Cunningham, Michael Gold, Moritz Strande, Linda LITERATURE REVIEW: SLOW PYROLYSIS OF FAECAL SLUDGE

Transcription

1 2016 Cunningham, Michael Gold, Moritz Strande, Linda LITERATURE REVIEW: SLOW PYROLYSIS OF FAECAL SLUDGE

2 Table of Contents LITERATURE REVIEW: SLOW PYROLYSIS OF FAECAL SLUDGE Introduction Faecal sludge as a pyrolysis feedstock Proximate analysis Ultimate analysis Organic fraction composition Unique characteristics of faecal sludge Conclusions Pyrolysis reactor design and operating variables Pyrolysis reactor operating parameters Pyrolysis reactor design variables Conclusions Regulations for pyrolysis and char land application Proposed heavy metal standards for char land application compared to regulations for biosolids land application Proposed standards for char PAHs and dioxin concentrations Gas emission regulations Land application and air emissions regulations in Tanzania Conclusions Pyrolysis end-product characteristics and applications Char Meta-analysis results Gas Liquid Conclusions Integration of pyrolysis into faecal sludge treatment Treatment plant overview Process heating demand as a function of feedstock %DM Slow pyrolysis process in combination with mechanical dewatering Conclusions

3 7. Existing pyrolysis reactors Alternative use of char compared to a soil conditioner Organic waste composting Cooking briquette production Solid fuel for industrial applications Agricultural fertilizer Conclusions Conclusions References Appendix Proposed char quality standards for land application Appendix USEPA Part 503 Rule Appendix

4 1. Introduction Pyrolysis, as shown in Figure 1, is the thermochemical decomposition of biomass in the absence of oxygen, which produces solid (char), gas and liquid products (liquids can be recovered from the condensable fraction of gas) (Basu, 2013; Lehmann and Joseph, 2015; Manyà, 2012). Gas (includes liquids) Feedstock Pyrolysis Solid (i.e. char) Figure 1. Char produced from rye grass, and chicken and pig manure (USDA, 2010). Slow pyrolysis has been traditionally used to produce cooking charcoal or recover tar from wood and other lignocellulosic biomasses (Lehmann and Joseph, 2015). The slow pyrolysis process operating conditions typically include a high heating temperature (HHT) of 300 to 700 C and heating rate of 5 to 100 C/min (Crombie et al., 2014). Table 1 compares the typical operating conditions and product yield distribution of slow pyrolysis with other thermo-chemical conversion processes of biomass such as intermediate and fast pyrolysis, gasification, and hydrothermal carbonization. Table 1. Comparison of slow pyrolysis with other thermochemical conversion processes (Libra et al., 2011). Process Reaction conditions (temperature Product distribution (%weight) ( C); vapor residence time) Solid Liquid Gas Pyrolysis: slow ~400; h-week Pyrolysis: intermediate ~500; 10-20s Pyrolysis: fast ~500; 1s Gasification ~800; ~10-20s Hydrothermal carbonization ~ ; no vapor residence time, ~1-12 h processing time

5 Char production has five potential benefits which provide the motivation for this analysis. These benefits are displayed in Figure 2 along with a simple way to think of each benefit in the context of faecal sludge management. The first purpose of faecal sludge pyrolysis in low-income countries would be waste management, with a particular emphasis on pathogen destruction. The benefits solid fuel and soil enhancement would provide revenue sources through fuel and soil amendment sales. Carbon sequestration and pollutant immobilization are added benefits, which could become a source of revenue in the future. It is expected that slow pyrolysis in low-income countries would focus on producing char, while combusting the gas and liquid fractions to maintain the reactor HHT and dry the pyrolysis feedstock. Figure 2. Five potential benefits of char production in the context of faecal sludge management (Jeffrey, Bezemer et al. 2015). The objective of this literature review is to assess if slow pyrolysis will cost effectively and safely treat faecal sludge in low-income countries at the municipal treatment plant scale. This task is completed by: identifying parameters that predict if faecal sludge char provides each of the five potential benefits based on the results of other feedstocks (chapter 2 & 4) determining the influence of operating conditions on achieving each of the five potential benefits (chapter 3 & 4) estimating if faecal sludge char heavy metal concentrations exceed proposed standards and existing regulations for land application (chapter 4 & 5) 4

6 discussing the integration of pyrolysis with existing faecal sludge treatment plants (chapter 6) evaluating existing slow pyrolysis units (chapter 7) reviewing char applications beyond the five potential benefits (chapter 8) identify future research needs (all chapters) 5

7 2. Faecal sludge as a pyrolysis feedstock This chapter identifies similarities and differences between characteristics of faecal sludge and other pyrolysis feedstocks (wastewater sludge, cow manure, septic tank faecal sludge, and lignocellulosic biomass). Extensive research has been conducted on the pyrolysis of lignocellulosic biomass (Crombie and Masek, 2014; Crombie et al., 2014; Cross and Sohi, 2013; Rajkovich et al., 2012; Scholz et al., 2014; Yamato et al., 2006) (Crombie and Masek, 2014) and wastewater sludge (Hossain et al., 2011; Méndez et al., 2012; Sanchez et al., 2007; Thipkhunthod et al., 2006; Yachigo and Sato, 2013). The pyrolysis of animal manure has also been thoroughly investigated (Cantrell et al., 2012; Liu and Li, 2014; Rajkovich et al., 2012; Sanchez et al., 2007). The pyrolysis of septic tank faecal sludge and faeces have only been assessed by Liu et al. (2014) and Ward et al. (2014) respectively. The characteristics of char produced from these feedstocks are further discussed in Chapter Proximate analysis Proximate analysis is the weight percent (wt%) of ash, volatile solids, and fixed carbon in solid fuels (Basu, 2013). Concentration of volatile solids in char can be an indicator whether pyrolysis was complete and fixed carbon can be a proxy for the pyrolysis char yield. Table 2 compares proximate analysis results for faecal sludge with faeces, wastewater sludge, cow manure and three lignocellulosic biomasses (pine wood chips and wheat straw pellets). Ward et al. (2014) did not provide detailed feedstock characteristics for the pyrolysis of faeces. However, based on personal communication the ash content was 15 weight percent (wt%) (Ward, "Personal Communication"). The lignocellulosic biomasses had the two highest fixed carbon content, cow manure was third, and wastewater sludge and septic tank FS tied for fourth. The effect of ash and fixed carbon content on char ability to meet the five potential benefits will be further discussed in Chapter 4. 6

8 Table 2. Proximate analysis results (%wt) of faecal sludge in comparison to faeces, wastewater sludge, cow manure and two lignocellulosic biomasses. FS Septic FS FS Dakar 1 Kampala #1 1 Kampala #2 2 Faeces 3,4 Wastewater Cow Pine Wood Wheat Straw Tank FS Beijing 5 Sludge 6 Manure 5 Chips 7 Pellets 7 Ash 47% 59% 45% 15-21% 17% 34% 14% 2% <1% Volatile Solids 53% 41% 44% NA 75% 50% 71% 76% 82% Fixed Carbon NA NA 9% NA 8% 8% 15% 22% 18% NA= not analyzed 1 (Gold et al., submitted); 2 (Bryne et al., 2015); 3 (Ward, personal communication); 4 (Schouw et al., 2002); 5 (Liu et al., 2014); 6 (Hossain et al., 2011); 7 (Crombie et al., 2014) Table 3. Feedstock high heating value (MJ/kg dry matter) and ultimate analysis (%wt). FS Dakar 1 FS Kampala #1 1 FS Kampala Faeces 3 Septic Tank #2 2 FS Beijing 4 Wastewater Sludge 5 Cow Manure 4 Pine Wood Chips 6 Wheat Straw Pellets 6 HHV (MJ/kg DM) NA C 29% 28% NA 44-55% 42% 32% 52% 51% 48% H 4% 4% NA NA 7% 5% 6% 4% 6% N 3% 3% NA 12% 6% 3% 3% <0.1% 2% O NA NA NA NA 43% 18% 37% 42% 44% P 1% 1% NA 2% NA NA 1% NA NA NA=not analyzed 1 (Gold et al., submitted); 2 (Bryne et al., 2015); 3 (Rose et al., 2015); 4 (Liu et al., 2014); 5 (Hossain et al., 2011); 6 (Crombie et al., 2014) 7

9 In the presented data, ash content varies greatly between feedstock types with faecal sludge from Kampala having the highest values (59%) and wheat straw pellets having the lowest values (0.2%). Faecal sludge from Dakar and Kampala has ash contents approximately three times greater than the septic tank faecal sludge from Beijing, which is potentially due to a different ash concentrations in the faeces and different design and operation of the onsite sanitation technologies. In Kampala and Dakar, treated faecal sludge was collected from drying beds after treatment in settling-thickening tanks (Gold et al., submitted). Anaerobic digestion in settling-thickening tanks and sand collected from the surface of drying beds could reduce volatile solids and increase the ash content. Other parameters that could influence the proximate analysis of faecal sludge include the type of on-site sanitation technologies (e.g. pit latrine versus septic tank), faecal sludge storage duration, inflow and infiltration (e.g. groundwater, greywater, faecal sludge) and the faecal sludge collection method (Niwagaba et al., 2014). This means that faecal sludge characteristics need to be determined on case-by-case basis. 2.2 Ultimate analysis Ultimate analysis is the elemental composition of a feedstock. However, elements included in ultimate analysis results vary between literature sources. Basu (2013) defines the ultimate analysis as the wt% of carbon, hydrogen, nitrogen, sulfur and ash. Literature on pyrolysis generally presents the carbon, hydrogen, and oxygen, sometimes including phosphorous but rarely lists nitrogen and sulphur. The high heating value (HHV) is sometimes also presented alongside the ultimate analysis results. Alternatively, publications will present a single table with feedstock characteristics containing some ultimate analysis results alongside physical and chemical parameters such as ph and cationic exchange capacity (CEC). Table 3 lists the carbon, nitrogen, oxygen, and phosphorous fractions, as well as the HHV, for feedstocks whose char will be discussed in Chapter 4. The faecal sludge samples from Kampala and Dakar contained a lower HHV than the faeces and septic tank FS samples from Beijing. This could be related to stabilization occurring over a long retention time within the septic tanks (Niwagaba et al., 2014). The nitrogen fraction was also lower for faecal sludge than faeces and septic tank FS. The author speculates this could also be attributed to solid substrate being hydrolyzed into dissolved substrate during anaerobic stabilization in on-site 8

10 sanitation technologies such as septic tanks (Batstone et al., 2002). The most surprising result from Table 2 is that the ultimate analysis results of faecal sludge from Kampala and Dakar were more similar to wastewater sludge than septic tank faecal sludge or faeces. The effect of the feedstock ultimate analysis results on char properties, and specifically the potential to use char as a solid fuel, is discussed in Chapter Organic fraction composition The main constituents of the organic fraction of lignocellulosic biomass are hemicellulose, cellulose and lignin (Lehmann and Joseph, 2015). The organic fractions of pine wood chips and wheat straw pellets are displayed in Table 4. On the other hand, the organic fractions of faeces, wastewater sludge, and animal manure are more commonly described as carbohydrates, proteins, lipids, and lignin. The particulate organic fractions of fresh human faeces are presented in Table 5. Organic fractions of wastewater sludge and animal manures are presented in Table 6. The large difference in carbohydrate and protein contents between wastewater sludge and manures is surprising. The organic fractions of feedstock can be used to predict char yield as a function of HHT because different fractions decay in different temperature ranges. However, it is more common to experimentally determine the char yield. This approach is recommended for faecal sludge because, as discussed in section 2.6, it is expected to contain large amounts of inorganic material. Table 4. Organic fraction composition (%wt) for pine wood chips and wheat straw pellets (Crombie et al., 2014). Pine Wood Chips Wheat Straw Pellets Cellulose 52% 23% Hemicellulose 21% 44% Lignin 13% 26% Table 5. Organic fraction composition of faeces (%wt) (Rose et al., 2015). Faeces Biomass 25-54% Protein 2-25% Carbohydrates 25% Lipids 2-15% 9

11 Table 6. Volatile solids concentration (g/kg) and organic composition (%wt) for select waste biomass (Winter, Hilpert et al. 1992). Wastewater Sludge Cow Manure Pig Slurries Poultry Manure Volatile solids >120 Nitrogen 7% 4% 10% 6-15% Protein 30-48% 17-22% 16-40% 20-40% Carbohydrates 30-41% 50% 20% 15% Lipids 20-27% 4-7% 10% 5% Lignin 20% 16-30% <5% 10-40% 2.6 Unique characteristics of faecal sludge In low-income countries, faecal sludge delivered to the treatment plant often contains some solid waste. Figure 3 displays the components of municipal waste found inside faecal sludge during a sampling project in Dakar, Senegal. Variability in faecal sludge characteristics presents another unique challenge. For example, the degree of stabilization is a function of solids retention time, which has a large range for on-site sanitation technologies. Less stable sludge contains more readily degradable organic matter and less complex molecules such as lignin and cellulose. Indicators of stabilization include total volatile solids (TVS), volatile suspended solids (VSS), biochemical oxygen demand (BOD), and chemical oxygen demand (COD). %wt 50% 40% 30% 20% 10% 0% Figure 3. Municipal waste measured in faecal sludge (%wt) in Dakar, Senegal (M'Voubou, 2004). 2.7 Conclusions Faecal sludge characteristics are highly variable. In comparison to faeces, cow manure and lignocellulosic biomasses, faecal sludge can have higher concentration of ash which are similar to wastewater sludge. Faecal sludge samples from Dakar and Kampala (Gold et al., submitted) (Bryne et al., 2015) that were collected before and 10

12 after drying beds are most similar to septic tank sludge faecal sludge from Beijing (Liu et al., 2014) followed by human manure, wastewater sludge, animal manure, and lignocellulosic biomass (Crombie et al., 2014; Hossain et al., 2011; Rose et al., 2015). The differences in feedstock characteristics should be considered when comparing the results from different chars in Chapter 4. Due to these differences, it is not possible to predict if trends observed for certain char properties from one type of feedstock will also be realized during the pyrolysis of faecal sludge. 11

13 3. Pyrolysis reactor design and operating variables This chapter will discuss common pyrolysis designs and operating variables. 3.1 Pyrolysis reactor operating parameters The pyrolysis process is divided into three phases: heating, reaction, and cooling (Garcia-Perez et al., 2010; Lehmann and Joseph, 2015). Heating: feedstock temperature increases to the HHT Reaction: HHT maintained for a specified hold time Cooling: feedstock (now char) cools to a temperature low enough for safe removal Figure 4 displays the reactor temperature for three pyrolysis batch experiment with different heating rates and hold times. The cooling time is not reported in these studies. In a batch reactor, the feedstock and the reactor are heated together from ambient to the HHT. A continuous reactor is typically held at a specific HHT and feedstock is conveyed through the reaction zone on a belt. 600 Reactor temperature ( C) Time (minutes) 450 C (5 C/min) 450 C (100 C/min) 500 C (15 C/min) Figure 4. Heating and reaction phase time and temperature profile in a batch reactor (Crombie et al., 2014; Hossain et al., 2011). Highest heating temperature (HHT) The HHT (highest heating temperature) is widely regarded to have the most significant effect on char structure. This is because the HHT affects the extent of physical changes that occur during the process, for example the release of volatiles 12

14 or the formation and volatilization of any intermediate melts. (Lehmann and Joseph, 2015) The HHT reactor temperature was shown to effect char yield, high heating value (HHV), fixed carbon content, ph, adsorption capacity, cationic exchange capacity (CEC), water retention, surface area, mechanical strength, elemental composition, extractable nutrients and carbon sequestration potential (Basu, 2013; Crombie et al., 2014; Lehmann and Joseph, 2015; Liu et al., 2014; Manyà, 2012; Ward et al., 2014). The effect of pyrolysis reactor HHT on the characteristics of chars produced from faecal sludge, wastewater sludge, animal manure, cow manure and lignocellulosic biomass will be discussed in Chapter 4. Heating rate The heating rate is the change in reactor temperature per unit of time (Basu, 2013). It has an effect on char physical and chemical properties such as particle size distribution and ph, as well as char yield (Crombie et al., 2014; Lehmann and Joseph, 2015). The effect of the heating rate on char yield and characteristics will be discussed in Chapter 4. Solids retention time (SRT) The solids retention time (SRT) is the time the feedstock spends in the pyrolysis reactor. It can be divided into three phases: heating, hold, and cooling. The heating phase time is a function of the HHT and heating rate, and decreases as heating rate increases. The hold time is the time spent at the HHT. The hold time reported in Manyà (2012) for experimental set-ups is 0 to 240 minutes; Ward et al. (2014) and Liu et al. (2014) used hold times of 40 and 120 minutes for pyrolysis of septic tank faecal sludge and faeces respectively. On the other hand, Crombie et al. (2014) proposed that the maximum hold time for an economically viable industrial scale continuous pyrolysis unit is 45 minutes. The cooling phase time is the amount of time until the char temperature is reduced to a safe level for handling (Garcia- Perez et al., 2010). The cooling phase time was not reported in any papers reviewed for this study. Therefore, it is not possible to estimate the feedstock SRT for these studies. The authors recommend listing the entire solids retention time and the time in each phase as part of future experimental work on faecal sludge pyrolysis. 13

15 Process control Process control based on smoke color is often employed for small scale batch reactors in low-income countries. These reactors are designed to maximize char production without gas or liquid fraction recovery. White smoke is emitted during the drying process, as water vapor is removed from the feedstock (Garcia-Perez et al., 2010; Lohri et al., 2015b). According to Garcia-Perez et al. (2010), black smoke indicates the pyrolysis process is under way; Lohri et al. (2015) describes this smoke as brown/yellow. Colorless smoke indicates the pyrolysis process is complete (Garcia-Perez et al., 2010; Lohri et al., 2015b). The air vents are then closed to begin the cooling phase and prevent oxygen from causing char combustion, while the reactor chimney is left open to allow the remaining smoke to escape and prevent gas pressure build-up within the reactor (Garcia-Perez et al., 2010). Process control based on operating parameters, such as HHT and heating rate, is a more precise method preferred in experimental setups (Crombie and Masek, 2014; Crombie et al., 2014; Cross and Sohi, 2013; Enders et al., 2012; Granatstein et al., 2009; Hossain et al., 2011; Inguanzano et al., 2002; Jeffery et al., 2015; Lehmann and Joseph, 2015; Liu et al., 2014; Shinogi and Kanri, 2003; Ward, personal communication). HHT and heating rate are also used to control pyrolysis reactors which have been implemented for agricultural wastes in low-income countries, as well as agricultural wastes or wastewater sludge in high-income countries (Green Charcoal International; Pyreg GmbH). Safety A pyrolysis reactor is designed to operate in an oxygen free environment. The accidental introduction of large amounts of air into the pyrolysis reactor creates unstable combustion, for example through leaks in the reactor (Garcia-Perez et al., 2010; Lehmann and Joseph, 2015). Exposing char to air before it has cooled could set the char on fire (Garcia-Perez et al., 2010). Char should be cooled to below 65 C before removing it from the reactor (Lohri et al., 2015b). Pressure Reactor pressure affects the char yield and gas composition. For example, the char yield from cellulose increased from 12% to 18% as reactor pressure increased from 0.1 to 1 MPa (Lehmann and Joseph, 2015). Pressure also increased the methane mass fraction of the gas produced during cellulose pyrolysis, while 14

16 decreasing the carbon dioxide mass fraction (Lehmann and Joseph, 2015). It is assumed that the pyrolysis reactors used in low-income countries will operate at atmospheric pressure, as this reduces process complexity and equipment. Thus, varying reactor pressure is not recommended as part of future experimental work on faecal sludge pyrolysis. 3.2 Pyrolysis reactor design variables Pyrolysis reactors are classified as kilns or retorts (Scholz et al., 2014). Kilns are traditional reactors that focus on char production with less emphasis on resource recovery from the gas products (Garcia-Perez et al., 2010). While improvements to kiln designs have increased efficiency and reduced emissions, there is a trend within the field of agricultural waste pyrolysis to replace kilns with retorts (Lehmann and Joseph, 2015). Retorts are reactors capable of recovering products from the liquid and gas products (Lehmann and Joseph, 2009). Product recovery can include gas combustion for energy production, or the conversion of liquids into biofuels (Du et al., 2013). Pyrolysis reactors are designed to achieve specific goals by changing the following variables (Garcia-Perez et al., 2010): Operating mode: batch, semi-batch or continuous Heating method: auto-thermal, direct or indirect Construction material: steel, brick or earth pit, etc. Portability: fixed or mobile Reactor position: horizontal or vertical Loading mode: manual or mechanical Energy supply: pyrolysis gas and/or external energy input Drying process: external or internal Operating mode and heating methods are discussed in more detail below. Faecal sludge pyrolysis should build on existing technologies. Therefore, the remaining design variables are not discussed. Operating mode Pyrolysis reactors can be operated in batch, semi-batch, and continuous mode (Basu, 2013; Lehmann and Joseph, 2015). In a survey of 142 pyrolysis reactors in low-income countries, 73% of respondents operate in batch mode, 7% operate in continuous mode and 20% did not specify (Scholz et al., 2014). In batch reactors, the feedstock must be heated to the reaction temperature using an external fuel source. This may lead to large operating cost. As discussed in 15

17 Chapter 2, emissions concentrations in batch systems never reach a steady state. This makes air pollution control more difficult. Batch reactors are not typically integrated with air pollution control equipment, hence volatile compounds, particulate matter and other pollutants in the pyrolysis gas may be emitted into the atmosphere. However, batch reactors typically have lower capital costs and operational complexity than semi-batch or continuous reactors. (Garcia-Perez et al., 2010; Lehmann and Joseph, 2015) Semi-batch reactors consist of a series of connected batch reactors. The first reactor is heated using an external fuel source. Then the gas produced in the first reactor is transferred to the second reactor, where it is combusted to provide the heat necessary to start the pyrolysis process. The gas from the second reactor is then transferred to the third reactor, and this pattern continues until the last reactor. Compared to the batch mode, the semi-batch mode reduces the requirement for external fuel and decreases air pollution. (Garcia-Perez et al., 2010; Lehmann and Joseph, 2015) In continuous reactors, feedstock travels through the reaction zone on a conveyer. The reactor is held at the HHT, hence multiple batches of feedstock are present in the reactor at different stages of decomposition simultaneously. The gas released from the feedstock helps to maintain the reactor heat. In some cases, gas is recycled in a process known as after-burning to reduce air particulate matter, carbon monoxide, and volatile organic carbon concentrations. Continuous reactors permit constant feedstock addition and end product production. They are typically fixed, and integrate well with air pollution control equipment. These reactors tend to have a higher energy efficiency, end-product quality consistency, operational complexity, and capital cost compared to batch and semi-batch reactors. (Lehmann and Joseph, 2015) Heating method Prior to pyrolysis, the feedstock must be dried. Thus, faecal sludge drying and pyrolysis are closely linked in the following ways: Average faecal sludge moisture content effects reactor design and energy balance. 16

18 Faecal sludge moisture content variability affects operating conditions and the need for external heat sources. Faecal sludge drying capacity determines pyrolysis reactor sizing and operating mode (i.e. continuous or batch). Feedstock moisture content and the desired reactor HHT determines the reactor heat demand (Liu and Li, 2014). Reactor heat demand can be partitioned into start-up and maintenance demands. The start-up demand is the external energy inputs required to process the first unit in a cycle. For a batch reactor, start-up energy must be provided for each batch of feedstock. The semi-batch process requires the start-up energy input for the first batch. Maintenance heat input is then needed only if the energy from gas combustion is not sufficient to maintain the reactor HHT. Continuous reactors require start-up energy input to initiate the pyrolysis process for the first batch of feedstock, and then ideally heat produced from pyrolysis gas combustion is sufficient to process feedstock until operation is terminated. For all operating modes, electricity is required for mechanical equipment used for feedstock loading, air pollution control, and process control/monitoring equipment. (Garcia- Perez et al., 2010; Lehmann and Joseph, 2015) Heat can be transferred to the feedstock by direct air addition (auto-thermal), direct contact with gas, or indirect contact with gas. Microwave heating is an alternative technology which requires electricity. (Garcia-Perez et al., 2010; Lehmann and Joseph, 2015) Small scale pyrolysis units typically use auto-thermal heating, which involves the partial combustion of feedstock with a controlled air/oxygen supply followed by closing air vents. This establishes the necessary temperature and low oxygen concentrations needed for pyrolysis to occur. The top lift updraft gasifier (TLUD), which utilizes auto-thermal heating, is a common reactor configuration in gasification cook stoves in low-income countries (Scholz et al., 2014). Direct contact with the gas eliminates the requirement for heat exchange equipment. TLUD is used in the pyrolysis unit under development by the Climate Foundation for faeces (Mitchell and Von Herzen, 2012). Indirect heat exchange requires an exchange surface, while also separating feedstock from the gas. (Lehmann and Joseph, 2015) Microwave heating has been effectively demonstrated at the laboratory scale for plastics, straw pellets, and wood chips (Mašek et al., 2013; Young, 2015). 17

19 Direct or in-direct contact with gas is the heating method most suitable for municipal-scale faecal sludge pyrolysis in low-income countries, as these are already used for community and industrial scale. Auto-thermal heating is more common at the household and single farm scale (Green Charcoal International; Pyreg GmbH; Scholz et al., 2014; Von Herzen et al., 2015). Microwave based technology does not appear to be suitable for low-income countries considering the high electricity demand and low char yields (Ludlow-Palafox and Chase, 2001). 3.4 Conclusions HHT, heating rate and solids retention time are operating parameters used to control continuous pyrolysis reactors. Continuous reactors are currently used in state of the art industrial scale pyrolysis of lignocellulosic biomass and wastewater sludge. A pyrolysis reactor used to conduct future research should have the capacity to safely and accurately vary HHT from 300 to 700 C, and implement multiple heating rates. This information would assist in completing feasibility analysis for implementing a state of the art reactor for faecal sludge pyrolysis in low-income countries. Training staff to ensure safe reactor operation is an important step before conducting further research on faecal sludge pyrolysis. 18

20 4. Regulations for pyrolysis and char land application This section will discuss regulations and proposed standards for emissions from pyrolysis and land application of char. Proposed standards for char heavy metal concentrations limits for land application will be compared with regulations implemented by the United States Environmental Protection Agency (USEPA) for biosolids land application. Furthermore, proposed standards for char Polycyclic Aromatic Hydrocarbons (PAHs) and dioxins concentration limits will be discussed. Gas produced during pyrolysis can be combusted for energy recovery. This creates a new source of air pollution, which is connected to air quality regulations. As part of this literature review, we were unable to find specific information on air emissions from pyrolysis of faecal sludge. Therefore, section 4.3 will present air pollution regulations developed for feedstock by the State of Massachusetts Department of Environmental Protection. Section 4.3 also contains an excerpt from air pollution emission requirements for United States wastewater sludge incineration facilities. 4.1 Proposed heavy metal standards for char land application compared to regulations for biosolids land application The following proposed standards to classify char quality for land application will be discussed in this section: British Char Initiative (BBI) International Char Initiative (IBI) Reducing Mineral Fertilizers & Chemicals Use in Agriculture by Recycling Treated Organic Waste as Compost and Bio-char Products (REFERTIL) These standards could be adopted by regulatory agencies to require manufacturers to produce char with pollutant concentrations below specific limits, as well as reporting select parameters to assist users in determining the char value for the five potential benefits. The three standards are included in Appendix 1. Table 7 compares heavy metal concentration limits proposed by these standards. Since the REFERTIL, IBI, and BBI standards have not been implemented by governments, the existing USEPA Part 503 regulations for biosolids land application will be compared to the proposed char regulations. The European Char Certificate is another set of standards (EBC, 2012); these were not included in this analysis. 19

21 Table 7. Char quality standards for select pollutants according to the BBI, REFERIL and IBI standards (IBI, 2013; REFERTIL, 2014; Shackley et al., 2014). BBI REFERTIL IBI Toxicant Units Max Permitted High Grade Standard Grade Organic Fertilizer Soil Improver Concentration Range Arsenic mg/kg Cadmium mg/kg Chromium mg/kg Cobalt mg/kg NL 1 NL 1 NL 1 NL Copper mg/kg Lead mg/kg Mercury mg/kg Manganese mg/kg 3500 NL 1 NL 1 NL 1 NL 1 Molybdenum mg/kg NL 1 NL Nickel mg/kg Selenium mg/kg NL 1 NL Zinc mg/kg PAHs mg/kg <20 < BETX mg/kg NL 1 NL 1 NL 1 NL 1 NL 1 Dioxins/Furans ng/kg < PCBs mg/kg TEQ 2 < NL= not listed; 2 Toxicity equivalence (TEQ) is the product of the pollutant concentration and the toxicity equivalent factor (TEF) (USEPA, 2013) 20

22 Char Quality Mandate (BQM) - British Char Initiative (BBI) The Char Quality Mandate (BQM), proposed by the BBI, would require manufacturer declarations for ten basic properties. The proposed standards are included in Table in Appendix 1. BQM also includes the test methods for twenty-three additional optional properties. Four of the basic properties that must be tested and reported assess char toxicity: Heavy metals Polycyclic aromatic hydrocarbons (PAHs) Benzene, toluene, ethylbenzene, and xylenes (BTEX) o Required when char could be used as animal feed Polychlorinated biphenyls (PCBs), dioxins and furans o Required when excessive concentrations of chlorine are present. Excessive concentrations is defined as greater than the concentration found in plants grown in a non-saline environment. However, a specific value is not listed. (Shackley et al., 2014) The maximum allowable concentrations for the pollutants listed above are displayed in Table 29 in the standard grade column. The criteria for obtaining the high grade quality label are displayed alongside the standard grade requirements. Char which meets the minimum quality criteria are sorted as Class 1 to 3 based on organic carbon and moisture content. International Char Initiative (IBI) The IBI standards and analysis methods are displayed in Table in Appendix 1. The IBI test categories are: Category A: required basic char properties Category B: required toxicant reporting Category C: optional advanced analysis and soil enhancement properties Reducing Mineral Fertilizers & Chemicals Use in Agriculture by Recycling Treated Organic Waste as Compost and Bio-char Products (REFERTIL) The REFERTIL draft set of parameters for char quality control are displayed in Table 30. Char which meets the minimum quality criteria is designated as soil conditioner. Char which meets the minimum quality criteria and has a total phosphorous concentration greater than 25% measured on a dry mass basis is designated as organic P fertilizer. However, a bioavailable/extractable phosphorous 21

23 declaration is not required to receive this designation. The char nutrients section in Chapter 4 shows that the ratio of bioavailable to total phosphorous is a function of feedstock characteristics and operating conditions (i.e. HHT). USEPA Part of 503 The USEPA part 503 rules regulate the land application of biosolids in four categories based on heavy metal and pathogen concentrations and the vector attraction potential. The heavy metals concentration limits are displayed in Table 8. The pathogen and vector attraction reduction requirements are displayed in Appendix 2. (USEPA, 1994) 1. Exceptional Quality (EQ) - Meets exceptional quality/pollutant control pollutant standards, vector attraction requirements, and achieves Class A pathogen reduction. 2. Pollution Control (PC) - Meets exceptional quality/pollutant control pollutant standards, vector attraction requirements, and achieves Class B pathogen reduction. 3. Cumulative Pollutant Loading Rate (CPLR) - Biosolids which exceed EQ/PC pollutant loads, but are below maximum concentrations. Biosolids can be applied to land, but each site must be monitored. 4. Annual Pollutant Loading Rate (APLR) - Pollutants exceed EQ/PC pollutant loads but below maximum concentrations. These biosolids can be given away in bags or containers. Special application instructions must be included to prevent excessive application. In a scenario where faecal sludge char is considered a biosolid and regulated under the USEPA Part 503 rule, this char would achieve Class A pathogen reduction due to the process operating temperature (>300⁰C) and product total solids concentration. The pyrolysis process satisfies the heat drying, heat treatment and pasteurization process requirements listed in Appendix 2. Char achieves the vector reduction requirements by meeting the volatile solids reduction and total solids concentration criteria. Char which meets the EQ and PC heavy metal limits displayed in Table 2 would therefore be classified as EQ biosolids, which is the highest quality designation for biosolids. Char which exceeds the EQ and PC limits but meets the maximum concentration would be classified under the CPLR or APLR regulation schemes and subject to restrictions regarding land-use application. (USEPA, 1994) 22

24 Table 8. Heavy metal limits for land application of biosolids in the United States (USEPA, 1994). Maximum Pollutant Concentration EQ and PC Limits CPLR Limits APLR Limits mg/kg mg/kg kg/hectare kg/hectare*year Arsenic Cadmium Chromium Copper Lead Mercury Molybdenum Nickel Selenium Zinc Comparison of char standards with biosolids regulations Generally the REFERTIL, IBI, and BQM standards contain the same parameters for char properties, organic pollutants and heavy metals. The REFERTIL limits are the most conservative for heavy metals, with maximum limits for soil improver and organic fertilizer being close to the lower limit of high grade char in the IBI and BQM standards. The IBI and BQM standards are similar, with the exception that the BQM limits for chromium in standard grade char is one order of magnitude lower than the maximum permitted by IBI. The USEPA biosolids EQ and PC limits were within the maximum concentration range proposed by the IBI. For cadmium, chromium, copper, mercury, and selenium, the EQ and PC limits were equal to the upper limit of the IBI range. The maximum allowable concentrations for biosolids proposed by the USEPA exceed the IBI limits for all metals except arsenic, lead, and nickel. The CPLR and APLR regulatory approach by the USEPA presented in Table 8 are particularly pertinent to char. Mixing char with other fertilizers or limiting the amount each farmer can apply is an alternative regulatory approach to limit metals resulting from application by maximum concentrations. Given char heavy metals concentrations, it is possible to estimate the maximum amount of char that can be land applied per hectare per year. 23

25 An estimate whether char produced from faecal sludge would meet these concentration limits is included in Chapter 5 for faecal sludge char from Dakar and Kampala. 4.2 Proposed standards for char PAHs and dioxin concentrations PAHs are a diverse set of hundreds of organic compounds which typically occur in combination (USEPA, 2008). Similarly, dioxins are a large set of chemical compounds that share certain chemical and biological similarities; they are formed during the incomplete combustion of biomass and fossil fuels, as well as industrial processes such as herbicide manufacturing (USEPA, 2015b). PAHs and dioxins are regulated due to their human health risk and persistence in the environment (USEPA, 2008; WHO, 2014). They have been measured by Hale et al. (2012) in char produced from lignocellulosic biomass, food waste, and dairy manure. The understanding of the mechanisms for PAHs and Dioxin formation during pyrolysis is limited; Hale et al. (2012) hypothesizes that Dioxin formation is positively correlated to feedstock chlorine concentrations and reactor heating rate. Table 9 lists concentration limits put forth by the BBI, REFERTIL, and IBI as part of proposed standards for char land application. Appendix 1 lists analytical methods for PAHs and Dioxins included in the BBI and IBI proposed standards. The combinations of PAHs and Dioxins are included in the measurement are defined in the analytical methods for example the USEPA method recommended by the IBI identifies 21 PAHs (IBI, 2013; USEPA, 2008). In Chapter 5, the PAH and Dioxin concentrations in char from cow manure (the feedstock most similar to faecal sludge in Hale et. al (2012)) are compared to the limits in Table 8. Table 9. Proposed PAHs (mg/kg) and Dioxin (ng/kg TEQ) in char by the BBI, REFERTIL and IBI for land application (IBI, 2013; REFERTIL, 2014; Shackley et al., 2014). Units BBI REFERTIL IBI PAHs mg/kg Dioxins ng/kg TEQ Toxicity equivalence (TEQ) is the product of the pollutant concentration and the toxicity equivalent factor (TEF) (USEPA, 2013) 4.3 Gas emission regulations Gas produced as part of the pyrolysis process is a potential source for air pollution. As of now, there are no official emissions limits or guiding documents at the national or international level for faecal or wastewater sludge pyrolysis reactors. The 24

26 Massachusetts Department of Environmental Protection issued interim regulations for farm scale pyrolysis reactors. The regulations were based on the emissions factors measured for industrial charcoal production from wood, which are displayed in Table 10. Emission factors, described in Equation 1, are an approximation for the amount of pollution released by a specific process per unit of end-product (USEPA, 2015c). For example, the kilograms of total particulate matter emitted per ton of charcoal produced are displayed in Table 10. Equation 1 also contains an emissions reduction efficiency (ERF) factor, which compares the effectiveness of a specific type of air pollution control equipment for each pollutant (USEPA, 2015c). The emissions factors for charcoal could be used as a first step to identify pollutants which could occur during faecal sludge pyrolysis and to compare the performance of an air pollution control device at reducing each emission. Equation 1. Calculation of emissions based on emissions factors (USEPA, 2015c). EE = AAAA EEEE (1 EEEEEE 100 ) E= Emissions mm pppppppppppppppppp tttttttt AR= Activity Rate mm iiiiiiiiii tttttttt EF=Emissions Factor mm pppppppppppppppppp mm iiiiiiiiii ERF= Emissions Reduction Efficiency (%) Table 10. Charcoal kiln emissions factors (USEPA, 1995). Process Pollutant Unit Charcoal Kiln Briquetting Total Particulate Matter kg/t Carbon Monoxide kg/t 145 ND Nitrous Oxide kg/t 12 ND Volatile Organic Carbon kg/t 135 ND Carbon Dioxide kg/t 550 ND ND=not declared, kg/mt= kilograms of pollutant per ton of charcoal produced 25

27 Biosolids incineration is another process which can be used as a reference for faecal sludge pyrolysis. According to the USEPA, the major pollutants emitted during wastewater sludge incineration are particulate matter, metals, carbon monoxide, nitrogen oxide, sulfur dioxide and non-combusted hydrocarbons. Table 11 lists air pollution regulations for biosolids incineration in two reactor configurations: Multiple hearth furnace and fluidized bed reactor. Fluidized bed incineration is able to achieve lower emissions, and is therefore subject to more stringent emission regulations (USEPA, 2003). Table 11. Proposed updated limits for existing wastewater sludge incineration units with emissions measured at 7% oxygen at dry basis under standard conditions USEPA (2015a). Incinerator Configuration Pollutant Units Multiple Hearth Furnace Fluidized Bed Incinerator Particulate Matter mg/m Hydrochloric Acid ppm Carbon Monoxide ppm Dioxins/Furans ng/m Mercury mg/m Nitrous Oxide ppm Sulfur Dioxide ppm Cadmium mg/m Lead mg/m The USEPA AP-42 database contains emission factors for multiple hearth furnace and fluidized bed incineration and ERF factors, described in Equation 1, for air pollution control equipment. The database can be used as a starting point for identifying air pollution control technology that would be installed alongside a pyrolysis gas combustion unit. 4.4 Land application and air emissions regulations in Tanzania Land application and air emissions regulations in Tanzania are taken as an example of regulations in low-income countries. In Tanzania, the Environmental Management Act passed in 2007 contains rules and regulations regarding process air emissions and biosolids land application. Tanzania has national standards for air emissions from large pollutants sources such as industries. Regulations for biosolids disposal are deferred from the national to the local government authorities. Regulations for air emissions are located in the Environmental Management (Air Quality Standards) Act Chapter 191 Sections 140, 145 and 230. The emissions 26

28 limits for the pollutants in Line 9 and 10 are summarized in Appendix 3. Regulations are given by reactor input fuel type (solid, liquid, and gas) and thermal output. Line 10 states: A person who undertakes an activity shall be required to comply with the highest permissible quantity of emission of sulphur oxides, carbon monoxide, hydrocarbon measured as total organic carbon, dust, nitrogen oxides or lead released into the air from a pollution source and respective test methods prescribed under the Second Schedule to these Regulations. (Government of Tanzania, 2007b) Regulations for the disposal of biosolids are found in the Environmental Management (Water Quality Standards) Regulation Chapter 191 Sections 143, 144 and 230, line 34: A person who violates guidelines or standards made by a local government authority on collection, transportation and disposal of wastewater and sludge commits an offence and shall be liable on conviction to a fine not exceeding five million shillings or to imprisonment for a term not exceeding two years or to both. (Government of Tanzania, 2007a) 4.5 Conclusions Measuring heavy metal, PAH, and Dioxin concentrations should be a part of future research into faecal sludge pyrolysis. Char which meets the heavy metal concentrations outlined by the USEPA Part 503 rule would be classified as excellent quality biosolids, and thus can be applied in agricultural. Further discussions on heavy metals in char are included in Chapter 5. It would be also useful to test faecal sludge char for the parameters listed in the proposed standards from the IBI, REFERTIL, and BBI. Air pollution should also be measured as part of further research. USEPA emissions factors provide guidance on which pollutants would be expected from a pyrolysis process. However, the emissions factors for wood charcoal production may have much different values to values measured as part of future research during faecal sludge pyrolysis. 5. Pyrolysis end-product characteristics and applications This chapter describes the parameters used to characterize pyrolysis solid (char), liquid, and gas end-products. It will discuss whether faecal sludge char is 27

29 expected to exceed the heavy metal, PAH and Dioxin standards proposed in Chapter 4. And finally, it will present the results of a meta-analysis on the change in crop production as a function of char feedstock, char application rate, crop type, and initial soil ph. 5.1 Char Beneficial uses of char include soil enhancement, solid fuel, pollutant immobilization, climate change mitigation, and waste disposal (Crombie et al., 2014; Enders et al., 2012; Jeffery et al., 2015; Ward et al., 2014). Table 12 lists selected parameters that can estimate to what level a char achieves each of the potential benefits. The parameter values are a function of feedstock characteristics described in Chapter 2 and reactor design/operating variables presented in Chapter 3. A set of trade-offs exist when optimizing a pyrolysis system to target a specific benefit (see Figure 5) (Jeffery et al., 2015). For example, maximizing soil enhancement and climate change mitigation potential through the pyrolysis of wood chips, wheat straw and wheat straw pellet at a HHT of 525 ⁰C reduces the benefit as a solid fuel (Crombie et al., 2014). Figure 5. Trade-offs between different char benefits (Jeffery et al., 2015). 28

30 Table 12. Char parameters used to predict potential benefits of char application. Potential Benefit Soil Solid Pollutant Carbon Organic Waste References Parameter Enhancement Fuel Immobilization Sequestration Disposal Yield Yes Yes Yes Yes Yes (Lehmann and Joseph, 2015) Proximate Analysis Yes Yes Yes (Basu, 2013; Ward et al., 2014) Ultimate Analysis Yes Yes Yes (Basu, 2013; Ward et al., 2014) ph Yes Yes (Jeffery et al., 2015) Total/Available Nutrients Yes Yes (Jeffery et al., 2015; Jeffery et al., 2011; Lehmann and Joseph, 2015; Rajkovich et al., 2012) Cation Exchange Capacity (CEC) Yes Yes (Crombie et al., 2014; Kloss et al., 2011; Yuan and Xu, 2011) Adsorption Yes Yes Yes (Granatstein et al., 2009; Kloss et al., 2011; Laird et al., 2010; Manyà, 2012; Sohi et al., 2010) Heavy Metals Yes Yes Yes Yes Yes (IBI, 2013; REFERTIL, 2014; Shackley et al., 2014; USEPA, 1994) Pathogens Yes Yes Yes Yes Yes (USEPA, 1994) High Heating Value Yes (Basu, 2013; Gold et al., submitted; Ward et al., 2014) Labile and Stable Carbon Yes Yes Yes (Crombie et al., 2014; Cross and Sohi, 2013) 29

31 Table 13 displays the trade-offs between parameters with a matrix of correlation coefficients calculated from the pyrolysis of bagasse, rice husks, activated sludge, and cow manure (Shinogi and Kanri, 2003). The sign of the correlation coefficient describes if one variable increases or decreases as the other variable changes value. For example, the char ash content and ph have a positive correlation coefficient, and therefore as ash content increases ph also increases (see Figure 7 as an example for septic tank faecal sludge). The correlation coefficient indicates how closely two variables are related. The closer the absolute value of the correlation coefficient is to one, the stronger the relationship between the variables. Table 13. Correlation coefficients determined from bagasse, rice husks, activated sludge and cow manure pyrolysis (Shinogi and Kanri, 2003). Surface Area (m 2 /g) Yield (%) Density (g/cm 3 ) Total Nitrogen (%) Total Carbon (%) ph Moisture (%) Ash (%) Volatility (%) Fixed Carbon % Surface Area Yield Density Total Nitrogen Total Carbon ph Moisture Ash Volatility Char yield The char yield, presented in Equation 2 (Lehmann and Joseph, 2015), is defined as the mass ratio of dry char and feedstock. The fixed carbon yield, presented in Equation 3, is the ratio of fixed carbon in char divided by the ash-free feedstock fraction. Equation 2. Char yield (Lehmann and Joseph, 2015). ηη cchaaaa = mm cchaaaa mm bbbbbb 100 ηη cchaaaa = bbbbbbbbhaaaa yyyyyyyyyy mm cchaaaa = mmmmmmmm oooo dddddd cchaaaa mm bbbbbb = mmmmmmmm oooo dddddd bbbbbbbbbbbbbb Equation 3. Char fixed carbon yield (Lehmann and Joseph, 2015). cc ffff ηη ffff = mm cchaaaa mm bbbbbb 1 bb aa ηη ffff = ffffffffff cccccccccccc yyyyyyyyyy 30

32 cc ffff = ffffffffff cccccccccccc cccccccccccccc bbbbbbbbhaaaa bb aa = aaaah cccccccccccccc feedstock Char and fixed carbon yield are a function of feedstock properties and reactor operating parameters. For char produced from faeces, septic tank faecal sludge, and wastewater sludge, the char yield decreases but the fixed carbon fraction increases with HHT as shown in Table 14 (Liu et al., 2014). A similar trend has been observed for char produced from lignocellulosic biomass and animal manures. The overall yield represents the mass of char which can be sold to generate revenue. Fixed carbon, as explained later in the stable carbon and carbon sequestration potential section, is correlated to the estimated char stable carbon fraction (as shown in Figure 12) (Crombie and Masek, 2014). The char yield and fixed carbon yields should be analyzed during future research on faecal sludge pyrolysis. Table 14. Proximate analysis results (%wt) for septic tank faecal sludge as a function of pyrolysis reactor HHT ( C) (Liu et al., 2014). HHT (⁰C) Source Parameter Units Initial Beijing, China Ash % 17.0% 26.3% 45.5% 62.5% Volatile Solids % 74.7% 60.5% 31.8% 6.3% Fixed Carbon % 8.3% 13.2% 22.7% 31.2% Proximate and ultimate analysis Proximate analysis is the char mass fraction of ash, volatile solids, and fixed carbon calculated by dry weight (Basu, 2013). The ultimate analysis is the elemental composition, which is the char carbon, hydrogen, nitrogen, sulfur and ash content (Basu, 2013). The feedstock proximate and ultimate analysis results depend entirely on the feedstock characteristics. The char proximate and ultimate analysis results depend on a combination of feedstock and reactor operating conditions. Ash is the inorganic solid residue that consists of mainly silica, aluminum, iron and calcium (Basu, 2013). The mass of ash is assumed to be conserved during the pyrolysis process; however the ash fraction of the char will change with different operating conditions (Lehmann and Joseph, 2015). For example in Table 14, the ash and fixed carbon fractions increase as a function of HHT while the volatile solids fraction decreases. The ultimate analysis results are similarly a function of feedstock characteristics and operating temperature. Information from the proximate and ultimate analysis can be used to estimate char suitability for a specific potential 31

33 benefit. For example, char ultimate analysis results can be used to estimate HHV and carbon dioxide intensity (carbon dioxide emissions/hhv) (Basu, 2013). While the proximate analysis results will help predict fuel ash content. Hence, the results of the two analyses can help to determine the value of char as a solid fuel based on the trade-off between desirable high heating value and undesirable ash content (Gold et al., submitted). Overall, proximate and ultimate analyses are a standard set of measurements which should be conducted on the feedstock and char as part of future research on faecal sludge pyrolysis. Cation exchange capacity (CEC) The char cation exchange capacity (CEC) is the ability to retain positively charged ions in a plant available and exchangeable form (Crombie et al., 2014; Yuan et al., 2011). CEC is an important parameter to assess the potential for char as as soil amendment and to retain artificial fertilizer. Char CEC is a function of feedstock properties and HHT. Figure 6 demonstrates that the relationship between HHT and CEC depends on the pyrolysis feedstock and is different for cow manure, poultry manure, and food waste. Food waste and poultry manure char CEC decreased with increasing HHT, while the CEC of cow manure char peaked at 500 ⁰C and then decreased. The CEC of char produced from wood chips, wheat straw and wheat straw pellets reached the maximum CEC at a HHT between 450 and 550 ⁰C, and then decreased with increasing HHT (Crombie et al., 2014). A reduction in CEC was observed for straw, spruce, and poplar as HHT increased from 400 to 525 ⁰C (Kloss et al., 2011). The char CEC is important because it affects the nutrient retention capacity for ions such as ammonium and therefore the suitability of char as a soil amendment in agriculture (Sohi et al., 2010). These results indicate that faecal sludge char CEC cannot be predicted from HHT and should be determined experimentally as part of future research on faecal sludge pyrolysis. 32

34 CEC (mmole/kg) Cow Manure Food Waste Poultry Manure HHT (⁰C) Figure 6. Cow manure, good waste and poultry char CEC vs. HHT (Rajkovich et al., 2012). Char Sorption Capacity Sorption is the immobilization of molecules that were in the liquid or gas phase through adsorption or absorption (Lehmann and Joseph, 2015). The sorption capacity depends on the char surface area and surface chemistry. Sorption capacity is related to oxygenated functional groups, which increase as a function of char aromaticity and surface area (Laird et al., 2010; Lehmann and Joseph, 2015). Char sorption capacity is tested by measuring the surface area and pore size; sorption capacity increases as a function of HHT (Manyà, 2012). Char with high sorption capacity is more effective at retaining water and mobile pollutants such as arsenic and cadmium, and capturing Polycyclic Aromatic Hydrocarbons (PAHs) (Kloss et al., 2011). Char can also sorb pesticides, which has a duel role of reducing run-off, but could also require higher pesticides application rates (Sohi et al., 2010). Likewise, ammonia and phosphorous sorption reduces nutrient run-off, but at high char application rates sorption can also reduce soil bioavailable nitrogen concentrations (Granatstein et al., 2009; Laird et al., 2010). The value of measuring the sorption capacity of faecal sludge char for agricultural applications is unknown. Rajovich et al. (2012) showed that changes in char surface area did not have a significant impact on biomass yield. ph Based on a review of 80 articles, chars produced from lignocellulosic feedstocks, animal manures and wastewater sludge generally have a neutral or 33

35 slightly basic ph (Lehmann and Joseph, 2015). Char ph increases with HHT, which is attributed to the simultaneous increase in ash content (Lehmann and Joseph, 2015). Figure 7 displays ph and ash content as a function of HHT for char produced from septic tank faecal sludge. Char ph % 12 80% % 6 40% % 0 0% HHT (⁰C) Char ash content (% weight) ph Ash Content Figure 7. ph and ash content as a function of HHT for char produced from septic tank sludge (Liu et al., 2014). Char application increases the ph of acidic soils through a liming effect, or acid neutralization (Jeffery et al., 2015). As shown in Figure 8, soil ph affects the bioavailability of nutrients essential to plant growth; plant growth inhibiting aluminum and manganese also become less bioavailable as ph increases (Ketterings et al., 2005; Martinsen et al., 2015; Yuan and Xu, 2011). Figure 9 shows the results from a greenhouse experiment where three doses (0, 2.5 and 5% weight char/weight soil) of kaiwe wood char were added to a soil with an initial ph of 4.6 (Klau Berek et al., 2011). Char addition increased plant yield and soil ph, while simultaneously reducing the exchangeable aluminum concentration. 34

36 Figure 8. Nutrient availability in soil as a function of soil ph (Ketterings et al., 2005). Figure 9. Effect of char application on plant growth, ph and exchangeable aluminum at 0% (left), 2.5% (center) and 5% (right) application rates (Klau Berek et al., 2011). The magnitude of soil ph change due to char application depends on char properties and application rate, and soil properties (Martinsen et al., 2015). The liming value, which is a measurement of the char acid neutralization capacity based on the inorganic ash fraction composition, is proposed as one of the main predictors for determining the potential of char produced from agricultural waste to increase soil ph (Jeffery et al., 2011; Lehmann and Joseph, 2015; Martinsen et al., 2015; Yuan and Xu, 2011). The liming value has been observed to increase as a function of HHT 35

37 (Yuan et al., 2011). The soil properties that most affect the magnitude of ph increase after char application are CEC and ph (Martinsen et al., 2015; Streubel et al., 2011). Yuan (2011) found change in soil ph to be closely correlated to char liming potential and soil initial ph (Yuan and Xu, 2011). A study on the effects of cocoa shell char application onto 31 acidic tropical soils found that CEC was a better predictor of final soil ph than initial soil ph, and that a lower initial soil CEC resulted in a larger increase in soil ph (Martinsen et al., 2015). Char ability to increase soil ph is one of the factors which determines its economic value as a soil amendment. Overall, understanding the effects of faecal sludge char application on site specific soil ph must be completed before recommending char as a soil amendment. Testing char liming value, CEC and ph, as well as soil CEC and ph are suggested as part of future research. Nutrient concentration and availability The total nutrient concentration is the sum of all nutrients contained in the char. The bioavailable nutrient concentration is the fraction of the total nutrients which plants can assimilate (Lehmann and Joseph, 2015). This can be estimated using nutrient extraction experiments with hydrogen chloride addition or Diethylenetriaminepentaacetic acid (DTPA) extraction (Hossain et al., 2011; Yachigo and Sato, 2013). The char total nutrient concentration is a function of feedstock and operating conditions (Hossain et al., 2011; Lehmann and Joseph, 2015; Shinogi and Kanri, 2003). Between 70 to 90% of total nitrogen is volatilized during the pyrolysis process, with volatilization increasing as a function of HHT (Hossain et al., 2011; Shinogi and Kanri, 2003; Ward et al., 2014). Of the remaining nitrogen, char commonly has a low available nitrogen fraction (Lehmann and Joseph, 2015). Char produced from faeces plus sawdust with an HHT of 500 ⁰C had a total nitrogen content of 3%, however only 0.02% was mineralized after 30 days (Von Herzen, personal communication; Von Herzen et al., 2015). The total phosphorous concentrations measured in char from faeces plus sawdust increased as the reactor HHT increased from 300 to 700⁰C (Von Herzen et al., 2015). As shown in Figure 10, the bioavailable phosphorous fraction was observed to be different between feedstocks, HHT, and heating rate (Crombie and Masek, 2014; Hossain et al., 2011). Due to unpredictable effect of feedstock 36

38 properties and operating conditions, it is recommended to analyze total and bioavailable nutrient concentrations in faecal sludge char as part of future research. Bioavailable phosphorous char/cioavailable phosphorous feedstock (%) 100% 80% 60% 40% 20% 0% HHT (⁰C) Sewage Sludge (10 ⁰C/min) Wheat Straw Pellets (5 ⁰C/min) Wheat Straw Pellets (100 ⁰C/min) Pine Wood Chips (5 ⁰C/min) Pine Wood Chips (100 ⁰C/min) Figure 10. Bioavailable char in wastewater sludge, pine wood chips and wheat straw pellets as a function of HHT and heating rate (Crombie et al., 2014; Hossain et al., 2011) Jeffrey et. al (2015) proposed that the majority of char benefits on crop productivity are from the liming potential and ability to retain nitrogen and phosphorous from other sources (Jeffery et al., 2015). In some cases, char increased nutrient retention by changing the physical, chemical, and biological properties of soil. Overall, an understanding of the mechanisms from char properties to nutrient availability and increased agricultural yields is still being developed (Lehmann and Joseph, 2015; Sohi et al., 2010). In the best case scenario, direct nutrient input and increased soil nutrient availability from char application will reduce nitrogen, phosphorous and potassium (NPK) mineral fertilizer application, improve nutrient utilization efficiency and increase crop yield (Scholz et al., 2014). Estimating the potential for faecal sludge char to provide the above benefits for farmers should be one objective of future research. However, plant growth trials in pots and test field plots may be the most effective way to ultimately present benefits through pictures and videos (Brown, "Personal Communication"). 37

39 Heavy Metals Heavy metals are a concern because of the potential for soil contamination and risks to human health (USEPA, 1997). Heavy metal concentrations in faecal sludge produced from char have not been analyzed. Therefore, this study estimates the heavy metal concentrations and the USEPA polluting loading rate (APLR) limits in faecal sludge char based on the ash content and heavy metals concentration in dry faecal sludge from Kampala, Uganda and Dakar, Senegal (Gold et al., submitted). The estimation is based on the following equations and assumptions based on the pyrolysis of septic tank faecal sludge by Liu et al. (2014): Char proximate analysis: volatile solids + fixed carbon = 1- ash. Ash was calculated based on the reduction in volatile solids and increase in fixed carbon. 20% of the initial volatile solids concentration are volatilized at HHT 300 ⁰C Volatile solids after pyrolysis at HHT 700 ⁰C is 10% of the initial volatile solids concentration. Fixed carbon after pyrolysis at HHT 700 ⁰C is five times the volatile solids concentration. Changes in fixed carbon and volatile solids between HHT 300 to 700 ⁰C were assumed to be linear. Heavy metal volatilization during the pyrolysis was assumed to be zero. For the purposes of this first estimate, this is considered to be the most conservative approach. This is not expected to be true under real operating conditions based on experiments conducted with pyrolyzed wastewater sludge (discussed below) (Hossain et al., 2011; Yachigo and Sato, 2013). The estimated faecal sludge char heavy metal concentrations are included in Table 15; light purple shade indicates the estimated concentrations. Estimated concentrations are below the USEPA maximum and excellent quality values, as well as the limits proposed by the International Char Initiative (IBI). Estimated char produced from faecal sludge from Kampala is expected to exceed the chromium limits proposed by the British Char Initiative (BBI) and the chromium and zinc limits proposed by the Reducing Mineral Fertilizers & Chemicals Use in Agriculture by Recycling Treated Organic Waste as Compost and Bio-char Products (REFERTIL) project. Estimated char produced from faecal sludge from Dakar is expected to 38

40 exceed the BBI Chromium limits and the REFERTIL chromium, copper and zinc limits. The results in Table 15 are considered to be a conservative estimate of heavy metal concentrations meant for illustrative purposes and a first step order of magnitude estimate. At this time, heavy metal concentrations are not considered a prohibitive issue to exclude faecal sludge as a pyrolysis feedstock. The uncertainties listed below, combined with the conservative numbers presented by the BBI and REFERTIL compared to the USEPA and IBI, underline that further research is required regarding heavy metals in faecal sludge char. The first source of uncertainty is the final ash fraction for faecal sludge char. The faecal sludge collected in Kampala and Dakar had an initial ash content approximately three times greater than the septic tank faecal sludge from Beijing by Liu et al. (2014). The initial ash content of faecal sludge from Kampala and Dakar was greater than the ash content of char produced from the same feedstock at a HHT of 500 ⁰C. Therefore, it is challenging to accurately approximate the increase in ash content from char produced from the Kampala and Dakar faecal sludge. The second source of uncertainty is heavy metal volatilization during pyrolysis. The analysis presented in Table 15 assumes zero heavy metal volatilization, however volatilization has been observed to occur during wastewater sludge pyrolysis (Hossain et al., 2011; Yachigo and Sato, 2013). Hossian et al produced char from wastewater sludge at HHT 300, 400, 500 and 700 ⁰C. Cadmium enriched for all char, while chromium, nickel, and lead concentrations reached a maximum at 500 ⁰C and then decreased for char produced at 700 ⁰C. Yachigo et al. (2013) compared char produced at HHT 300 and 800 ⁰C and observed copper and zinc enrichment, but cadmium volatilization (Yachigo and Sato, 2013). These conflicting results support the research needs to assess heavy metal concentrations for faecal sludge char produced at different HHT. The reduced bioavailability of heavy metals (discussed above) could provide a safety factor to allow for an increase in char heavy metal concentration limits for land application compared to biosolids. Experimental results suggest that wastewater sludge char has a lower heavy metal bioavailability than wastewater sludge (Hossain et al., 2011; Méndez et al., 2012). Mendez et al. (2012) specifically determined 39

41 through a soil incubation study that the risk of copper, zinc and nickel leachability was lower for wastewater sludge char than wastewater sludge (Méndez et al., 2012). The bioavailability of heavy metals in wastewater sludge char was found to decrease for manganese, iron, cadmium and zinc (Hossain et al., 2011). Copper bioavailability measurements showed inconsistent results as Yahcigo et al. (2013) showed bioavailability to be higher in a char produced at HHT 800 ⁰C than char produced at 300 ⁰C, while in Hossain et al. (2011) bioavailability reached a maximum at HHT of 500 ⁰C and then decreased (Hossain et al., 2011; Yachigo and Sato, 2013). This could potentially mean that for certain agricultural applications, there is a lower heavy metal bioavailability and leaching potential for faecal sludge char versus dried faecal sludge. It also indicates that comparing the results of experiments which used different nutrient extraction methods can be problematic. The suitability of faecal sludge char in regards to heavy metals can also be assessed by using the USEPA biosolids annual pollutant loading rate (APLR) limits, and then estimate the mass of faecal sludge char that could be applied per year (USEPA, 1994). The results of this calculation are displayed in Table 16. Zinc, highlighted in yellow, was estimated to be the limiting pollutant for both faecal sludge chars. Note, that the measurements provided for faecal sludge feedstock were not accurate enough to calculate the APLR limits for chromium or mercury. Still, the maximum char annual application rates listed in Table 16 greatly exceed the typical char application rates of 2.6, 6.5 and 26 MT char per hectare (corresponding to 0.2, 0.5 and 2.0% char/soil weight to weight ratio (Raijkovich, Enders et al. 2012). 40

42 Table 15. Estimated faecal sludge (Kampala and Dakar) char proximate analysis results compared to septic tank sludge in Beijing (Liu et al., 2014). Estimated heavy metal concentrations compared to regulatory limits (Gold et al., submitted; IBI, 2013; Liu et al., 2014; REFERTIL, 2014; Shackley et al., 2014; USEPA, 1994). HHT (⁰C) Regulatory Limits Source Parameter Units Initial USEPA Max USEPA EQ IBI BBI REFERTIL Ash % 59% 64% 73% 82% Volatile Solids % 37% 30% 16% 3% Fixed Carbon % 4% 7% 11% 15% Arsenic mg/kg Cadmium mg/kg <2 NA NA NA Kampala, Uganda Chromium mg/kg Copper mg/kg Lead mg/kg Mercury mg/kg <0.9 NA NA NA Nickel mg/kg Zinc mg/kg Ash % 47% 53% 62% 71% Volatile Solids % 48% 38% 21% 5% Fixed Carbon % 5% 8% 16% 24% Arsenic mg/kg Cadmium mg/kg <1.8 NA NA NA Dakar, Senegal Chromium mg/kg Copper mg/kg Lead mg/kg Mercury mg/kg <0.8 NA NA NA Nickel mg/kg Zinc mg/kg

43 Table 16. Maximum char application rate without exceeding USEPA APLR limits (USEPA, 1994). Source HHT Arsenic Cadmium Chromium Copper Lead Mercury Nickel Zinc MT/hectareyear hectare-year hectare-year hectare-year hectare-year hectare-year hectare-year hectare-year MT/ MT/ MT/ MT/ MT/ MT/ MT/ Unit C NA NA Kampala, NA NA Uganda NA NA NA NA Dakar, NA NA Senegal NA NA

44 PAHs and dioxins As discussed in section 4.2, the pyrolysis process can potentially lead to the formation of Polycyclic Aromatic Hydrocarbons (PAHs) and dioxins; however, limited research has been carried out on the effect of feedstock characteristics and reactor operating conditions on char PAH and dioxin concentrations (Hale et al., 2012). Hale et al. (2012) measured the PAH concentrations in 28 feedstocks. Based on the information presented in Chapter 2, cow manure is the most similar feedstock to faecal sludge. The PAH and dioxin concentrations measured in Hale et al. (2012) for different reactor HHTs are presented in Table 17. Table 17. Measured PAHs and Dioxin concentrations in cow manure pyrolyzed at various HHTs (Hale et al., 2012). HHT ( C) PAHs (mg/kg) Dioxins (ng/kg TEQ) NL NL NL Compared to Table 9, the measured PAHs concentration in cow manure was two orders of magnitude lower than the concentration limits proposed by the BBI and IBI, and one order of magnitude lower than the IBI proposed standards. Pyrolysis process HHT does not have a clear effect of the PAH concentration. The measured dioxin concentration in cow manure was two order of magnitude lower than the concentration limits proposed in the BBI and REFERTIL standards, and one order of magnitude lower than the concentration limits in the IBI proposed standards. As mentioned in section 4.2, the mechanisms for PAHs and dioxin formation during char production are still being researched. Based on the one to two order of magnitude difference between cow manure char PAHs concentrations and the proposed concentration limits, it is expected that faecal sludge char PAHs concentrations would be below the proposed concentration limits for HHTs 300 to 600 C. Hale et al. (2012) hypothesized that dioxin concentrations could be a function of feedstock chlorine concentrations. Fresh cow manure has a chlorine content of 1.6 %wt, while faecal sludge measured in Kampala had a chlorine content of %wt (Bryne et al., 2015; Sweenten et al., 1986). Therefore, it is also expected that faecal sludge char dioxin concentrations would be below the proposed concentration limits. 43

45 Pathogens Faecal sludge should be considered to contain high concentrations of pathogens. Pathogen inactivation is an important step to reduce environmental and human health risks prior to use of faecal sludge char. The USEPA developed the Part 503 rules to regulate biosolids land application. Within the Part 503 rule, biosolids with pathogen concentrations less than 3 most probable number (MPN) of Salmonella sp. per 4 grams of biosolids or less than 1,000 MPN of faecal coliforms per gram of biosolids are designated as Class A biosolids. Class A biosolids are considered safe and can be applied to any type of land as freely as any other fertilizer or soil amendment. Class A biosolids status can be achieved by meeting specific treatment temperatures and time requirements outlined in Chapter 5. Operating temperature of pyrolysis exceed the requirements needed to achieve Class A biosolids status. Therefore, it can be assumed that char produced from faecal sludge would achieve Class A biosolids disinfection levels and therefore can be land applied for agricultural applications with a very low risk of infection from pathogens. (USEPA, 1994) Other pollutants Char addition has reduced crop yields in some instances due to salinity, aluminum and/or sulfur content (Lehmann and Joseph, 2015). A reduction in corn yield observed at increasing char application rates for char from food waste was attributed to high char salinity (Rajkovich et al., 2012). An understanding of soil, char, and crop properties, as well as ecosystem conditions, helps to determine char compatibility as an agricultural input (Lehmann and Joseph, 2015). High heating value (HHV) The high heating value (HHV) represents the potential of char to release thermal energy upon combustion (Basu, 2013). As shown in Figure 6, the HHV in char produced from faeces decreases as HHT increases. In the absence of equipment to measure the HHV through calorimetry, Equation 3 provides an approximation for the HHV based on faeces elemental analysis (Channiwala and Parikh, 2002). Equation 4 was shown to be an acceptable approximation for the HHV of char produced from municipal organic waste (Lohri et al., 2015a). Assessing if faecal sludge char HHV can be approximated by Equation 3 or Equation 4 should be a component in future research, because using these equations would save time and equipment resources. 44

46 Equation 4. Approximate HHV as a function of elemental composition for faeces (Channiwala and Parikh, 2002). HHHHHH = 349.1CC HH SS 103.4OO 15.1NN 21.1AAAAh Equation 5. Approximate HHV as a function of proximate analysis results for municipal organic waste (Parikh et al., 2005). HHHHHH = FFFF VVVV AAAAh HHV (MJ/kg) HHT (⁰C) Figure 11. Faeces char HHV as a function of HHT (Ward et al., 2014). The HHV provides an approximation for the value of char as a solid fuel, with a higher HHV corresponding to better potential as a solid fuel. However, a trade-off must be considered between the HHV and combustion efficiency of faeces char. Ward et al. (2014) proposed the combustion efficiency of charcoal briquettes produced from human faeces improved with lower hydrogen to carbon (H:C) and oxygen to carbon (O:C) ratios as indicated by reduced carbon dioxide, water vapor and smoke production. For the pyrolysis of faeces, the oxygen and hydrogen concentrations decreased at a faster rate than carbon; therefore, the H:C and O:C decreases as HHT increases (Ward et al., 2014). Stable carbon and carbon sequestration potential The carbon sequestration potential of char depends on the char stability in the soil over time (Manyà, 2012). Multiple methods have been used to approximate char stability: proximate analysis to identify fixed carbon and volatile solids, ultimate analysis to calculate the hydrogen to oxygen (H:C) and oxygen to carbon (O:C) ratios, and chemical oxidation (Crombie et al., 2014; Cross and Sohi, 2013; Enders et al., 2012; Spokas, 2010). The results from the five analysis methods presented in Figure 8 indicate carbon stability was found to depend on HHT and feedstock type 45

47 (Crombie et al., 2014; Cross and Sohi, 2013; Manyà, 2012). The HHT had a greater influence on carbon stability than the feedstock type for char produced from pine wood chips, wheat straw pellets, and rice husks; the stable carbon fraction for char produced at 550 ⁰C ( %) was greater than the stable fraction for char produced at 300 ⁰C ( %) (Crombie et al., 2014; Cross and Sohi, 2013). Proximate analysis could potentially overestimate fixed carbon fraction by underestimating the ash fraction; ash content underestimation occurs because of limited phosphorous, potassium, sulfur and carbonate volatilization at the high heating temperature during analysis (Crombie et al., 2014; Enders et al., 2012). The volatile solids are similarly measured using the proximate analysis method. The chemical oxidation method uses hydrogen peroxide to mimic the oxidation of char over the first 100 years in the soil (Cross and Sohi, 2013). The benefit of this test is that it can be carried out in the relatively short time period of one week (Cross and Sohi, 2013). Figure 12 compares the different analysis methods. The O:C ratio was shown to have a higher correlation across all samples than the H:C; the H:C scatter was attributed to the differences in feedstock properties (Crombie et al., 2014). As expected, increased fixed carbon and reduced volatile fractions increased the stable carbon fraction. Overall, the results presented in Figure 12 illustrated an overall trend in stabile carbon content for select agricultural biomass products. However, the variability in the data indicates that the proximate and ultimate analysis results cannot be directly generalized for all feedstocks to approximate stable carbon content because of the differences due to feedstock properties. Combining the chemical oxidation method with the elemental and ultimate analysis results are recommended to analyze faecal sludge char produced under different HHTs. Although the chemical oxidation method has mostly focused on agricultural feedstocks, the results for faecal sludge could be most directly compared to the stability recorded for chicken manure at an HHT of 500 ⁰C (Cross and Sohi, 2013). 46

48 Figure 12. O:C, H:C, volatile solids, and fixed carbon analysis methods compared to chemical oxidation method for determining stable carbon content (Crombie et al., 2013). 4.2 Meta-analysis results A meta-analysis from Lehman et al. (2015) includes the results of 60 studies to assess the effect of char application rate (Figure 13), crop type (Figure 14), char feedstock (Figure 15), and initial soil ph (Figure 16) on the change in crop productivity. The results of the meta-analysis indicate that char application tends to increase crop yield. However, the change in crop yield is highly variable, as evidenced by the large range of results. This is due to soil, crop, ecosystem and char 47

49 interactions that must be considered prior to recommending char for agricultural application (Kloss et al., 2011; Lehmann and Joseph, 2015). Therefore, careful analysis and testing of faecal sludge char is important prior to promoting its use as a soil enhancement product. Figure 13. Change in crop productivity as a function of char application rate (t/ha) (Lehmann and Joseph, 2015). Figure 14. Change in crop productivity as a function of crop type (Lehmann and Joseph, 2015). 48

50 Figure 15. Change in crop productivity as a function of char feedstock (Lehmann and Joseph, 2015). Figure 16. Change in crop productivity as a function of initial soil ph (Lehmann and Joseph, 2015). 4.3 Gas Gas produced during the slow pyrolysis process consists of hydrogen, moisture, carbon dioxide, methane, aliphatic hydrocarbons, benzene and toluene as well as small amounts of ammonia, hydrochloric acid and hydrogen sulfide (Basu, 2013). Gas should not be confused with syngas, which is hydrogen and carbon monoxide separated from gas; high purity syngas with low particulate and hydrocarbon content can be converted to diesel or gasoline using the Fischer- Tropsch Synthesis (FTS) process (Basu, 2013; Garcia-Perez et al., 2010). The gas yield increased as a function of HHT during pine wood chip, wheat straw, wheat straw pellets, straw pellets and wood pellet char production (Crombie and Masek, 49

51 2014). Since the pyrolysis process is assumed to be a closed system, it is expected that gas production will increase as faecal sludge char yield decreases. In previous research, the gas composition was associated with the thermal decomposition of different fractions of the feedstock; in the case of lignocellulosic biomass, the organic fractions are presented as lignin, cellulose and hemicellulose (Crombie and Masek, 2014). It is also expected that faecal sludge gas composition will vary as a function of HHT as different fractions of the feedstock decompose. This literature review focuses on combusting the gas produced during pyrolysis to produce heat to support the pyrolysis process and/or assist in faecal sludge drying. The two most important parameters for combustion are gas energy content and pollutant concentrations. The gas yield and energy content will directly influence the pyrolysis process energy balance discussed in Chapter 6. Pollutants present in the gas are particulate matter, heavy metals, carbon monoxide, nitrogen oxide, sulfur dioxide and unburned hydrocarbons. Designing and implementing an air pollution control system will be an important task to ensure work safety, regulatory compliance and environmental protection for the implementation of a faecal sludge pyrolysis reactor. Experiments on faecal sludge pyrolysis should assess the gas calorific value and pollutant concentrations. 4.4 Liquid The liquid fraction, commonly referred to as tar, consists of the condensable vapors formed during the pyrolysis process (Inguanzano et al., 2002). The tar is a mixture of complex hydrocarbons and water (Basu, 2013). Tar tends to condense in low temperature zones of equipment, which presents a maintenance challenge as well as a potential opportunity to recover another endproduct (Basu, 2013). The capture and extraction of high value products from the liquid stream produced from pyrolysis has been investigated for various feedstocks including microalgae, rice husks and wood (Czernik and Bridgwater, 2004; Du et al., 2013; Isahak et al., 2012). A review by Du et al. (2013) proposes that fast pyrolysis is the most common and effective operating mode for maximizing tar yield (Du et al., 2013). Ongoing research is being conducted to develop methods to convert tar into fuel that can replace fossil fuels; however such projects are challenging due to the tar corrosiveness, thermal instability and high viscosity (Zhang et al., 2007). In slow-pyrolysis reactors, the tar is usually not extracted. Instead it is valuable to keep it in the gas and combust it to 50

52 produce energy for initiating/maintaining pyrolysis and/or drying of pyrolysis feedstock s. 4.6 Conclusions This chapter proposed a set of parameters to predict whether faecal sludge pyrolysis could safely and cost effectively provide the following benefits: soil enhancement, solid fuel, pollutant immobilization, carbon sequestration, and organic waste treatment. Trade-offs exist when producing char to maximize a specific benefit. It is recommended that as part of future research, correlation coefficients are calculated for faecal sludge char parameters by completing experiments with a homogenous feedstock under different closely controlled operating conditions specifically various heating rates and HHT. These results would determine the optimum operating conditions to maximize each of the five potential benefits. Based on faecal sludge characteristics from samples in Kampala and Dakar, it is expected that faecal sludge char heavy metal concentrations will remain below the limits proposed by the IBI, REFERTIL, and BBI as well as the USEPA regulations for biosolids land application. Based on the results of a single study on cow manure char, it is expected faecal sludge char PAHs and dioxin limits will also be below the IBI, REFERTIL, and BBI proposed standards. It is recommended to measure these parameters in faecal sludge produced under different operating conditions. Finally, the meta-analysis results indicate that changes in crop yield vary highly as a function of pyrolysis feedstock and application rate, crop type, and initial soil ph. Finally, the pyrolysis process also produces gas and liquid end-products. It is expected that these end-products would be combusted for heat recovery at a faecal sludge treatment plant in low-income countries. 51

53 6. Integration of pyrolysis into faecal sludge treatment This section investigates the challenges and benefits of integrating pyrolysis into an existing faecal sludge treatment plant. A mass and energy balance is estimated based on measured faecal sludge characteristics and estimated char yields. Next, the energy balance was revaluated for a scenario where six faecal sludge dewatering technologies are installed before pyrolysis. 6.1 Treatment plant overview The Cambèréne Wastewater and Faecal Sludge Treatment Plant (Figure 17) in Dakar, Senegal and the Bugolobi Wastewater Treatment Plant (Figure 18) in Kampala, Uganda were used as case studies based on Gold et al. (submitted). The treatment process described below for Bugolobi does not represent the actual operation of the treatment plant but the one used by Gold et al. (submitted) for research. At the Cambèréne Wastewater and Faecal Sludge Treatment Plant, faecal sludge is deposited from trucks into a bar screen. The faecal sludge then flows into a settling-thickening tank. The effluent from the settling-thickening tank is co-treated with wastewater. The settled faecal sludge is pumped to drying beds. The feedstock for a pyrolysis unit could hypothetically be fed faecal sludge from the drying beds. (Gold et al., submitted) At the Bugolobi Wastewater Treatment Plant, faecal sludge is unloaded from trucks into a bar screen. The screened faecal sludge is pumped to drying beds. Similar to the Cambèréne Wastewater and Faecal Sludge Treatment Plant case, the pyrolysis unit would be installed after the drying beds (Gold et al., submitted). A faecal sludge mass balance was calculated for the Cambèréne Wastewater and Faecal Sludge Treatment Plant (Table 18) and Bugolobi Wastewater Treatment Plant (Table 19) based on estimated faecal sludge delivered per year, estimated treatment process efficiency and measured faecal sludge characteristics. Tables 20 and 21 display the estimated char production, and faecal sludge mass reduction, at the Cambèréne Wastewater and Faecal Sludge Treatment Plant and Bugolobi Wastewater Treatment Plant for different HHTs. The char yield is based on the yields estimated in Table 15. The method for calculating the char yield is described in the heavy metals section of Chapter 4. 52

54 Figure 17. Cambèréne process flow diagram (Gold et al., submitted). Table 18. Cambèréne faecal sludge flow chart (Gold et al., submitted). Flow in t TS/day. Flow # 1 (Liquid FS) 2 (Liquid FS) 3 (FS effluent) 4 (thickened FS) 5 (dewatered FS) 6 (FS char) 7(effluent) From Vacuum truck Screen Settling-thickening Settling-thickening Drying Bed Pyrolysis Other Treatment To Screen Settling-thickening Co-treatment Drying Bed Pyrolysis End-use End-use Flow See Table 20 Not Listed Figure 18. Bugolobi process flow diagram (Gold et al., submitted). This is not the actual process flow at Bugolobi but the one used by Gold et al. (submitted). Table 19. Bugolobi faecal sludge flow chart (Gold et al., submitted). Flow in t TS/day. Flow # 1 (Liquid FS) 2 (Liquid FS) 3 (Dewatered FS) 4 (FS char) From Truck Screen Drying Bed Pyrolysis To Screen Drying Bed Pyrolysis End-use Flow See Table 21 53

55 Table 20. Estimated char yield and faecal sludge mass reduction for pyrolysis at the Cambèréne Wastewater Faecal Sludge and Treatment Plant (Gold et al., submitted). HHT Mass (t TS/year) Faecal sludge % mass reduction Feedstock C % 500 C % 700 C % Table 21. Estimated char yield and faecal sludge mass reduction for pyrolysis at the Bugolobi Wastewater Treatment Plant (Gold et al., submitted). HHT Mass (t TS/year) faecal sludge % mass reduction Feedstock C % 500 C % 700 C % An energy balance was calculated based on feedstock HHV and % dry matter (%DM), and reactor HHT. The feedstock was divided into water and dry matter; Equations 6 and 7 were used to calculate the amount of heat needed to raise each fraction to the reactor HHT. Table 22 contains the constants used in Equation 6, 7 and 8. Feedstock characteristics are displayed in Table 23. The total heating demand, shown in Equation 8, is the sum of the water and feedstock heating demand plus heat losses. Heat loss was disregarded for this calculation, but should be included in future calculations. The process model developed by Liu (2014) assumed a heat loss of 0.05% of the total heat demand. Equation 6. Water heating requirement during pyrolysis (Liu and Li, 2014). QQ HH2 OO = CC HH2 OO(ll) TT HH2 OO (ll) + RR HH2 OO + CC HH2 OO (gg) TT HH2 OO (gg) QQ HH2 OO = HHHHHHHH dddddddddddd oooo wwwwwwwwww MMMM kkkk CC HH2 OO(ll) = HHHHHHHH cccccccccccccccc oooo llllllllllll wwwwwwwwww MMMM kkkk KK CC HH2 OO(gg) = HHHHHHHH cccccccccccccccc oooo ssssssssss MMMM kkkk KK RR HH2 OO = wwwwwwwwww heeeeee oooo vvvvvvvvvvvvvvvvvvvvvvvv MMMM kkkk TT HH2 OO (ll) = bbbbbbbbbbbbbb pppppppppp aaaaaa ffffffffffffffffff tttttttttttttttttttttt dddddddddddddddddddd ( KK) TT HH2 OO (gg) = HHHHHH aaaaaa bbbbbbbbbbbbbb pppppppppp tttttttttttttttttttttt dddddddddddddddddddd ( KK) 54

56 Equation 7. Dry matter heating requirement during pyrolysis (Liu and Li, 2014). QQ DDDD = CC DDDD TT DDDD) QQ DDDD = HHHHHHHH dddddddddddd oooo dddddd mmmmmmmmmmmm MMMM kkkk CC DDDD = HHHHHHHH cccccccccccccccc oooo dddddd mmmmmmmmmmmm MMMM kkkk KK TT DDDD (ll) = HHHHHH aaaaaa ffffffffffffffffff tttttttttttttttttttttt dddddddddddddddddddd ( KK) Table 22. Constants for Equations 6 and 7 (Liu and Li, 2014). Variable Units CC HH2 OO(ll) MJ/kg*K CC HH2 OO(gg) MJ/kg*K CC DDDD MJ/kg*K RR HH2 OO 2.26 MJ/kg Water Boiling Temperature 373 K Equation 8. Pyrolysis Heat Demand (Liu and Li, 2014). QQ tttttttttt = QQ HH2 OO + QQ DDDD + QQ llllllll Table 23. Faecal sludge feedstock characteristics (Gold et al., submitted). Source Feedstock Energy content (MJ/kg DM) %DM (%DM) Cambèréne % Bugolobi % The heat supply into the reactor for this system would be from combusting the gas produced during pyrolysis. The gas energy content was calculated by dividing the feedstock energy content between the gas and char end-products based on the char yield (Sanchez et al., 2007); Figure 19 shows the estimated gas and char energy contents for faecal sludge from Cambèréne pyrolyzed at 300 C. The amount of energy in pyrolysis gas increases as a function of HHT, which corresponds to a reduction in the organic fraction remaining in the char as discussed in Chapter 4. Table 24 and 25 contain the estimated energy balances for Cambèréne and Bugolobi; the results estimate a positive energy balance. 55

57 Char 0.77 kg (11.8 MJ) Feedstock 1 kg (13.4 MJ) Gas (1.6 MJ) Figure 19. Example energy estimate for faecal sludge pyrolysis at 300 C. Table 24. Estimated energy balance pyrolysis process Cambèréne Wastewater Faecal Sludge and Treatment Plant. HHT Gas Energy Content (MJ/kg DM pyrolyzed) Heat Demand (MJ/kg DM pyrolyzed) Balance (MJ/kg DM pyrolyzed) 300 C C C Table 25. Estimated energy balance pyrolysis process at Bugolobi. HHT Gas Energy Content (MJ/kg DM pyrolyzed) Heat Demand (MJ/kg DM pyrolyzed) Balance (MJ/kg DM pyrolyzed) 300 C C C Process heating demand as a function of feedstock %DM Further analysis was completed to investigate the effect of feedstock moisture content on the energy balance of a reactor using faecal sludge at Cambèréne. The results, displayed in Figure 20, indicate that heating demand needed to pyrolyze the equivalent of 1 kg dry matter of faecal sludge increases significantly as the feedstock %DM decreases. The intersection between the heat demand curve and the estimated gas energy indicate the %DM where the energy demand is equal to the supply; this corresponds to 58, 62 and 70% DM for HHTs 700, 500, and 300 C. These values are within the range of the Pyreg reactor minimum requirements 65% DM for a feedstock with 10 kj/kg DM pyrolyzed at ~600 C (Pyreg GmbH). Figure 20 also shows that the difference in energy demand for different HHT, which is the distance between the three curves, decreases as the %DM increases. The current processes at Cambèréne and Bugolobi utilize drying beds to increase faecal sludge %DM. Drying beds can achieve a 90 %DM in dry arid climates 56

58 such as Dakar, which as displayed in Figure 20 will result in a positive energy balance (Seck et al., 2015). However, in Kampala achieving 90% DM was not feasible due to climate conditions and operating practices (Gold et al., submitted). Furthermore, drying beds require a large area and retention time in the order of weeks to months; rain can prolong drying time and filter bed material inert material such as sand can adhere to the faecal sludge and thereby increase the faecal sludge ash content (Gold et al., submitted; Niwagaba et al., 2014). 50 Heat Demand (MJ/kg DM) % 10% 20% 30% 40% 50% 60% 70% 80% 90% 100% Feedstock % DM HHT 300 C HHT 500 C HHT 700 C Gas Energy 300 C Gas Energy 500 C Gas Energy 700 C Figure 20. Reactor heating demand as a function of HHT and feedstock %DM. 6.3 Slow pyrolysis process in combination with mechanical dewatering Mechanical dewatering technologies are an alternative to drying beds which, in combination with a homogenization tank, could be integrated into the Cambèréne Faecal Sludge and Wastewater Treatment Plant (Figure 17) after the settling-thickening tank. The output %DM content for six mechanical dewatering technologies and the corresponding estimated energy requirements are displayed in Table 26. Table 26 indicates the dewatering technologies have a large range of output %DM values, which the author hypothesizes is a function of feedstock characteristics and operating conditions (such as the addition of chemical coagulants prior to dewatering). The average output %DM for these technologies is 57

59 approximately 30%. Based on Figure 20, 30% DM is insufficient to operate the pyrolysis reactor with faecal sludge from Cambèréne at HHTs between 300 and 700 C without the addition of supplemental fuel. The analysis was repeated for septic tank faecal sludge from Kumasi, which had the highest calorific value of the sludges analyzed in Murray Muspratt et al. (2014), and even with this higher calorific value feedstock supplemental fuel is required for pyrolysis. The analysis indicates sludge would need to have a calorific value of 37 MJ/kg for pyrolysis to be carried out without supplemental fuel after mechanical dewatering to 30% DM. Table 26. Output %DM, energy intensity per cubic meter of thickened faecal sludge, and estimated annual electricity demand for the Cambèréne Wastewater and Faecal Sludge Treatment Plant for different dewatering technologies (Albertson et al., 1987; Andreasen and Nielsen, 1993; Andreoli et al., 2007; Atherton, 2006; BDP- Industries, 2015; Boehler et al., 2003; Huber-Technologies, 2013; Jung and Nellenschulte, 1995; Leschber et al., 1996; Lohmann, 1974; Metcalf and Eddy, 2003; Reifentuhl, 2013; Van Kleeck, 1938; Wakeman, 2007). Pre-coat Vacuum Output DM (%) Electricity Requirement (kwh/m 3 ) Range Average Range Average Estimated Electricity Demand (kwh/year) Filter 20-40% 30% ,800 Belt Press 16-44% 30% ,970 Membrane Filter Press 16-50% 33% ,320 Bucher HPS 22-41% 32% ,320 Solid Bowl Centrifuge 18-40% 29% ,220 Screw Press 13-40% 27% ,520 There are multiple options to still integrate mechanical dewatering and faecal sludge pyrolysis in the Cambèréne Faecal Sludge and Wastewater Treatment Plant without supplemental fuel combustion. The first option is to integrate mechanical dewatering with a thermal or solar drying process (corresponds to the purple arrow in Figure 21). A second option is to increase the energy content of the feedstock by adding high energy feedstock or removing inert fractions from faecal sludge such as gravel or sand. These three changes would increase the energy content in the pyrolysis gas end-product (the blue dashed line in Figure 21). These two options can work in combination; an example is thermal drying after mechanical dewatering and then adding wood chips or saw dust to the feedstock prior to pyrolysis. 58

60 50 Heat Demand (MJ/kg DM) % 20% 40% 60% 80% 100% Feedstock % DM HHT 700 C Gas Energy 700 C Increase Feedstock HHV Figure 21. Effect of changing feedstock characteristics on energy balance for HHT of 700 C. 6.3 Conclusions Based on the energy balance, gas produced during the pyrolysis of dried faecal sludge at the Cambèréne and Bugolobi can meet the process heat requirements. Pyrolysis with faecal sludge with a %DM of 58, 62 and 70 at HHTs of 700, 500, and 300 C would be sufficient to meet process heat demands. This means sludge could be removed from the drying beds earlier, thus increasing existing process capacity. Another alternative is to install mechanical dewatering. Mechanical dewatering technologies could produce sludge with 30% DM; however the %DM has a large range. As shown in Figure 20, the energy in gas produced from pyrolysis of faecal sludge from the Cambèréne and Bugolobi at 30% DM is insufficient to meet the process heat demand. There are three potential solutions to this energy deficit: combust supplemental fuel to meet the heat demand, mix high energy and/or low moisture feedstock with faecal sludge or combine mechanical dewatering with a thermal/solar drying technology. 59

61 7. Existing pyrolysis reactors This chapter will provide an overview of the Pyreg GmbH ( Oekozenturm Langenbruck ( and the Climate Foundation ( pyrolysis technologies. To the author s knowledge, these are among the most promising pyrolysis technologies on the market (Pyreg GmbH) or in development (Oekozentrum Langenbruck, Climate Foundation) for wastewater sludge. However, none of the technologies have yet been tested with faecal sludge as a pyrolysis feedstock. All three reactors described below operate in continuous mode. Table 27. Pyrolysis Reactor Overview (Lohri, 2015; Rensmann, personal communication; Von Herzen, personal communication). Company/Organization Stage of Development Scale Operational complexity Pyreg Gmbh Commercialized Municipal Treatment Plant Most Oekozentrum Langenbruck Field Tested Municipal Treatment Plant/Community Medium Climate Foundation Field Tested Community Least The Pyreg reactor has been implemented for agricultural wastes and wastewater sludge in Germany and Switzerland. The system includes a gas combustion unit, air pollution control equipment and char cooling. The company has indicated that they have no current activity in low-income countries. The equipment requires maintenance to be completed by trained specialists (Rensmann, personal communication). The Oekozentrum Langenbruck reactor has been tested successfully with agricultural waste (e.g. coffee pulp, wood chips). The system also includes a gas combustion unit; however it is unclear if the unit comes standard with air pollution control equipment. Oekozentrum is interested in marketing the reactor for low-income countries. It was designed to provide a solution for coffee producers in Peru. (Lohri, 2015) The Climate Foundation s reactor is currently being tested using a feedstock of faeces and sawdust in Bangalore. The reactor contains a gas combustion unit and air pollution control equipment (Von Herzen, personal communication). 60

62 Capital cost estimates for the three reactors are presented in Table 28. It is not possible to estimate the annual operating and maintenance costs for faecal sludge pyrolysis with the current available information. The author estimates these costs will depend on the process energy balance, labor costs, and feedstock characteristics. Similarly, the equipment lifetime with faecal sludge as a feedstock is not possible to estimate at this point. Feedstock characteristics of faecal sludge would be expected to effect the equipment lifetime (for example high sand content would wear down metal parts faster). Table 28. Pyrolysis unit capital costs, annual capacity, and cost per capacity (Lohri, 2015; Rensmann, personal communication; Von Herzen, personal communication). Developer Capital Costs (USD) Capacity (DM t/year) Cost per ton Capacity (USD/DM t*year) Pyreg 700,000 1, Oekozentrum Lange 24, Climate Change Foundation 95,

63 8. Alternative uses of char This chapter presents alternative ways to use char apart from a soil conditioner. For example, char can be loaded with compost and nitrogen, phosphorous and potassium (NPK) fertilizers or briquetted for use as a solid fuel (Lehmann and Joseph, 2015). 8.1 Organic waste composting Composting is the aerobic biological conversion of organic solid waste into a soil amendment, carbon dioxide, ammonia, water, and heat. The process also reduces the organic waste volume. A well operated composting process includes a prolonged thermophilic phase (temperature greater than 60 C), which inactivates pathogens. In the soil, compost can inhibit plant growth if nitrogen immobilization occurs when the carbon to nitrogen ratio is too low (C: N) or if ionic nitrogen concentrations are excessive; mixing char with compost can alleviate this by adsorbing nitrogen to prevent ammonia volatilization or excessive ammonium concentrations. (Lehmann and Joseph, 2015) Similar to the relationship between soil, char, and crops discussed in Chapter 4, the effects of char addition on the composting process are expected to vary based on char and compost feedstock characteristics, char and composting process operating conditions, and char application rates. Char application to the composting process has been observed to increase efficiency measured as increased microbial activity indicated by a higher heating rate (temperature change over time explained in chapter 2) and carbon dioxide emissions. Other observed benefits include reduced ammonia volatilization, methane emissions, and odor formation. The observed changes in char properties due to the composting process are reduced CEC and copper and zinc mobility and extractability. (Lehmann and Joseph, 2015) Interestingly, the studies listed in Lehmann et al. (2015) add char produced from lignocellulosic biomass into a manure or wastewater sludge composting process. As discussed in Chapter 2, lignocellulosic biomass has high concentrations of lignin; additionally, it has low water and nutrient contents. Therefore, lignocellulosic biomass degrades at a very slow rate. Typical composting feedstock such as manures have higher nutrient concentrations and degradation rates compared to lignocellulosic biomass. As part of future research related to char and composting, it 62

64 would be suggested to consider char produced from lignocellulosic biomass could be added to a faecal sludge composting process. (Lehmann and Joseph, 2015) 8.2 Cooking briquette production Char produced from faecal sludge could substitute wood-based charcoal as a cooking fuel. Ward et al. (2014) found briquettes produced with faeces char pyrolyzed at a HHT of 300 C meet the minimum HHV defined by the Food and Agriculture Organization (FAO) for charcoal. The faeces char briquettes had a HHV similar to commercially available charcoal. Increased pyrolysis reactor HHT reduced faeces char briquettes HHV, with briquettes produced at 300 C having nearly twice the energy content of those produced at 750 C. However, faeces char produced at lower HHTs had higher hydrogen to oxygen ratio; this is expected to result in lower combustion efficiency and higher smoke production. Overall, Ward et al. (2014) showed that briquettes produced from faeces char are a promising substitute for wood-based charcoal (Ward et al., 2014). Faecal sludge char could be used as an input or co-input in briquette production. Selling faecal sludge char to cooking briquette manufacturers would provide another revenue source to support faecal sludge pyrolysis at a centralized treatment plant. The price paid by briquette manufacturers is expected to be greater than the price farmers would be able to pay to use char as a soil amendment. Faecal sludge char could replace wood as a briquette manufacturing input, which would reduce the associated impacts from large scale deforestation. Further investigation should be conducted to determine the technical parameters which make char an effective cooking fuel, such as HHV, moisture content, and strength. 8.3 Solid fuel for industrial applications Faecal sludge char could potentially be used as a solid fuel in industries with large and consistent fuel demands such as coal power plants and cement kilns. One of the main potential benefits is increased revenue; in Kampala, Uganda, dried faecal sludge sells for $10 per ton as a soil conditioner while coconut husks and sawdust sell for $150 per ton as a solid fuel (Gold et al., submitted). Concerns about using faecal sludge char as a solid fuel are the same with using dried faecal sludge or wastewater sludge ash content and heavy metals. Gold et al. (submitted) proposed that dried faecal sludge is comparable to wastewater sludge; industries currently combusting wastewater sludge should be able to directly substitute dried 63

65 faecal sludge. However as discussed in Chapter 4, the pyrolysis process increases the ash content and heavy metal concentrations in char compared to the feedstock. The ratio of fuel value (HHV) to the ash and heavy metal concentration is lower in faecal sludge char than in dried faecal sludge (Liu et al., 2014; Thipkhunthod et al., 2006; Ward et al., 2014). Therefore, we propose that faecal sludge char is an inferior industrial solid fuel to dried faecal sludge. 8.4 Agricultural fertilizer Char can be added to soil in combination with nitrogen, phosphorous, and potassium (NPK) fertilizer. The objective of char application is to increase fertilizer use efficiency; this is done by increasing the nutrient retention either in plants to due crop yield or in the soil. The char properties which increase crop yield are described in Chapter 4, while increased retention in soil occurs due to cation adsorption (a function of CEC) or improved soil water retention capacity that reduces run-off volume (Lehmann and Joseph, 2015). Wood char application increased the nitrogen retention in highly weathered tropical soil to from 10.9% to 18.1% (Steiner et al., 2008). Similar to adding char as a stand-alone soil enhancement product as discussed in Chapter 4, the potential benefits of adding char in combination with organic fertilizer are expected to depend on crop type, soil properties, char application rate, and environmental conditions (Lehmann and Joseph, 2015; Scholz et al., 2014). Faecal sludge char co-application with NPK fertilizer could decrease the amount of fertilizer needed for specific crops. However, making specific recommendations based on the crop type, soil properties and char characteristics will require further experimental work. Field trials would also be recommended to provide visual evidence to farmers that this is effective. 8.5 Conclusions The applications for char beyond a standalone soil amendment are ways to generate increased revenues. Each of these applications is completed in combination with another product or process. Obtaining faecal sludge properties as a function of different operating conditions is a first step before estimating which of these applications could increase project viability in specific markets. 64

66 9. Conclusions In conclusion, the goal of future research should be to better understand the potential of faecal sludge pyrolysis to safely and cost effectively achieve the five potential benefits in low-income countries. This work should begin with bench scale experiments; an experimental plan is proposed in Figure 22 below. The operating conditions which should be varied are HHT, heating rate, hold time, and solids retention time. A small tube furnace, shown in Figure 23, would allow for experiments which vary these operating conditions. A tube furnace can also be integrated into an experimental set-up where an inert gas flows through the tube to capture the pyrolysis gas. Analyze then pyrolyze faecal sludge Vary pyrolysis operating conditions Analyze char properties Compare results with other chars Propose operating conditions for 5 PBs Figure 22. Proposed experimental plan to assess the potential of faecal sludge pyrolysis. Figure 23. Thermo scientific tube furnace suitable for bench-scale pyrolysis experiments (Thermo Scientific). 65

67 Figure 24. Example experimental pyrolysis set-up (Liu et al., 2014). Looking at the bigger picture, the trade-offs of implementing pyrolysis as a faecal sludge treatment process in a low-income country have been summarized with a strengths, weaknesses, opportunities, and threats (SWOT) analysis. The results are displayed in Figure 25. Pathogen destruction and volume reduction are key strengths of the pyrolysis process at the HHTs between C (Liu et al., 2014; USEPA, 1994). The treatment plants could increase existing drying bed capacity by removing and pryolyzing sludge at less than the current output %DM. This would reduce the retention time in Cambèréne from days to an estimated 14 days (Sandec, unpublished data), and thereby increase the drying bed capacity by 14-57%. Finally, treatment costs can be off-set by additional revenue through the five potential benefits defined in Chapter 1: waste management, solid fuel, soil enhancement, carbon sequestration and pollutant immobilization (Jeffery et al., 2015). Weaknesses are increased process complexity, air pollution and potential supplemental fuel requirements. Process complexity will vary as a function of the reactor design and operating mode (i.e. batch, semi-batch, or continuous). Ensuring that a treatment plant has the necessary knowledge and equipment to operate the reactor and perform the necessary maintenance is essential to ensuring stable and safe long-term success. Meeting the energy demand necessary to heat the feedstock will depend on feedstock characteristics, energy content, %DM and reactor HHT. In the case where pyrolysis gas cannot meet the reactor heat demand, supplemental fuel would need to be combusted. Another weakness is the new source of air pollution resulting from the on-site combustion of pyrolysis gas, which is expected to emit pollutants described in Chapter 4. These pollutants can be managed through the installation of the appropriate air pollution control equipment. More information is required to estimate capital and operating costs for a faecal sludge pyrolysis process in a low-income setting. 66