Keywords

1 Introduction

There is appreciation in using anaerobic digestion (AD) as a step at treatment plants managing faecal sludge (FS) from communities or towns. Small-scale digesters are more practical for use in faecal sludge treatment plants (FSTPs) in low- and middle-income countries (LMIC) due to low complexity, low capital and maintenance costs as opposed to large-scale digesters at centralized wastewater treatment plants (Tayler 2018). Unlike on-site biodigester toilets, where fresh excreta (faeces and urine) are used as the feedstock in institutions and public places, biodigesters at decentralized scale receive FS previously stored in on-site sanitation facilities (pit latrines, public toilets, septic tanks, aqua-privies and container-based/portable toilets) and are expected to behave differently. The biodegradable characteristics of FS from these sources do not only vary with different technology options but also other geographical and environmental factors such as groundwater infiltration, emptying frequency, user habits, constituent materials, type, concentration of contaminants and location of sanitation facilities (Still and Foxon 2012). For example, biomethane potential (BMP) is very low in FS from septic tanks since it is partially stabilized but high from public and container-based toilets, where the FS retention period is short (Rose 2015). In addition, FS from septic tanks at the household level desludged after 5 years is expected to be more stabilized compared to that from septic tanks in public places such as schools and hotels, desludged in less than 1 year (Schoebitz et al. 2014). The difference in retention times causes variation in BPM of FS depending on the source. This makes AD of FS from containment technologies of varying retention times feeding the same decentralized digester technology (at treatment plant) complex. However, there is limited information on technical and operational feasibility, as well as opportunities for AD of FS at decentralized scales. This chapter presents the potential of AD technology at decentralized FS treatment plants by analysing the biodegradability characteristics of FS from different sources, BMP, co-digestion, operation and maintenance as well as management options of the produced slurry.

2 Faecal Sludge as a Feedstock

Faecal sludge treatment facilities mainly receive partially stabilized FS, which has stayed for varying durations in different types of on-site containments. However, considerable amounts of organics have been realized in various types of FS received at treatment facilities as reflected by the higher fraction of volatile solids in Table 14.1. The purpose of the AD is stabilization of the partially digested, fresh or raw FS, pathogen and odour reduction, as well as production of biogas energy and slurry.

Table 14.1 Characteristics of faecal sludge from various containments
Full size table

The optimum pH for AD for high biogas yield ranges between 6.5 and 7.5 (Vögeli et al. 2014), which is generally the pH range for the various types of raw and partially digested FS. There is no need of adding chemicals to raise or lower pH for the operation of the plant. However, container-based FS exhibits low pH ranges, which necessitates mixing with FS from other containments to improve characteristics for a good biogas substrate. This can be achieved by introducing a mixing/buffer tank before the digester to aid in homogenization of FS from different containments.

Total solids (TS) for FS from most containments average >6%, which is required for unstirred fixed-dome digesters, in order to limit solids settling (Sasse 1988). In addition TS in the range of 5–10% is optimal for operating anaerobic digesters without addition of extra water (Nijaguna 2002). However, for TS > 4%, there is a need to have fixed-dome digester base slanting towards the middle for easy collection and removal of any settled sludge. The volatile solids (VS) of FS from various sources are >50% TS, which is required for application of fixed-dome digesters. FS temperature in most tropical countries is in the mesophilic range (20–40 °C), which is resistant to operational challenges since this can be achieved at minimal or no extra energy input.

The C/N ratio of FS from most containments is below the optimal requirement (16–25) for AD. This may lead to ammonia accumulation in the digester, which is toxic to methane-forming bacteria. Therefore, it may necessitate raising the C/N ratio during operation, through various ways such as co-digestion with organic feedstocks of a higher C/N ratio. FS can potentially be co-digested with other organic waste streams such as market wastes, municipal solid wastes, brewery waste or primary sludge to improve AD (Englund and Strande 2019).

3 Biomethane Potential for Faecal Sludge

Biomethane potential (BMP) measurements provide evidence for biogas production performance from substrates based on hydrolysis and degradation rates (Raposo et al. 2012; Hagos et al. 2017). BMP test is crucial before the full-scale design of anaerobic digesters to predict biogas production. However, since biogas production in real-time conditions faces design limitations, the expected values are lower than laboratory-scale values (Rose 2015). This is because the methane gas yields are under optimal conditions (such as mixing and constant temperature), whereas actual yields are reduced due to various uncertainties in full-scale operation.

BMP experiments, on the other hand, allow comparison between different substrates, and more reliable information can be obtained through setting up pilot-scale digesters (Englund and Strande 2019). FS presents higher theoretical BMP values when degradability is not accounted for. Therefore, fresh FS from container-based sanitation technologies presents higher BMP values, which is more suitable for AD, as compared to other sources of FS. Studies of different types of FS have indicated methane production completed within 10 days, implying that sludge holding times exceeding this period don’t improve the yield of biogas (Rose 2015). However, a minimum hydraulic retention time (HRT) of 20 days has been reported for odour reduction (Englund and Strande 2019; Tayler 2018).

4 Co-digestion of Faecal Sludge with Organic Waste Streams

Co-digestion involves the use of more than one waste stream in anaerobic degradation, aimed at circumventing the limitation of using single substrates, thereby improving biogas production (Hagos et al. 2017). This is a promising technology in most cities or towns of LMIC, where management of different waste streams poses a number of challenges. AD process can co-manage more waste streams, improving biodegradability and nutrient balance.

4.1 Case Studies on Co-digestion of Faecal Sludge and Organic Waste Streams

Various researchers have determined the implications of co-digesting FS with other waste streams as summarized in Table 14.2. Majority of the studies reviewed were performed in laboratory experiments, with only one that used a field-installed biodigester. The experimental reactors were all batch fed and operated mainly under mesophilic conditions. The feedstocks/substrates added to FS in these studies included organic food waste, garden waste, cattle and poultry manure, sludge and effluent from wastewater treatment plant (WWTP). The performance/dependent variable for the experiments was mainly biogas generation.

Table 14.2 Case studies on co-digestion of faecal sludge and other waste streams
Full size table

From these studies, the volume of biogas produced increased when FS was digested with other wastes than when digested alone. Hoang and Nguyen (2020) noted better biogas quality when co-digested, where a high rise in composition of CH4 was observed to reach 71.5%. The optimal biogas generation/yield is linked to feedstock type, co-digestion, particle size reduction, operational temperature, C/N ratio and pH. Hoang and Nguyen (2020) obtained a biogas yield of 13 mL/g dry matter (DM) when FS was digested alone, but the yield increased to 18 mL/g DM when FS was mixed with poultry manure (PM), cow manure (CM) and sewage sludge (SS). However, mixing two substrates produced more gas than mixing four substrates. For example, FS with PM produced a biogas yield of 28 mL/g DM, FS and CM produced 25 mL/g DM and FS with SS produced 25 mL/g DM.

Anaerobic digesters operating under thermophilic temperature ranges take shorter time to produce gas compared to those in mesophilic temperature range (Burka et al. 2021). The favourable pH range for AD is near neutral; hence, an initial drop in pH, resulting from acidogenesis and acetogenic oxidation, inhibits biogas generation. However, low pH can be solved through addition of sodium bicarbonate as a buffer (Afifah and Priadi 2017). Another product of acidogenic fermentation that often causes inhibition of anaerobic degradation is ammonia.

Overall, it can be deduced that while biogas can be recovered from FS, a co-feeding substrate is necessary for enhancing AD, thereby improving the quantity and quality of biogas recovery (Table 14.2). This is more so, considering the fact that FS undergoes partial stabilization during collection in on-site containment systems, resulting into a degraded quality with decreased biogas potential. Containments filling for longer time are more affected than those which are filled for shorter time, where FS from CBS systems preferred for biogas production in decentralized FSTPs.

5 Anaerobic Digestion Products

The leading AD products (biogas and bio-slurry) produced from digestion and/or co-digestion of FS and other substrates such as organic solid waste, cattle, pig and buffalo dung have immense benefits. Biogas offers an alternative clean and modern energy source that can replace dirty biomass fuels, with the potential to contribute to poverty alleviation. Bio-slurry can boost agriculture production with recyclable sustainable nutrients. Utilization of both biogas and bio-slurry from AD can contribute towards alleviating climate change-induced impacts (Warnars and Oppenoorth 2014). The products of AD are discussed in the following sections.

5.1 Biogas Use Alternative

Biogas can be put to use in various ways such as cooking, lighting or driving engines of vehicles or other machineries. The latter is applicable for large-scale systems or treatment plants, where the produced heat can also be put to use. At decentralized FSTPs, biogas can be used to meet the cooking requirements of the workers at the plant and heat requirements for stabilization of equipment for the plant laboratory.

The biogas production patterns at the FSTPs do not match consumption. Gas usage mainly happens during the day, but production continues throughout the night. In cases of low biogas usage at the treatment plant, there may be a need to package the gas in gas storage bags to be used at a different location. However, biogas has a limitation of low energy density (6 kW h/m3), which necessitates large storage volumes unless it is compressed. The option of storage in compressed medium to high-pressure gas cylinders is not feasible for decentralized FSTPs due to high costs involved (Vögeli et al. 2014).

In cases where packaging is not feasible and/or gas utilization patterns are interrupted such as non-functioning gas stoves, biogas needs to be flared in order to control methane release to the environment. Therefore, a gas burner for use in flaring needs to be considered in the design of FSTPs based on AD technology.

5.2 Bio-slurry Management and Uses

The effluent from the digester after gas production is known as bio-slurry or digestate. Like most digesters in developing countries, which operate under mesophilic temperature zone, the generated slurry at FSTPs still has high levels of pathogens; hence it cannot be used in the same ‘as generated’ state unless further treatment is done.

Bio-slurry has high water content; hence there is a need to be dewatered before use. Dewatering can be done using the common existing technologies such as sand drying beds. The liquid effluent from the drying beds percolates downwards through the filter media, while the solid fraction is retained on the beds, which is later dried. Dried bio-slurry has calorific value in the same range as FS; hence it can be put to similar uses such as production of fuel briquettes, soil conditioner, vermi-compost, animal feeds and protein-rich supplement or bricks for construction work (Semiyaga et al. 2015). However, cost (capital, operation and maintenance) assessment of the required technologies for various products needs to be considered, since this has been reported to be challenging in sustaining the decentralized plant operations (Massoud et al. 2009).

Where agricultural activities exist without food crops to be eaten raw, or in tree farming, bio-slurry can be applied without further treatment. It can also be applied in agriculture with deep row entrenchment. Furthermore, pathogens are not assimilated in plant material (roots, shoots, leaves or fruits). Therefore, health risks associated with the use of microbiologically contaminated bio-slurry can be averted by implementing a multi-barrier approach (WHO 2006) in combination with the sanitation safety planning approach (WHO 2016). The advantage with the use of bio-slurry in agriculture is that it contains plant nutrients, which are already mineralized and therefore readily available to the crops. This is as opposed to unstabilized organics, which, when applied in agriculture, must undergo several conversion processes before they are available to the crops. Moreover, slurry application in agriculture, as a replacement to synthetic fertilizers, not only replaces finite resources but also adds humic substances to soil. Humic substances are not present in synthetic fertilizers. The end result of using bio-slurry is to replenish soil fertility, which contributes to cycling of plant nutrients, leading to sustainable agriculture.

The liquid effluent stream after dewatering joins the liquid line of the FSTP, which is later discharged or reused after treatment. Liquid effluent is a plant nutrient-rich irrigation resource although it should not be applied on plants which are eaten raw such as vegetables and root crops such as cassava and potato, since the treatment system at most FSTPs does not eliminate viruses and bacteria. Therefore, the most practical means of managing liquid effluent stream is through disposal into the environment after treatment; hence the presence of a sink such as a wetland is crucial when locating a treatment plant, based on AD.

6 Case Studies on Decentralized Scale Anaerobic Digesters

Although available literature indicates the viability of AD, in sanitation, solid waste management and energy recovery, only a few deals with its application in FS treatment at the decentralized scale. This section provides an overview of twelve (12) documented case studies, which are limited to geographic areas where FS management is prominent, such as sub-Saharan Africa and South Asia. Nine (9) documented cases of AD plants are in operation at the full scale, and two (2) are pilot scale, while one (1) is an experimental digester (Table 14.3). Most FSTPs have adopted a fixed-dome biogas digester, which is preceded by a screen chamber that receives the FS from the desludging vehicles. FS collected from pit latrines is reported to have high municipal solid waste content, which is problematic to AD processes. Therefore, application of screening stage before the digester unit at the treatment plant holds the key (Zziwa et al. 2016). Digesters handling FS should be positioned after the reception and preliminary treatment facilities. Ideally, the extraneous materials such as solid wastes and grit need to be removed by screening and grit removal units, since these waste streams do not contribute to the biogas production. In addition, FS after screening has particle sizes of less than 5 mm, which is reported to be in a range required to have onset of AD (Semiyaga et al. 2017).

Table 14.3 Case study faecal sludge treatment plants using anaerobic digestion as a treatment step
Full size table

After screening and/or grit removal, a homogenization/feeding tank is applied in a number of treatment plants to prevent shock loading of the digesters, since FS arriving at the treatment facility has varying characteristics. For example, at the Devanahalli (India), a treatment plant receives FS from septic tanks and soak pits into a feeding tank, where settling takes place. The anaerobic digester only receives the settled solid faction, while the liquid stream is treated in other proceeding units (Rao et al. 2020). The anaerobic digesters in this case are not stirred. On the other hand, the two case studies cited in China make use of continuously stirred tank reactors (CSTR) (Shikun et al. 2017). Stirring helps in shortening the hydraulic retention time; hence it can be ideal where large volumes of FS are to be digested. However, there is more energy involved in operation of the stirred reactors.

Most anaerobic digesters are reported to operate under mesophilic conditions, with the exception of the CSTR that operated either under mesophilic or thermophilic conditions. The biogas digesters vary in size with retention time of the FS ranging from 4 h (for the UASB) to 20–45 days in case of unstirred reactors. From the cases reported in Table 14.3, two value propositions are noted: (1) biogas that is often used for heating and lighting at the treatment plant and (2) bio-slurry that is processed to form a compost or soil conditioner. In one case (Nashik, Maharashtra, India), where FS and organic solid wastes are co-digested, the biogas is purified and used to generate electricity.

Finally, two cases reported successful operation and maintenance (O & M) of the plants by municipalities (Rao et al. 2020; Rath and Schellenberg 2020). However, some of the challenges cited from the O & M include non-operation of the plants to their full capacity owing to small volumes of FS collected from the communities. Commercialization of biogas generated presented a challenge, as its cost is found to be higher than the use of alternative energy sources. As such biogas is often used within the treatment plants. The sale of compost is also limited, which was attributed to limited farmlands within the plant location. Therefore it was opted for use in landscaping within the municipalities.

7 Operation and Maintenance of Biodigesters at FS Treatment Stations

There is a need to develop and implement an operation and maintenance (O & M) strategy that includes a task schedule and allocation of responsibilities and having control mechanisms for proper checking of the completed duties. Some of the specific O & M considerations for anaerobic reactors at FSTPs start from feeding digesters, regular monitoring and periodic maintenance.

The anaerobic digester is fed regularly to maintain stable gas production; hence, the design has to be adequate for the routinely delivered FS quantities and co-digestion substrates (if any). The alternative organic wastes for co-digestion should be pre-treated to remove impurities (such as metals, glass and plastics) and reduce particle sizes to <5 cm. This is necessary to raise the surface area for microorganisms to access and degrade the material faster. This is relevant for AD, where the microorganisms are slow degrading.

For periodic maintenance purposes, solids that settle or accumulate at the digester bottom are not easy to remove, particularly in fixed-dome digester type. Tayler (2018) proposed periodic removal of the settled solids (sludge) using vacuum trucks, leaving the digester for several days to reduce risk from dangerous gases and then manually emptying the residual sludge. This can be achieved by considering design of two digesters operating in parallel. However, precaution should be taken by the workers to use appropriate personal protective equipment (PPE) with breathing apparatus.

Other general monitoring activities can be regularly checked, such as gas tightness of the pipes and dome, pipe blockages, slurry levels in the expansion chamber and biogas stove (where cooing is an option) (Vögeli et al. 2014).

8 Conclusions

Faecal sludge treatment plants in LMIC have a huge AD feedstock potential. This potential is driven by the majority of the population in these areas using on-site sanitation technologies, where FS is generated. Most of the existing FS treatment plants do not make use of the available FS feedstock to recover energy in the form of biogas. The characteristics of FS such as TS, pH, temperature and BMP depict potential for biogas production. The fresher the FS, the higher is the potential to generate large quantities of FS. AD will suit the treatment of FS, if other benefits from the process such as organic matter stabilization as well as reduction of pathogen and odours are aggregated. To achieve increased biogas production from FS, its low C/N ratio should be boosted by co-digestion with other organic waste materials. The logistics of collecting source-separated organic solid wastes and delivery to where the AD plant is located should be analysed in view of the costs involved and the benefits to be realized. The use of bio-slurry in agriculture, although advantageous, since AD mineralizes nutrient content, making it readily available to crops, is riddled with its pathogenic content due to insufficient sanitization occurring, in AD. The microbial health risks involved can be circumvented by applying in fertilizing crops not to be consumed raw, deep row entrenchment and implementation of multi-barrier approaches in combination with sanitation safety planning.