that are less commonly applied include ammonia stripping, ion exchange, solar drying, etc. 3.4 Thermal Processing of the Digestate's Liquid Fraction 3.4.1 Thermal Drying Background (Andreoli et al., 2007) The thermal drying process is one of the most efficient and flexible ways of decreasing the moisture content from dewatered organic industrial and domestic sludges. Thermal drying may be used for different sludge types, either primary or digested, and a feeding sludge solids content of 15%–30% is recommended. Under ideal conditions, 2,744 kJ (0.76KW) of energy are needed to evaporate 1 kg of sludge water , and it is usual to increase this value up to 100% for normal operational conditions. The total energy demand will depend 1) on the efficiency of the selected equipment and 2) on the type of the processed sludge. Mainly the energy comes from external sources, such as fuel oil, natural gas etc. Biogas generated in AD may provide an alternative energy source. As the heating power of biogas is 22 MJ/L and burners can usually work at 70% efficiency, under ideal conditions, 0.17 liters of biogas are required to evaporate 1 kg of water. Besides this, energy losses (through walls, air, etc.) must also be accounted for, together with the energy required to increase the sludge temperature to slightly above 100 ◦C, when the evaporation process starts. The suppression of the biological stabilization stage significantly reduces capital costs, and favors the production of pellets with high organic matter content and heating value, which make the product marketable in agriculture or as fuel. The main advantages of sludge thermal drying are: 1) significant reduction in sludge volume; 2) reduction in freight and storage costs of the sludge; 3) generation of a stabilized product suitable to be easily stocked, handled and transported; 4) production of a virtually pathogen-free final product; 5) preservation of biosolids fertilizing properties; 6) no requirements of a special equipment for land application;
  16. 16. 12 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.7, Principles of drying processes, drying by convection (left) and drying by contact (right) (Fuchs and Drosg, 2010) 7) sludge is suitable for incineration or landfilling; 8) product may be put into sacks and distributed by retail dealers. The main limitations of thermal drying processes are: 1) production of liquid effluents; 2) release of gases into the atmosphere; 3) risk of foul odours and disturbing noise The major types of thermal drying systems are: • Direct contact dryers (convection): where hot air has direct contact with the sludge, drawing away moisture, gases and dust (Direct dryers are typically rotary- drum, flash, moving-belt dryers, or centridryer types.) • Indirect contact dryers (conduction): where heat is transmitted through heat exchange plates (Indirect dryers are thin-film, rotary-disc, or rotary-tray dryers.) Both processes are illustrated in Figure 7, In convection, we have heat transfer by direct contact of the wet sludge and the hot gases. The heat of the inlet gas provides the latent heat required for evaporating the liquid from the sludge. The vaporized liquid is carried by the hot gases. Under equilibrium conditions of constant-rate drying, mass transfer is proportional to 1) the area of wetted surface exposed, 2) the difference between the water content of the drying air and saturation humidity at the wet-bulb temperature of the sludge–air interface, and 3) other factors, such as velocity and turbulence of drying air expressed as a mass transfer coefficient (Tchobanoglous et al. 2003). Direct dryers are the most common type used in thermal drying of sludge. Flash dryers, direct rotary dryers, and fluidized-bed dryers use this method. In conduction, we have heat transfer by contact of the wet sludge solids with hot surfaces. A metal wall separates the sludge and the heating medium (usually, steam or oil). The vaporized liquid is removed independent of the heating medium. Indirect dryers for drying municipal sludge include horizontal paddle, hollow-flight or disk dryers, and vertical indirect dryers. (Turovskiy & Mathai, 2006).
  17. 17. 13 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 8, Schematic flow of a Rotary Dryer (Frischman, 2012) Liquid effluent is usually less than 1% of the total flow and can be recycled to the plant headworks. Both systems require equipment for enclosure and treatment of the water vapor and the dust which are released from the dryers to avoid odour and particle emissions to the atmosphere. Indirect processes produce pellets with up to 85% solids concentration. For solids contents higher than 90% and possible production of organo-mineral fertilizers, direct drying processes should be preferred. The drying cost varies among different technologies chosen with a range of US $65- 80/ton DS. Utilizing the waste heat from the burning of sludge is reducing the cost significantly. For example, flue gas drying preceding incineration can save as much as 60% on the cost compared with direct incineration in large sludge plants. Similarly, mechanical compression of the vapors generated from indirect dryers can improve the energy efficiency considerably. Cogeneration-sludge drying units might be an economically attractive option to consider in large wastewater treatment plants (Mujumdar, 2015) Sludge drying is not an isolated issue. It has to be addressed along with other economical, environmental, and safety concerns. Over the last decade, there has been a significant increase in investment in environmental protection including for sludge processing. Innovative drying technologies with higher thermal efficiencies, lower emissions, less operator involvement, cheaper capital costs, and better final products are needed by the market. 3.4.2 Direct contact dryers a. Rotary Drying (Frischman, 2012) The design of a Rotary Dryed is illustrated in Figure 8, In a rotary drier digestate fibre is coming in contact with hot gases (convection). The rotary drier consists of a cylindrical drum which is rotated about its axis. Flights within the drying drum pick up and cascade the digestate. The drum is mounted on a
  18. 18. 14 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 9, Schematic flow of a Belt Dryer (Frischman, 2012) slight slope from the horizontal to transport the dried product along its length. The feed to the drier is blended with dried product to give a feed of approximately 65% dry solids (DS) to improve movement within the drum. Waste gases are passed through a cyclone to recover solids before further treatment, with large amounts of ammonia may contained within the exhaust gas stream. Final product DS of up to 95% can be achieved. Screening can be added to give a homogenous product pellet size. A rotary drying system in operation in Louisville, Kentucky, has its primary sludge anaerobically digested first and then blended with thickened waste activated sludge. The sludge mixture is then dewatered in centrifuges to about 26% solids and fed into four drying trains, each comprising a rotary drum dryer. Each dryer is sized to evaporate water from dewatered sludge at a rate of 8500 kg/h. Total installed evaporative capacity is 34 metric tons/h. The methane generated from the digesters provides half of the energy required by the dryers. Heat recovered from the dryers is used to heat the anaerobic digesters. (Turovskiy &Mathai, 2006) Pros: 1) Reduced volume of digestate for transport and storage; 2) Improved marketability as a fertilizer/soil conditioner; 3) Effective pathogen kill. Cons: 1) High energy requirement; 2) High temperature operation; 3) Large capital investment; 4) Reduced nutrient content of final product; 5) Gas treatment required; 6) Risk of explosive atmosphere within drying plant. b. Belt Drying (Frischman, 2012) Belt dryers are used in the water and wood pulp industries. The viability of this technology is be dependent on the installation and the end use of the dried product. Figure 9, illustrates the basic concept design, In a belt dryer digestate is contacted with hot gases (convection). The digestate fibre is evenly distributed over the drying belt by an extruder. The extruder produces digestate strands in order to increase surface area and provide a uniform size. The drier belt passes through a series of successive chambers of increasing temperature. At the end of the belt digestate is dropped onto a second belt which runs back
  19. 19. 15 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 10, Schematic flow of a Solar Dryer (Frischman, 2012) through the drier, underneath the first belt, to complete the drying process and cool the product. Dried product is discharged from the drier at the end of the second belt. Product is discharged at up to 90% DS and at a temperature below 40°C. As the digestate is not agitated during the drying process the risk of creating an explosive dust atmosphere within the drier is significantly reduced. This improves the safety and operability of the process. Pros: 1) Reduced volume of digestate for transport and storage; 2) Improved marketability as a fertilizer/soil conditioner; 3) Effective pathogen kill. Cons: 1) High energy requirement; 2) Large capital investment; 3) Reduced nutrient content of final product. c. Solar Drying (Frischman, 2012) This technology provides a low operational expenditure (Opex) (the day-to-day management costs) solution for dewatering digestate, however it requires a large land area. Most operational plants of this type are located in warm climates. Figure 10 depicts the basic design of a solar system, Solar drying uses a combination of forced ventilation and solar energy to de-water digestate. Waste heat from a CHP can also be used via underfloor heating. The feed to the process can be fed with either whole digestate or de-watered fibre. The digestate is fed into greenhouses where it is distributed across the drying bed. Digestate is turned and ventilated to increase efficiency and reduce odours. The greenhouses operate as a batch or semi-continuous process. The final product is a dried digestate exceeding 50% dry solids. Pros: 1) Increased concentration. 2) Reduced transport volume; 3) No liquor treatment required. Cons: 1) Large surface area required. 2) Cold climates may restrict application (most current applications are in Spain or Southern France).
  20. 20. 16 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 11, Design of a Flash Drying System (Mujumdar, 2015) d. Flash Dryer (Mujumdar, 2015) Flash drying is the fast removal of moisture by spraying or injecting the sludge into a hot gas stream. In a flash drying system (see Figure 11) the wet sludge cake is blended with previously dried sludge in a mixture to improve pneumatic conveyance. The blended sludge and hot gases from the furnace at 704°C are mixed ahead of a cage mill, and flashing of the water vapor begins. Gas velocities on the order of 20 to 30 m/s are used. The cage mill mechanically agitates the sludge–gas mixture, and drying is virtually complete by the time the sludge leaves the cage mill, with a mean residence time of a few seconds. The dried sludge is conveyed to a cyclone pneumatically. The sludge at this stage has moisture content of only 8 to 10%. The sludge is then separated from the spent drying gases in the cyclone. Flash dryers have high energy and operation and maintenance costs. Today, other types of dryers are preferred.
  21. 21. 17 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 12, Schematic flow of Horizontal indirect dryer system (Mujumdar, 2015) 3.5 Indirect contact dryers (Mujumdar, 2015) Indirect dryers produce less gas, so most of their designs are closed loop with heat recovery and odour removal units. Because indirect dryers depend on heat being transferred from a heated surface and the dewatered sludge is still relatively wet (around 25% solid content for activated sludge), the interfacial behavior of the sludge and the heated surface is an important issue. While there is no air flow to disperse or disintegrate the wet sludge, mechanical agitation has to be designed to prevent the heating surface from being fouled, especially in the sticky zone with the solid content ranging between 55% - 70%. This technology has been established in the processing of other products. For sludge processing, usually a horizontal agitated thin film evaporator is selected. Its drying rate lies between 20 and 160 kg/m2 /h. When higher final solid contents are desired, a rotary paddle or disc dryers may be used either alone or as the second stage following a thin-film evaporator. Indirect drying with simultaneous sludge drying and pelletizing emerged in the last decade in Europe as a result of environmental and energy conservation concerns. a. Horizontal Indirect Dryer Horizontal indirect dryers for drying municipal wastewater sludge include the paddle dryer, hollow-flight dryer, and disk dryer. Figure 12 is a schematic diagram of a horizontal indirect dryer. The dryer consists of a horizontal jacked vessel with one or two rotating shaft fitted with paddles, flights, or disks which agitate and transport the sludge through the dryer. The heat transfer medium (usually, steam) circulates through the jacketed shell and through the hollow-core shafts and hollow agitators (paddles, flights, or disks). A weir at the discharge end of the dryer ensures complete submergence of
  22. 22. 18 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 13, Vertical indirect dryer by Pelletec (Mujumdar, 2015) the heat transfer surface in the material being dried. The steam is discharged as condensate after transferring its available energy to the sludge. Dryers that use hot water or oil as the heat transfer medium are constructed internally in a manner different from those required for steam. Dewatered sludge is fed into the vessel continuously, with or without mixing with any recycled dried product. The transfer of heat from the heat transfer medium raises the temperature of the sludge and evaporates the water from the sludge solids surface. The water evaporated is transported out of the dryer by low-volume sweep gases or exhaust vapors. If dried product is mixed with dewatered sludge, the moisture of the feed sludge can be reduced by 40 to 50%. The blending prevents agglomeration and fouling of the heat transfer surface. Dryers that dry unblended feed sludge should have internal breaker bars and must provide enough horse power to turn the agitator shafts to break up the clumps. Horizontal indirect dryers are capable of drying sludge with less than 10% moisture. b. Vertical Indirect Dryer A vertical indirect dryer, such as the Pelletech dryer shown in Figure 13, dries and pelletizes sludge simultaneously. It is a vertically oriented multistage unit that uses steam or thermal oil in a closed loop as the heat transfer medium to achieve a dry solids content of 90% or more. Dewatered sludge cake blended with dried product is fed at the top inlet of the dryer. The dryer is equipped with several trays heated by the heat transfer medium. The dryer has a central shaft with attached rotating arms. The rotating arms are
  23. 23. 19 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 14, Schematic flow of a Surface Scraped Heat exchanger (Frischman, 2012) equipped with adjustable scrapers that move and tumble the sludge in thin layers from one tray to another in a rotating zigzag motion until it exists at the bottom as a dried pelletized product. The process minimizes the formation of dust and oversized chunks. The dryer’s exhaust consists of water vapor, air, and some pollutants. After the water vapor is condensed, only a small amount of gases, mainly moist air, remains to be treated. These gases are vented from the dryer to an odor control unit for thermal destruction of odor-causing compounds. 3.6 Evaporation If we want to concentrate the digestate or increase its dry solids content, we can use evaporation. Evaporation uses thermal energy (heat) to release the moisture from the digestate. However, unlike the drying techniques, evaporation preserves the nutrients and a percentage of the moisture. Evaporation is typically used for liquor or whole digestate treatment. The final solids concentration is dependent on the desired product, but concentrations of up to 20% DS can be achieved. High temperatures will cause ammonia to be released, this can be solved by decreasing the pH of the digestate, usually with acid dosing, before the evaporation. This allows the digestate liquor to be converted into a concentrated fertilizer. a. Surface Scraped Heat Exchanger (HRS) (Frischman, 2012) This technology provides a viable method for treating digestate liquor and producing a balanced fertilizer product. Consideration will need to be given to the acidic nature of the final product and the affect this may have on agricultural appliance. Figure 14 illustrates the schematic flow, Evaporation uses waste heat from the CHP. Surface scraped evaporators are designed not to be affected by fouling issues by the evaporation of the digestate. The evaporators use a shell and tube configuration. The interior surface of the heat exchanger tubes is constantly cleaned by internal scrapers to reduce fouling and increase heat transfer efficiency. The digestate liquor is dosed with acid prior to evaporation to prevent ammonia loss within the evaporator. The volume of acid
  24. 24. 20 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.15, Schematic flow of Gasification (Frischman, 2012) dosed is dependent on the digestate and the desired retention time. Within the evaporator the liquor is concentrated to approximately 20% dry solids. This concentrate can then be mixed with the (previously) separated digestate fibre to produce a nutrient rich solid fertilizer. Trials have indicated that the condensate from the process is suitable for direct discharge to ground water, although further treatment may be required for certain applications. Alternatively it can be recycled as process water. Pros: 1) Reduced transport volume; 2) Potentially no further treatment of condensate required; 3) Concentrated nutrient rich product; 4) Use of heat eligible for RHI. Cons: Acidic product may limit available land bank. 3.7 Gasification (Frischman, 2012) In the gasification process, the oxygen supply is limited to enable partial combustion of organic matter within the feed in order to produce a synthesis gas (syngas). Syngas is a mixture of mainly carbon monoxide and hydrogen, which can be burnt to produce energy (Perry, 1997). For the process to operate efficiently, the feed digestate must have a low moisture content and ideally be in a dry pelletized form. In Fig.15, we have the schematic flow of the process, Gasification is applied to convert organic matter to a mixture of gases consisting mostly of carbon monoxide and hydrogen, known as syngas. Reactions take place at high temperatures with carefully controlled amounts of oxygen, air or steam. The syngas can be burned in a gas engine to produce heat, and the ash/ char from the process can be used for road construction, production of concrete, or sent to landfill. Gasification of traditional fuels such as wood and coal is well established, and the process has also been used for municipal solid waste. Full scale gasification of dried sewage sludge has also been shown to be economic. However the use of gasification for digestate is not well documented. During digestion, most of the organics have already been released, making the value of the digestate as a fuel for gasification, relatively low. Digestate must be dried and ideally
  25. 25. 21 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Table 3, Calorific power of different sewage sludge (Andreoli et al., 2007) pelletized before it can be gasified, adding an additional energy demand on the process. Pros: 1) Volume reduction; 2) Destruction of pathogens and toxic compounds; 3) Renewable heat and energy generation. Cons: 1) High operating cost; 2) Complex operation; 3) Ash disposal to landfill; 4) Loss of fertilizer potential. 3.8 Incineration (Andreoli et al., 2007) Incineration provides the greatest volume reduction. The remaining ashes volume is usually less than 4% of the dewatered feed sludge volume. Incinerators can use sludge from several treatment plants (Table 3) and are usually designed with capacities higher than 1 ton/h. Incineration destroys organic substances and pathogenic organisms through combustion, using excess oxygen. Incinerators must use sophisticated filter systems to significantly reduce pollutant emissions. Gas emissions released are regularly measured to ensure operational efficiency and safety. Incinerator design requires detailed mass and energy balances. Despite the high organics concentration in dewatered sludge, combustion is only autogenous when solids concentration is higher than 35%. If need be, we can use auxiliary fuels, such as boiler fuel with low sulphur content. The calorific value of sludge crucial in the amount of fuel consumption. Products from complete combustion of sludge are water vapour, carbon dioxide, sulphur dioxide and inert ashes. There are two types of incinerators that are currently in use for sewage sludge: • multiple chamber incinerator • fluidised bed incinerator A multiple chamber incinerator is divided into three distinct combustion zones. The higher zone, where final moisture removal occurs, the intermediate zone where
  26. 26. 22 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 combustion takes place and the lower or cooling zone. Should supplementary fuel be required, gas or fuel oil burners are installed in the intermediate chamber. A fluidized bed incinerator consists of a single-chamber cylindrical vessel with refractory walls. The organic particles of the dewatered sludge remain in contact with the fluidized sand bed until complete combustion. Fluidized bed incinerators are usually preferred over multiple chamber furnaces, due to lower operational costs and lower gas emission. However, despite the significant reduction in sludge volume, incineration cannot be considered a final disposal route, as residual ashes require additional processing. Inadequate ashes disposal could lead to possible leaching of metals and their absorption by plants. Landfill is the preferable destination for the ashes. Incineration is well established technology but investment and operating costs are high and viable only for large plants or where agricultural application of digestates is not possible because of the digestate quality or land bank availability. Pros: 1) Volume reduction; 2) Destruction of pathogens and toxic compounds; 3) Possible energy recovery. Cons: 1) High operating cost; 2) Complex operation. 3) Potential environmental impact of residuals (exhaust air); 4) Loss of fertilizer potential; 5) Public perception. a. Multiple Chamber Incinerator (Turovskiy &Mathai , 2006) The flowchart of a system with a multiple hearth furnace is presented in Figure 16. The furnace shell is a vertical steel cylinder 6 to 8 m in diameter lined internally with refractory brick or heat-resistant concrete. The furnace is divided vertically into seven to nine refractory hearths. A vertical rotating shaft passes through the center of the furnace, to which the horizontal frames of the rake mechanisms, made of heat-resistant cast iron, are affixed. Each hearth has material transfer openings located alternatively on the periphery of one hearth and in the center section of the adjacent hearthThe sludge moves by conveyors into the charging hopper and then onto the uppermost hearth of the furnace. The sludge is moved by rakes into the transfer openings, it drops to the next lower hearth, and continues its travel to the lower hearths. This provides continuous movement of the sludge mass in the opposite direction to the hot combustion air. The use of rake mechanisms to move and break up the clumps in the sludge intensifies the drying and combustion processes.
  27. 27. 23 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.16, Flowchart of multiple hearth incinceration (Turovsky & Mathai, 2012) Pros: 1) combusting both primary and secondary sludge, as well as trash from screens, scum from settling tanks and oil separators, dirty grit from grit chambers, and industrial wastes; 2) they are characterized by their simplicity of service and by the reliability and stability of operation during significant variations in the quantity and quality of sludge treated; 3) The furnaces can be installed in the open air. Cons: 1) high capital cost; 2) large area required; 3)presence of rotating mechanism in the high- temperature zone; 4) frequent failure of the rake devices. b. Fluidized Bed Incinerator (Turovskiy &Mathai , 2006) Fluidized-bed furnaces are adapted in the industry as a drying and roasting technology in a wide range of fields. The furnace, a vertical steel cylinder lined internally with refractory brick or heat-resistant concrete, consists of a cylindrical furnace chamber, a lower conical section with an impermeable air distribution grate, and dome-shaped crown. Heat-resistant quartz sand is placed on the grate.
  28. 28. 24 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.18, Flowchart of Pyreg (Frischman, 2012) Fig.17, Flowchart of fluidized bed incineration (Turovskiy &Mathai , 2006) The turbulent (fluidized) bed in the furnace is formed when air is blown through the distribution grate at a rate at which the sand particles move in a turbulent manner and appear to boil in the flow of gas. The design of the furnace depends on the composition of the sludge and thus its thermal balances for the combustion process, establishing the geometric dimensions of the furnace elements, and the quantities of auxiliary fuel, air, and exhaust gases. Figure 17 illustrates the flowchart of an incineration system with a fluidized bed furnace. 3.9 Pyrolysis (Frischman, 2012) Pyrolysis processes heat the digestate without oxygen, breaking down the organic content into char and syngas. For an efficient operation the feed digestate must have a low moisture content and ideally be in a dry pelletized form. Pyrolysis decreases the digestate mass by 70%, lowering the transport costs. The char produced can be used as a soil amendment or as a partial replacement for peat in growing media production. a. Pyreg Process Pyreg (Figure 18) is a proven technology for biomass feedstocks at full scale and digestate at pilot scale. Its size and modular design make it applicable to a range of plant sizes. Nevertheless the digestate must be dried before processing. The viability of Pyreg is dependent on the market for the biochar.
  29. 29. 25 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 By 'biochar' we meant the char coming from the pyrolysis of organic matter. In Pyreg approximately 70% of the feed mass is destroyed within the reactor and the remainder is a mix of carbon-rich char and ash. The possibility of installing a turbine to recover electricity from the exhaust gasses, and make the process entirely self-sufficient, is still tested. The possibility of increasing the biogas yield and ease of dewatering by mixing biochar with the digester feedstock is under study as well. Pros: 1) Carbon capture; 2) Soil amendment - reduced application of compound fertilizers; 3) Biochar value as a growing media constituent; 4) Renewable heat generation; 5) Small footprint; 6) Modular units. Cons: 1) Acceptance of the new technology; 2) Securing a market for biochar; 3) Establishing a PAS and QP for biochar; 4) Limited process experience; 5) High feed solids content required. 3.10 Wet Air Oxidation (Andreoli et al., 2007) Initially wet air oxidation was designed for the paper industry's residues treatment, but it was adapted for sewage sludge treatment later on. Wet oxidation is recommended when the sludge is 1) too diluted to be incinerated, and 2) too toxic to be given to the biological treatment plant. Low-pressure wet air oxidation is used to decrease the sludge volume and increase its dewaterability for thermal treatment. Intermediate and high-pressure oxidation are used to decrease sludge volume through oxidation of volatile organic matter into CO2 and water. Sludge organic matter, may be considered easily oxidizable (proteins, lipids, sugars and fibers, , which are approximately 60% of the total organic matter) or not easily oxidizable. Wet air oxidation takes advantage of the dissolved or particulate organic matter to be oxidized at temperatures in the range of 100◦C–374◦C (water critical point). The temperature of 374◦C stabilizes the water in a liquid form, even at high pressures. Oxidation is accelerated by the high solubility of oxygen in aqueous solutions at high temperatures. Wet air oxidation is efficiently applied in destruction of organic matter of effluents with 1%–20% solids concentration, allowing enough organic matter to increase the reactor internal temperature through heat generation without external energy supply. The upper 200 g/L (20%) solids concentration limit avoids the surplus heat to raise the temperature above the critical value, which could lead to complete evaporation of the liquid. Wet air oxidation of organic matter can be described by the following eq., CaHbOcNdSeClf + O2 → CO2 + H2O + NH4 + + SO4 2− + Cl− (1)
  30. 30. 26 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 19, Conventional wet air oxidation system with a vertical reactor (Andreoli et al., 2007) Equation (1), is exothermic, so the wet air oxidation process is able to produce sufficient energy to maintain a self-sustaining process. For this to happen, the influent COD concentrations should be higher than 10 g/L. The latest developments in technology and environmental legislations for the final sludge disposal in several countries, have rekindled the interest for sludge stabilization by wet air oxidation. Figure 19 shows a vertical reactor wet air oxidation system. The influent sludge is pumped towards the Wet Air Oxidation (WAO) reactor, passing through a heat exchanger to raise its temperature. The WAO reactor effluent goes through a phase splitter, routing the sludge for dewatering, whereas the liquid flows back through the heat exchanger, where part of the heat is transmitted to the incoming sludge. The gaseous effluent is released into the atmosphere after being treated by an electrostatic precipitator and filtered for solid particles and odorous substances removal. Wet air oxidation may use air or pure oxygen as oxygen supply. Compressed air as an oxidizing agent is usually found in wastewater treatment plants. Pros: 1) able to process sludge too diluted to be incinerated, and too toxic to be given to the biological treatment plant; 2) less external energy required; 3) solid produced is sterile, not putrescible, settles readily and may be easily mechanically dewatered. Cons: 1) foul odours; 2) corrosion of heat exchangers and reactors; 3) required power consumption to start-up the oxidation process; 4) high COD in liquid effluent;
  31. 31. 27 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 20, Dependence of the volatility of ammonia in water on temperature and pH (Fuchs & Drosg, 2010) 5) high metal content in residual ashes; 6) highly sophisticated, requiring skilled personnel for operation and maintenance. 3.10.1 Ammonia stripping and scrubbing (Fuchs &Drosg et al., 2015) In gas stripping volatile substances are extracted from a liquid by gas flow through the liquid, recovering N (in the form of NH3) from the liquid. The volatility of ammonia in an aqueous solution is increased by raising the temperature and the pH (as shown in Figure 20). Excess heat can be used for heating up the digestate and the pH can be increased by degassing to remove CO2 or by the addition of alkali. For ammonia stripping of digestate, there are two mainly applied processes: 1) air stripping and 2) vapour stripping. In air stripping (see Figure 21) heated digestate enters a stripping column. As a pre-treatment CO2 is removed, this lowers the buffer capacity. In a subsequent stripping column filled with packing material to increase surface area available for the ammonia mass transfer, ammonia is transferred from the liquid digestate to the stripping gas stream. After this, ammonia is recovered from the gas phase by a sulphuric acid scrubber, where a valuable commercial-grade ammonium sulphate fertiliser is produced. The cleaned gas can be reused in the stripping column. For vapour stripping, a much higher temperature is needed to produce the vapour. The setup can be comparable to Figure 21, only that there is no need for a final scrubber, as the ammonia can be directly condensed together with the vapour to produce ammonia water with a concentration of up to 25 – 35 % ammonia.
  32. 32. 28 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.21, Ammonia air stripping including CO2 removal and ammonia recovery by sulphuric acid scrubbers (Fuchs & Drosg, 2010) Fig.22, Details of a simplified in-vessel stirring process without stripping columns (Bauermeister et al., 2009) The usage of packed columns is a big problem for the process, because residual solids can clog the column, so a prior solid–liquid separation along with a high maintenance and cleaning effort. Nevertheless, a stripping method performed in simple stirred tank reactors (see Figure 22) has obtained positive results. A first large-scale facility using such a type of process principle is already in operation (Bauermeister et al., 2009). Pros: 1) Gaseous emissions free from toxins and particulates; 2) Greener image than incineration; 3) Total oxidation achieved; 4) Renewable heat and energy generation; 5) a standardised, pure nitrogen fertiliser product can be recovered. In addition, such a fertiliser liquid can be used to enrich other digestate fractions in digestate processing to a standardised nitrogen concentration, and this can increase their marketability. Cons: 1) Relatively high temperature and pressure; 2) The usage of packed columns is a big problem for the process, because residual solids can clog the column.
  33. 33. 29 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 3.11 Economics of digestate processing for nutrient recovery (Fuchs &Drosg et al., 2015) Detailed cost analysis of 6 digestate processing scenarios for a model biogas plant In a study conducted by KTBL (KTBL, 2008), a model biogas plant (50 % manure, 50 % corn silage) is considered. For the reference scenario (no digestate processing), it is assumed that about half of the digestate can be applied on agricultural land around the biogas plant and the other half has to be transported to remote areas. For the cost analysis both machinery and storage facilities are included. For the digestate products, a theoretical economic value is assumed according to their nutrient content (N, P2O5 and K2O). The following scenarios are investigated: I. Reference – direct land application II. Separation (screw press) and separate land application of solid fraction and liquid phase III. Separation (screw press) and drying of the solids with a belt dryer IV. Separation (decanter centrifuge) and purification of the liquid phase by ultrafiltration and reverse osmosis V. Separation (decanter centrifuge) and concentration of the liquid phase by evaporation VI. Separation (decanter centrifuge) and further treatment of the liquid phase by nitrogen removal (NH3- stripping and precipitation) The results of the study can be seen in Figure 23,
  34. 34. 30 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig. 23, Comparison of specific costs for digestate processing at a model biogas plane (KTBL, 2008) Fig.24, Comparison of cost ranges for specific treatment options versus costs for digestate disposal (Fuchs &Drosg, 2013) Figure 23 depicts that the viable implementation of digestate processing is depending strongly on the specific site. Local conditions cause big differences in the individual expenses and savings, e.g. for reduced storage facilities or revenues from the marketing of the processes products, causing large variations of the total costs. However, typical cost for different digestate treatment can be provided and compared with the respective costs of digestate disposal. An overview is provided in Figure 24,
  35. 35. 31 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 The transportation and disposal costs come from a study which looked into the economics of large scale industrial biogas plants (Baernthaler et al., 2008). Costs include capital and operational costs as well as a theoretical market value for the products. Most technologies only partially lower the amount of digestate for disposal, so the costs refer to the amount of digestate saved by each processing. 3.12 Design of a Multi Criteria Decision Tool The operational costs for to the treatment and disposal of digestate are often underestimated. Digestate treatment might appear as a minor within the system, but it has a major impact on the operational costs per year, and thus to the viability of a biogas installation. So modeling for the evaluation and optimization of the technology we choose is important. When we run an evaluation, we have to take different types of data and boundary conditions into account. We should 1) have reliable and real-case technical data from several methods of digestate treatment processing; 2) take into account the legal constraints and applicable values. In this context it is important to not only to the final disposal of the digestate,, but also on the composition of the input of the biogas-installation because it has a great impact on the way the digestate has to be be treated and disposed; 3)have the regional data because, the costs for the transportation and disposal, vary wildly, even within the same region; 4) get the input of the biogas installation, like the nutrients (N,P) or DM content because they will influence the final composition of the digestate. In our case, in order to simplify the problem, we won't work with any regional data. Also, due to the fact that the author could not get the technical data needed from the companies that manufacture the processing technologies, the Thesis will rely on a comparative study (Frishcman, 2012). For modeling the decision process, comparative programming is going to be used, due to its simplicity and speed of process. 3.12.1 Compromise Programming Compromise programming is a distance based method which determines how much, different alternatives are from an ideal point (Fig.25) . The smaller the gap, the higher the alternative is ranked. When the decision factors are chosen and their importance is weighted, we use Equation 2, di =[ Σak * (1 -nik)p ]1/p (2) where ak are the weights for the decision factors k (Σak = 1), nik is the performance value of the criterion k for alternative i and p is a compensation factor between 2
  36. 36. 32 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.25, Geometric scheme of distance based methods and 10. The larger the compensation factor, the more a bad performance influences the final result. 3.12.2 Selection of the best processing method The selection of the weights is a pretty complex problem since every country and every region values differently each and every one aspect. In my effort to design a general case, I have attributed the values (remember that the total sum of the values is 1.0): 1) Feed Solids (%) DS: 0,1. The importance to handle DS matter is important because otherwise a prior separation of the liquid and solid fractions is needed. 2) Reliability: 0,1. The uneventful operation of the treatment process is a major factor for the design of the installation. 3) Power Usage: 0,2. The energy consumption is of paramount important for the financial viability of every project and a chance for energy savings could make the cut between two choices. 4) Odour Potential: 0,05. Odours can be a serious problem to the workers and the locals around the installation. 5) Chemical Usage: 0,15. Chemicals in the industry can get pretty expensive and influence the yearly operational costs. 6) Noise: 0,05. Same as Sound 7) Hazard (T, P, Chem.): 0,15, same as Sound 8) Carbon footprint: 0,2. The legal constraints are getting tighter with each year and there are heavy fines for untreated effluents. For the selection of the compensation factor, I chose the value 5. For other values the results may differ wildly.
  37. 37. 33 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Table 4, Ranking of the available digestate treatment methods according to their individual score in each of the following categories; 1) Feed Solids, 2) Reliability, 3) Power Usage, 4) Odour, 5) Chemical Usage, 6) Noise, 7) Hazard (T, P, Chem.), 8) Carbon footprint. 5 is the highest score and 1 the lowest. Feed Solids (%) DS Reliability Power Usage Odour Potential Chemical Usage Noise Hazard (T, P, Chem.) Carbon footprint Method score Rank Rotary Drying 2 2 5 5 1 3 2 5 114.92 4 Belt Drying 2 2 5 5 1 3 3 5 115.10 3 J-Vap 1 4 3 5 1 1 2 2 63.31 9 Solar Drying 1 3 5 3 1 1 1 1 100.06 5 Surface Scraped Heat Exchanger (HRS) 1 4 3 3 3 1 1 3 71.31 8 Incineration 5 3 5 5 1 3 2 5 115.29 2 Gasification 4 5 5 5 1 3 2 5 115.38 1 Wet Air Oxidation (WAO) 1 3 3 3 3 3 4 3 76.05 7 Pyreg (slow pyrolysis) 4 3 3 5 5 1 3 1 80.94 6 Fig.26, Flow diagram from the Digestate Treatment System GNS as example of a system with 2 stripping reactors, (Bauermeiter et al., 2009) After formulating our algorithm we use a comparative study (Frischman, 2012) to get Table 4, So according to the comparative programming method for a compensation factor of 5, gasification is the best choice with pyrolysis being second. As stated before this is a totally objective result and real-life installation designs should take into account more and regional data. 3.13 Schematic Flow of a Digestate Thermal Process Here, is presented, the mass flow schematic of a digestate thermal process. Since a gasification process was chosen in chapter 3.12, the author will present a case from a study by Bauermeiter et al. (2009), from a modified stripping process by GNS (gns- halle.de), where the ammonium nitrogen is removed from the digestate by using only exhaust heat from the CHP without the use of bases, acids or external stripping media (Fig.26). The presented data are from the ANAStrip - Plant, BENAS of the Biogas Plant in Ottersberg from 2007/2008.
  38. 38. 34 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 3.14 Properties of Aqua Ammonia - Effects of pH and temperature As already stated in the previous chapters, each of the current nitrogen recovery methods suffer from its own drawbacks. Given that the financial budgets and legal regulations become more and more tight, we need to find alternative digestate processing methods which can provide us with more efficient procedures. In order to do that, we need to take a look again at the NH3 properties and its water solution, aqua ammonia. Ammonia is a colorless gas with a characteristic odour, highly soluble in water. Its aqueous solutions are alkaline and have a corrosive effect to metals and tissue. The pH in an aqueous solution of 0.1 M is 11.2, which is characteristic of a weak base (pKa = 9.3). Ammonia is used primarily 1) as a nitrogen source for producing fertilizers; 2) as refrigerant for producing nitric acid and other chemical reagents such as sulfuric acid, cyanides, amides, nitrites and intermediaries dyes; 3) as a nitrogen source in the production of synthetic fiber monomers and plastics; 4) as corrosion inhibitor in oil refining and other industries such as paper, extractive, food, fur and pharmaceutical industries. Some of the aforementioned processes produce wastewater with dissolved toxic gases, ammonia among others, in small concentrations. For example, in the production of urea fertilizers the wastewater created has dissolved ammonium in the range of 500 to 2000 ppm. The removal of this substance before the effluent release, is crucial for two important reasons:  It is extremely toxic to marine life (concentrations < 0.01 ppm have negative effects on fish, while 0.1 ppm can be lethal to other species).  It can be bio-oxidized by nitrifying organisms to nitrites and nitrates. The ammonia in wastewater effluents exists in two forms, as volatile ammonia (NH3) and ammonium ions (NH4 + ). Ammonia recovery processes try to maximize as possible the volatile ammonia component. That depends mainly on two factors, 1) the temperature (T) and 2) the pH value of the aqueous solution. The T has a direct effect on the solubility of ammonia in water, which is reducing as the temperature rises. For example, one volume of water can dissolve 1200 volume of ammonia at 0 ◦C (atm. pressure); at 20 ◦C this solubility falls down to 700 vol. of ammonia per 1 volume of water. Nevertheless, raising the temperature cannot release by itself all the dissolved ammonia in the solution, because much of its quantity dissociates right away in water to form unstable NH4 + solutions, based on the following chemical reaction:
  39. 39. 35 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Figure 27, Equilibrium Ammonia-Ammonium dependent on pH (Segura, 2012) NH3 + H2O ↔ NH4 + + OH- (K1 →, K2 ←) (3) At 25◦C, the equilibrium constants for this chemical reaction are, K1 = 1.8*10−5 towards NH4+ formation and K2 = 5.6*10−10 towards ammonia formation (K1/K2 ≈ 3.2×104 ). Increasing pH drives the equilibrium towards the side of ammonia and the aqueous solution has a higher ammonia concentration which means more volatile ammonia molecules instead of ammonium ions, which results in higher removal efficiencies. It should be mentioned that the vapor pressure of aqueous ammonia is higher than that of water. Raising the ammonia concentration in water, would increase significantly the vapor pressure of the solution. (Bourawi et al., 2007). For a 10 wt% ammonia solution, the total vapour pressure increases from 12.1 KPa at 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
  40. 40. 36 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Figure 28, Influence of temperature and pH on the ratio of ammonia partial pressure to total ammoniacal nitrogen (TAN) concentration (Zarebska et al., 2014) content in our distillation product, a moderate temperature combined with a high pH should be chosen (Fig.28). (Zarebska et al., 2014) From Fig.28 we can see that the NH3 vapor pressure to total ammonia nitrogen (TAN) ratio is influenced more from the pH rather than by temperature. 3.15 Membrane Distillation 3.15.1 Fundamentals of Membrane Distillation (MD) MD is mainly used for desalination purposes but many have gained interest for it as an advanced treatment of wastewater for water reuse. Researchers have also studied the viability of MD for brackish water desalination, process water treatment, and resource concentration for industrial uses. The MD process can use the advantages of anaerobic processes, while the mesophilic (20-45°C) or thermophilic (41-122°C) operating conditions that are usually needed to run fermentation processes (AD) can fulfill the waste heat requirement for the following MD process. MD has attracted attention for the removal of volatile compounds like ammonia because of its possibly low energy need. It can potentially recycle and reuse the industrial wastewater, and can process wastewater streams having a high temperature but relatively low levels of volatile organic compounds and ammonia. In MD the driving force for the transport of ammonia across the membrane is the difference in the partial pressure of ammonia on each side of the membrane. (Xie et al., 2009). In chapter 6 we explained why by raising the content of volatile ammonia in its aqueous solution, we increase its vapour pressure, thus making MD an ideal candidate process for the task of ammonia removal.
  41. 41. 37 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Currently, MD is one of the few membrane processes that use a thermal concept. Energy need is, theoretically, the same as in traditional evaporation processes. Nevertheless, the operating temperature is much lower than that of a conventional process because it is not required to heat the solution above its boiling point. The process can be operated at temperatures typically < 70 °C, and driven by low temperature difference (20 °C) of the hot and the cold solutions. Thus, low-grade waste or alternative energy sources (solar and geothermal) can be coupled with MD systems to drop the cost and raise the efficiency of the separation system. Consequently, MD might overcome not only the limits of thermal systems drawbacks but also the ones of the other membrane systems such as reverse osmosis (RO) or nanofiltration (NF). MD is not significantly affected by concentration polarization, so high recovery rate can be achieved, when compared with RO. MD has also all the other properties of the membrane system (easy scale-up, easy remote control and automation, no chemicals, low environmental impact, high productivity/size ratio, high productivity/ weight ratio, high simplicity in operation, flexibility, etc.) (E. Drioli et al., 2015) 3.15.2 MD Membranes MD process is mainly driven by the vapour pressure gradient which is created by a temperature difference across the membrane. As the latter is not a pure thermal driving force, MD can be operated at a much lower temperature than conventional thermal distillation processes. The hydrophobic nature of the membrane prevents water coming through due to surface tensions, unless a transmembrane pressure higher than the membrane liquid entry pressure (LEP) is applied. So, liquid/vapour interfaces are formed at the entrances of each pore. The transfer of water through the pores, is performed in three steps: (1) formation of a vapour gap at the hot feed solution–membrane interface; (2) transport of the vapour phase through the microporous system; (3) condensation of the vapour at the cold side membrane–permeate solution interface. MD membranes are usually made from 1) polytetrafluoroethylene (PTFE), 2) polyvinylidene fluoride (PVDF), and 2) polypropylene (PP). PTFE has the highest hydrophobicity, good chemical and thermal stability, and oxidation resistance. PVDF and PP also show good hydrophobicity and thermal/chemical resistance and can be easily developed into membranes with versatile pore structures.
  42. 42. 38 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Table 5. Specification of hydrophobic microfilters used in the membrane distillation (Hyun-Chul Kim et al., 2015) Recent studies have researched new membrane materials such as carbon nanotubes, fluorinated copolymer materials, and surface modified polyethersulfone (PES) that achieve good mechanical strength and high porosity. Hydro-repellent membranes allow for the complete rejection of non-volatile solutes (e.g., macromolecules, colloidal fraction, ionic species and so forth). Typical feed T is in the range of 30-60o 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.
  43. 43. 39 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 However, as the maximum pore size decreases, the mean pore size of the membrane decreases and the permeability of the membrane becomes low. 2. High permeability. The flux will “increase” with an increase in the membrane pore size and porosity, and with a decrease of the membrane thickness and pore tortuosity. In fact, molar flux through a pore is related to the membrane's average pore size and other characteristic parameters by: N≈(<ra >*ε)/(τ*δ) (5) where ε is the membrane porosity, τ is the membrane tortuosity, δ is the membrane thickness, 〈rα 〉 is the average pore size for Knudsen diffusion (when α=1), and 〈rα 〉 is the average squared pore size for viscous flux (when α=2). To obtain a high permeability, the surface layer that drives the membrane transport must be as thin as possible and its surface porosity as well as pore size must be as large as possible. In order to do so we see from eq. (5) that we need, in terms of molar flux, to maximize the membrane porosity and pore size, while minimizing the transport path length through the membrane, (τ, δ). However, thermal efficiency in MD increases gradually with increasing the membrane thickness and an optimization between the two requirements has to be researched. 3. Low fouling problem. Fouling is one of the main problems when using porous membranes. In the gas–liquid contact processes, since there is no convection flow through the membrane pores, the contactors are less sensitive to fouling. Nevertheless, when we have gas and liquid streams with large content of suspended particles (e.g. industry), we can get clogging of the membrane pores. Ideally pre- filtration is necessary for an efficient operation. 4. High chemical stability. The life sustainability of the membrane depends on any reaction between the solvent and membrane material, which could possibly affect the membrane matrix and surface structure. Liquids with high content of acid gases are corrosive, which make the membrane material less resistant. 5. High thermal stability. When operating with high temperatures, the membrane material may be affected by degradation or decomposition. Any change in the nature of membrane depends on the glass transition temperature Tg for amorphous polymers or the melting point Tm for crystalline polymers. The factors that increase the crystallinity (Tg/Tm) of a membrane can improve its chemical and thermal stability. For processes at high temperatures, fluorinated polymers are used due to their high hydrophobicity and chemical stability.
  44. 44. 40 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.29, Schematic representation of MD configurations (Onsekizoglu, 2012) 3.15.3 MD configurations MD processes can be categorized into four basic configurations (Fig. 29), depending on how vapour is condensed in the permeate side. In all four configurations there's direct contact of one side of the membrane with the feed solution. Direct contact membrane distillation (DCMD) is the most researched because of its simplicity. Vacuum membrane distillation (VMD) can be used for high output, while air gap membrane distillation (AGMD) and sweep gas membrane distillation (SGMD) have low energy losses and high performance ratio. New configurations with improved energy efficiency, better permeation flux or smaller foot print have started to be looked into, such as material gap membrane distillation (MGMD), multi-effect membrane distillation (MEMD), vacuum-multi-effect membrane distillation (V- MEMD) and permeate gap membrane distillation (PGMD). 1. In direct contact membrane distillation (DCMD), water with lower temperature than the liquid feed, is used as condensing medium in the permeate side. In this configuration, the liquid in both sides of the membrane is in direct contact with the hydrophobic microporous membrane. DCMD is the most process to set up in a laboratory. Direct contact of the membrane with the cooling side and poor
  45. 45. 41 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Table 6, Advantages, disadvantages and application areas for MD configurations (Kullab, 2011) conductivity of the polymeric material though, causes heat losses throughout the membrane. So in DCMD, the thermal efficiency (fraction of heat energy used only for evaporation), is relatively smaller than the other three configurations. 2. In air gap membrane distillation (AGMD), water vapour is condensed on a cold surface that has been set apart from the membrane with a stagnant air gap which reduces the heat losses. 3. In sweeping gas membrane distillation (SGMD), we use a cold inert gas in the permeate side for sweeping and carrying the vapour molecules to outside the membrane module where the condensation takes place. Although we have a relatively low conductive heat loss with a reduced mass transfer resistance, the extra operational costs of the external condensation system make SGMD the least applied configuration. 4. In vacuum membrane distillation (VMD), the process is driven by applying vacuum at the permeate side. The applied vacuum pressure is lower than the equilibrium vapour pressure. Therefore, condensation takes place outside of the membrane module. Each of the MD configurations has its own advantages and disadvantages for a given application (Table 6),
  46. 46. 42 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.30, Different resistances to heat and mass transfer in (a) DCMD, (b) VMD, (c) AGMD, (d) SGMD (Andreoli et al., 2007) In Fig.30 is illustrated the heat and mass transfer in the 4 configurations, Recently, MEMSYS (memsys.eu) patented an integration of vacuum with multi- effects in their module designing for MD. V-MEMD is a modified form of VMD that integrates the concept of state-of-the-art multi-effect distillation into the VMD. In the process, the vapors that are produced in each stage are condensed during the subsequent stages. Vapors are generated in steam raiser working under vacuum by exchanging the heat provided by external source. The vapors are introduced in the first stage where these are condensed by exchanging the heat with feed via a foil. The vapors generated in the first stage are transported through the membrane and collected on the foil in the second stage. The flow of different streams in a single stage has been illustrated in Fig. 31.
  47. 47. 43 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.31, Schematic illustration of streams in V-MED module (Andreoli et al., 2007) Fig.32, Frame and stages used by MEMSYS (i) a simple frame, (ii) single stage consisting of welded frames and covering plates, (iii) multiple stages (Andreoli et al., 2007) The company claims that this configuration has a high feed to output ratio which is a important parameter for industrial applications. A condenser is used to condense the vapors generated in the final stage. The vapor pressure in each stage is less than its preceding stage. A schematic diagram with the module fabrication is depicted in Fig. 32.
  48. 48. 44 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.33, Variation of feed ammonia concentration in HFMC, DCMD and MDCMD (Dan Q et al., 2013) 3.16 Performance of MD in Ammonia Recovery 3.16.1 Comparison between MD Configurations In a comparative study (Dan Qu et al., 2013), a modified direct contact membrane distillation (MDCMD) with receiving solution in permeate was developed for accelerating ammonia extraction from a water solution (Fig. 34). Its efficiency was then compared to DCMD and to hollow fiber membrane contactor (HFMC). Also there was researched the effects of feed pH, temperature, flow rate and concentration on ammonia extraction efficiency and the permeate flux in MDCMD process. The developed MDCMD process had the highest ammonia stripping efficiency. The ammonia removal efficiency of DCMD, HMC and MDCMD was 52%, 88% and 99.5% within 105 min, respectively, proving MDCMD to have an advantage over the other two methods and a good option as an alternative process. In the MDCMD process, feed pH value was the parameter with the highest influence. Increasing feed pH value was increasing ammonia removal efficiency as well as the permeate flux, but only up to 12.20, after which it gave no noticeable effect. Increase of feed temperature and velocity led to an increase of the ammonia removal efficiency, ammonia mass transfer and the permeate flux. Initial feed ammonia concentration didn't have a significant effect on ammonia extraction efficiency (Fig.33).
  49. 49. 45 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Table 7, Properties of the PVDF membrane used in the experiment (Dan Q et al., 2013) Fig.34, Experimental setup for ammonia removal (Dan Q et al., 2013) Vacuum membrane distillation (VMD) had the highest mass transfer but the lowest selectivity, while direct contact membrane distillation (DCMD) had the highest selectivity and moderate mass transfer. The sweeping gas membrane distillation (SGMD) gave moderate selectivity and the lowest mass transfer. Also, in a DCMD process for ammonia stripping, water vapor as well as ammonia can both transfer across the membrane to the permeate side due to the temperature difference, which may lead to wastewater volume minimization. The ammonia removal efficiency (R) could be defined as: R= (1 - Ct/Co) * 100% (6) The ammonia mass transfer coefficient (Ka) was determined experimentally as follows: Ka = Vf/At * ln(Co/Ct) (7) Therefore, plotting ln(Co/Ct) vs. t yielded a straight line. And the ammonia mass transfer coefficient, Ka, can be calculated from the slope of the line.
  50. 50. 46 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.35, Variation of feed ammonia concentration and average permeate flux at different feed pH (Co=1.5 g/L, Tf=50oC, Tp=28C, uf=0.5 m/s, up=0.1 m/s),(Dan Q et al., 2013) The ammonia removal efficiency of DCMD, HFMC and MDCMD was 52%, 88% and 99.5% within 105 min, respectively. Obviously, compared with the HFMC and DCMD, the MDCMD gave the highest Ka and R with an acceleration of receiving solution to ammonia removal efficiency. Also, higher feed temperature leads to a higher NH3 diffusion rate both in the bulk solution and membrane pores which results in a higher mass transfer coefficient. Effect of operating parameters in MDCMD process 1.Effect of the feed pH Figure 35 illustrates the values of feed ammonia concentration and average permeate flux versus time at different feed pH values. The average permeate flux increased with increasing pH values till 12.2, and no significant changes of the permeate flux were observed in the case of pH 12.2 to 13.2. It also can be seen from Fig.35 that the ammonia concentration in feed was lowered more quickly at higher pH values. The ammonia removal efficiency reached to 99.5% when the pH was higher than 12.20. Fig.34 plots the Ka against feed pH values. The Ka increased from 1.89 to 6.29 × 10−5 m/s with increasing pH value from 10.0 to 12.2.However, as the pH value increased to 13.2, the ammonia transfer coefficient was only raised from 6.29 to 6.74 × 10−5 m/s.
  51. 51. 47 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.36, Effect pf feed pH on ammonia mass transfer coefficient (Co=1.5 g/L, Tf=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*
  52. 52. 48 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.37, Variation of feed ammonia concentration and average permeate flux at different feed T (Co=1.5 g/L, Tf=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.
  53. 53. 49 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.39, Variation of feed ammonia concentration and average permeate flux at different feed flow rates (Co=1.5 g/L, Tf=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%.
  54. 54. 50 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.40, Schematic diagram of treatment 1, 2 and 4 showing the recovery of NH3 by the gas-permeable membrane manifold as it was governed by the balance in Eq.2 that depended on manure pH (M.C. García-Gonzalez et al., 2015) Fig.41, Mass of ammonia recovered in the acid concentrator tank for aerated, not aerated and chemically amended manure treatments. A second order eq. and R 2 are represented. The error bars are the standard deviation of duplicate experiments (M.C. García-Gonzalez et al., 2015)
  55. 55. 51 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 Fig.42, Experimental set-up for fermentative wastewater treatment with AMBBR and subsequent distillation using hydrophobic microfilters. The biogas was analyzed in a gas chromatograph (Hyun-Chul Kim et al., 2015) 3.16.2 Performance of (PTFE), (PVDF) and (PP) membranes in ammonia recovery In a study researching MD combined with an anaerobic moving bed biofilm reactor (AMBBR) for treating municipal wastewater for the treatment of domestic wastewater (Fig.42), (Hyun-Chul Kim et al., 2015). The viability of using a membrane separation technique for post processing of anaerobic bio effluent was studied. Three different hydrophobic 0.2 mm membranes made of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polypropylene (PP) were used in this study. From the three different hydrophobic membranes studied, the highest permeate flux was recorded for the PTFE membrane. A gradual but noteworthy decline of the flux was seen in the MD treatment with the PTFE membrane for which the permeate flux decreased up to 84% of the initial value after the 45h distillation. The initial permeate flux of the PVDF and PP membranes had 81% and 41% respectively. A longer-term treatment took place, using the MD module with a flat-sheet PVDF membrane for the reuse of anaerobically treated wastewater. Waste heat leaving the AMBBR was used as the dominant factor for the mass transfer in the MD process. The characterization of effluent organic matter (EfOM) using liquid chromatography - organic carbon detection (LCOCD) verified that almost all of the
  56. 56. 52 Nitrogen recovery from nitrogen rich wastewaters and slurries by thermal treatment processes, Christos Charisiadis 2015 EfOM was rejected by a macro-porous filter placed in the MD module. MD treatment also lead to the complete rejection of total phosphorus (TP) from the feed wastewater. 3.17 Handling of the Aqua Ammonia At the end of processing the sludge/digestate, we have a water solution with high N content that we still need to extract/use. There are a lot of chemical methods that produce mainly fertilizing products but the problem is that they use acids during their process. Acids in the industry are expensive and we would like if possible to find routes that avoid using them. In this chapter we're concentrating on chemical conversion of ammonia to Urea, Ammonium Carbonate and Ammonium Sulfate. 3.17.1 Ammonia Conversion to Urea (Fertilizers Europe, 2000) Urea is made from ammonia and carbon dioxide. The two substances are reacting at high pressure and temperature, and the urea is formed in a two step reaction, 2NH3 + CO2 ↔1 NH2COONH4 (ammonium carbamate) ↔2 H2O + NH2CONH2 (urea) (8) The urea contains unreacted NH3, CO2 and ammonium carbamate. When pressure is reduced and heat is applied, NH2COONH4 decomposes to NH3 and CO2. The ammonia and carbon dioxide are recycled. The urea solution is then concentrated to give 99.6% mass fraction ( wt.%) molten urea, and granulated for use as nitrogen- rich fertilizer and as a component in the manufacture of resins for timber processing and in yeast manufacture. The conversion to ammonium carbamate (Reaction 1) is fast and exothermic and is almost complete under the industrial reaction conditions. The decomposition to urea (Reaction 2) is slower, endothermic and) is usually in the order of 50-80%. The conversion increases with increasing temperature and NH3/CO2 ratio and decreases with increasing H2O/CO2 ratio. Modern urea technology installations vary in size from 800 to 2,000 t/d, have very similar energy requirements and nearly 100% material efficiency with slight differences in energy balances. The production outputs include, 1) Urea; 2) Process condensate water which can be used as boiler feed water after treatment; 3) Steam/ turbine condensate which are exported to the battery limits for polishing and re-use as boiler feed water; 4) Low pressure steam. The LP steam produced in the carbamate condenser is used for heating purposes in the downstream sections of the plant. The excess may be sent to the CO2 compressor turbine or CO2 booster or exported for use in other site activities.

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