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1Anaerobic treatment systems for treating faecal sludgeAmandeep Singh, M.sc WATENVAbstract:Faecal sludge is waste collected from toilets and contains high organic contents compared to general domestic wastewater, hence treatment of the faecal sludge as wastewater is not economical and efficient.Anaerobic treatment of faecal sludge is efficient as there is enormous generation of methane (CH4) during the digestion process which can be converted to biogas, as a source of energy.Introduction:Faecal Sludge and its characterisctics:Faecal sludge (FS) is raw or partially digested, slurry or semisolid, and results from the collection, storage or treatment of combination of excreta and blackwater, with or without household drainage. FS is highly variable in consistency, quantity, and concentration. It contains a lot of matter, solids and nutrients collected from septic tanks and pit latrines, as well as from low or zero-flush, unsewered public toilets. Faecal sludge defecated without treatment spreads pathogenic bacteria’s into onsite and open places that may results to health hazards to living organisms and human beings. (Linda Strande 2014)Anaerobic treatment for Faecal Sludge:Anaerobic treatment of faecal sludge is the most apt method employed in decentralized sanitation systems especially when the objective is resource conservation and reuse. Anaerobic digestion is suitable for faecal sludge treatment in all environmental conditions2even when faecal sludge is highly concentrated. The anaerobic effluent is generally required to be processed further for reuse or discharge. (Kujawa-Roeleveld, Zeeman 2006)Anaerobic conditions are characterized by the lack of oxygen. Anaerobic degradation occurs anywhere in FSM systems where oxygen has been depleted, for example anaerobic and facultative waste stabilization ponds, septic tanks, and settling tanks. Anaerobic fermentation can also be employed for the treatment of sludge. Anaerobic digesters can provide a beneficial method of stabilizing FS, as it also results in the production of biogas that can be used for energy generation. Biogas is a mixture of mainly methane (55- 75%) and carbon dioxide (30-45%) (Arthur et al. 2011).Due to the less energetically favorable nature of anaerobic metabolisms, less sludge (i.e. microbial biomass) is produced during anaerobic digestion. Anaerobic digestion is a complex process characterized by hydrolysis, fermentation, acidogenesis, acetogenesis and methanogenesis. Hydrolysis is an enzymatic process through which particulate matter and more complex organic compounds are degraded. At the same time, proteins, lipids, and polysaccharides are converted into amino acids, fatty acids, and monosaccharides. During fermentation (or acidogenesis) acidogenic microorganisms further degrade amino acids, sugars, and fatty acids to methogenic substrates (e.g. H2, CO2, methanol, methylamines, and acetate). Organic molecules are used as both electron donors and accepters. Therefore, methanogen, archea can be characterized as chemoorganotrophs. During methanogenesis, one group of archea split acetate into methane and carbon dioxide, while another group produces methane through the use of hydrogen and carbon dioxide. Methanogenesis occurs more readily at mesophilic (30- 38°C) and thermophilic (49-57°C) temperatures. Methanogenic processes are presented in the following equations. (Benjamin Hemkendreis, Manuel Henseler and Karin Güdel 2008b)Equation 1: 4H2+ CO2 g CH + 2H2O3Equation 2: CH3OH + H2 g CH4 + H2OEquation 3: CH3COO- + H2O g CH4 + HCO3 (Madigan et al., 2003)Process in Anaerobic treatment:Acidogenic and methanogenic microorganisms have a syntrophic relationship. The methanogens use the hydrogen produced by acidogens, maintaining an optimal partial pressure for the acidogens. Hence, the slow growth rate of methanogens is the limiting step in the process. If this process slows down, then the volatile fatty acids produced by acidogens will build up in the reactor, resulting in a lowered pH, and a further disruption of the methanogenic activity. Because of this carefully balanced microbial relationship, it is important to ensure that consistent operation and monitoring are occurring, with pH monitoring being the most convenient and useful method. Methanogens are also strongly inhibited by the presence of oxygen, free ammonia, heavy metals, and sulfides. Anaerobic digestion treats organic waste in airtight chambers to ensure anaerobic conditions. Anaerobic digestion has been widely applied in decentralised wastewater treatment facilities for the digestion of primary sludge and waste activated sludge, typically with plug flow (PFR) or continuously stirred reactors (CSTRs). Anaerobic treatment technologies also include upflow anaerobic sludge blanket (UASB) reactors, anaerobic baffled reactors (ABRs) and anaerobic filters. Anaerobic treatment is also well known and developed for industrial wastes and highly loaded wastewater treatment plants (e.g. agro-industries, Arthur et al., 2011).Anaerobic digestion also occurs in settling-thickening tanks, mainly in the thickened layer. The extent of digestion relies on the amount of the initial stabilisation of FS, the temperature, and on the retention time inside the tank. This process degrades a part of the organic matter and generates gases. Operational performance has shown that fresh FS that is not stabilized (e.g. from public toilets that are emptied frequently) does not settle well. This is because anaerobic digestion of fresh FS contributes to an increased upflow from gas bubbles, and FS4that is not stabilized also contains more bound water FS that is a mixture of stabilized and fresh sludge are more appropriate for treatment in settling-thickening tanks (Heinss et al., 1998; Vonwiller, 2007).Figure 1: The flow chart below depicts the process involved in the anaerobic treatment of waste.Design parameters for anaerobic treatment systems:The main design parameters for anaerobic digesters are the hydraulic retention time (HRT), the temperature and the loading pattern. Operating conditions that play an important role in the design and operation of anaerobic digesters include solids retention time SRT), HRT, temperature, alkalinity, pH, toxic substances, and availability of nutrients and trace elements.When designing an anaerobic reactor, it is important to know the organic load that can be expected, in order to allow for a long enough HRT for degradation to occur. For systems without recycling, the SRT is equal to the HRT (e.g. plug flow reactors). The anaerobic reaction steps are directly related to the length of the HRT: an increase or decrease in the HRT leads to an increase or decrease in the degree of hydrolysis, acidification, fermentation and methanogenesis (Metcalf and Eddy, 2003). It is therefore important to keep track of the HRT to prevent reactor failures. The temperature also plays an important role, especially on5the degree and rate of hydrolysis and methane formation. At the same time, temperature affects physical and chemical parameters in reactors such as gas exchange and solubility of salts, and inactivation of pathogens.Difference between aerobic & anaerobic systems:Anaerobic bacteria transform organic matter in the faecal sludge into biogas that contains large amounts of methane gas and carbon dioxide whereas Aerobic process use the bacteria then break down the sludge into organic compounds. Anaerobic treatment does not use air to decompose the organic matter whereas aerobic treatment does. Anaerobic treatment has high potential to produce energy with the help of Biogas (methane) that can be used as an ideal fuel. Collecting methane is better rather than to release methane as methane is a greenhouse gas and has a bad odor.Advantages of biogas accumulation by anaerobic systems:Well-functioning biogas systems can yield a wide range of benefits for their users, society and the environment in general: production of energy (heat, light, electricity).Transformation of organic waste into a high quality fertilizer; improvement of hygienic conditions through reduction of pathogens, worm eggs and flies; mainly for women, in firewood collection and cooking; environmental advantages through protection of soil, water, air, and woody vegetation; micro-economical benefits through energy and fertilizer substitution, additional sources of income and increasing yields from animal breeding and agriculture; macro-economical benefits through decentralized energy generation, import substitution and environmental protection. (Afifah, Priadi 2017)6Anaerobic co-treatment of faecal sludge:The co-treatment of FS and wastewater in anaerobic processes is an alternative for sludge stabilization, volume reduction and increased dewaterability. Possibilities include upflow anaerobic sludge blanket reactors (UASB), anaerobic digesters and anaerobic ponds. Anaerobic treatment can offset treatment costs through the production of biogas, which can be used for heating or for the generation of electricity. Pathogen reduction can also be achieved with thermophilic digestion (Metcalf & Eddy 2003) . The characteristics of FS need to be carefully considered, as fresh or less stabilized FS will have higher concentrations of biodegradable organics. Although the biogas production and utilization is an attractive benefit, there are currently limited applications and technologies. Therefore, research is needed for the development of anaerobic systems for the co-treatment of high strength FS (Strauss et al., 2006). FS from septic tanks (digested FS) may not be appropriate for anaerobic co-treatment, depending on the level of stabilization it has undergone. In this case, the low concentrations of biodegradable organics in digested FS will lead to low biogas production but high solids accumulation resulting in significant operational costs with limited benefits (Still and Foxon, 2012). For any anaerobic treatment process, probably the most important operational aspect is the feeding. It needs to be supplied gradually and continuously to avoid overloading and shocks (Heinss and Strauss, 1999; Metcalf and Eddy, 2003; van Lier, 2008). For FS co-treatment in UASB reactors, the maximum OLR of design must not be exceeded in order to avoid the overloading of the system.in particular, high strength .FS needs to be carefully handled since the high organic content can easily overload the system. In this study, 0.25% high strength fresh FS (approximately 10 tankers of 5 m3 per day) had an organic load equivalent to around 139,000 p.e. that led to the overloading of a 100,000 p.e. UASB plant. Anaerobic digesters appear to be more robust for the anaerobic co-treatment of FS. Permissible loading rates for mesophilic digesters (operated at 35?C) depend7on the operational conditions but can reach up to 1.6-2.0 kgVSS/m3/d (Heinss and Strauss, 1999; Metcalf and Eddy, 2003). Also, the feeding, including FS, needs to be limited to the maximum daily feed rate of design which depends on the applied SRT. Thermophilic anaerobic digesters (49-52C) are an alternative that can lead to faster hydrolysis rates (the rate limiting step in anaerobic digestion of wastewater and FS) resulting in higher biogas yields (Angelidaki et al., 1993). However, they are susceptible to small temperature variations and also operating and maintenance costs are higher compared to mesophilic digesters, which make them unattractive for low-income countries (Heinss and Strauss, 1999).Ponds appear to be cost-effective technologies for FS co-treatment when operated as low loaded systems (0.6 kgBOD5/m3/d). However, their implementation needs to be carefully evaluated because the initial investment and operational costs could be high since they have substantial land requirements and high operational costs as a consequence of the frequent desludging needed. Moreover, they can involve important environmental issues if methane is lost into the atmosphere. (Linda Strande 2014)Resource Recovery:Research in this area includes the potential of anaerobic treatment, which is to get resource recovered from the treatment of faecal sludge and to utilize a reliable and environment friendly natural resource. The project is identifying innovative methods of resource recovery, and is also focusing on scaling up the use of dried sludge as a fuel.Creativity is essential in every aspect of technology, management and planning to continue to advance solutions that are globally transferable and applicable for the currently 2.7 billion people worldwide served by onsite sanitation technologies and the billions more that will need to be served in the decades to come. Keeping an open mind will be key to developing innovative and optimal solutions, learning from the past, but also not limiting future8possibilities through biases of what has or has not worked in the past in other situations (Linda Strande 2014)Conclusion:In this era, there is a high need for clean, reliable and environmental friendly treatment technologies due to the increasing stress on energy production. The anaerobic treatment resolves problems of the developing countries, considering the efficiency, costs, space required and dis posal. In addition to that, we can preserve and create energy from biogas which is an end product of faecal sludge. Anaerobic treatment should be widely adopted to eliminate the complex methods used in aerobic treatments to treat faecal sludge.ReferencesAfifah, Ukhtiy; Priadi, Cindy Rianti (2017): Biogas potential from anaerobic co-digestion of faecal sludge with food waste and garden waste. In. RENEWABLE ENERGY TECHNOLOGY AND INNOVATION FOR SUSTAINABLE DEVELOPMENT: Proceedings of the International Tropical Renewable Energy Conference (i-TREC) 2016. Bogor, Indonesia, 26–28 October 2016: Author(s) (AIP Conference Proceedings), p. 20032.Benjamin Hemkendreis, Manuel Henseler and Karin Güdel (2008a): Faecal Sludge management. Dübendorf, Swi: Eawag/Sandec (Department of.Benjamin Hemkendreis, Manuel Henseler and Karin Güdel (2008b): Sandec Training Tool 1.0 – Module 5 – Faecal Sludge management. eawag aquatic research. Dübendorf, Switzerland.Kujawa-Roeleveld, Katarzyna; Zeeman, Grietje (2006): Anaerobic Treatment in Decentralised and Source-Separation-Based Sanitation Concepts. In Rev Environ Sci Biotechnol 5 (1), pp. 115–139. DOI: 10.1007/s11157-005-5789-9 .Linda Strande (2014): Faecal Sludge Management. Systems Approach for Implementation and Operation: IWAP London.Metcalf & Eddy, Inc (2003): Wastewater engineering. Treatment and reuse: Fourth edition / revised by George Tchobanoglous, Franklin L. Burton, H. David Stensel. Boston : McGraw-Hill, 2003 ©2003.Word Count : 1997

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