The Major Types of Anaerobic Digesters and Their Uses in Various Waste Management Programmes - Term Paper Example

Environmental Biotechnology Anaerobic digestion is carried out in a closed equipment/reactor through bacteria, and the outcome of this process in the form of gas and inorganic constituents are usually used further. The applicability of anaerobic digestion is usually for large-scale operations involving a series of processes. This discussion is based on identifying major types of anaerobic digesters and their uses in various waste management programmes. A brief account of principles/functions employed in anaerobic digestion has been outlined. Classification of anaerobic digesters has been made in many ways such as wet and dry, batch and continuous, single and multiple stages depending upon the desired outcome. Dry systems include the Dranco and Valorga processes; secondly, wet systems include single stage systems based on Waasa process, Linde process, and double stage systems based on Hitachi design, IBVL design and the TERI’s enhanced acidification and methanation process. Earlier anaerobic digestion was employed to treat sewage and sludge, which eventually progressed to be used for degradation of industrial wastes on much larger scale. The basic functioning of an anaerobic digester can be understood from The Dranco Anaerobic Digestion Plant for Biodegradable Waste (See Appendix 1). Anaerobic digesters can be operated using a variety of inorganic compounds that act as terminal electron acceptors (Shammas, Liu & Wang, 2009). Existing anaerobic digesters are mainly based on two principles, i.e. those that work on dispersed-growth bacteria and the others on attached growth bacteria (Polprasent, 2007). The attached-growth digesters are more efficient for higher organic loadings and lower retention time. Anaerobic digesters utilize four main groups of bacteria namely, acid-forming (hydrolytic and fermentative) bacteria, acetogenic (acetate and H-producing) bacteria, acetoclastic (methane forming) bacteria, and Hydrogen-utilizing methane bacteria (Polprasent, 2007; p.153). The most commonly used anaerobic digesters have been grouped into six categories: Anaerobic contact reactors: These function on high-rate processes retaining the sludge and concentrating solids in a separate reactor; subsequently the solid residual is returned to the influent. The most commonly used anaerobic contact reactors include the continuously stirred tank reactors (CSTR) and completely mixed contact reactors (CMCR); these are mostly employed in treatment of municipal sludge. A more sophisticated form of contact reactor is the sand-bed filter reactor, mostly used for wastewater treatment. Sludge blanket reactors (UASB and ESGB): Upflow anaerobic sludge reactors (UASB) work on enhanced acidification and methanation process in which organic content extracted from the waste through acidification process is treated in high-rate methanogenic digester which leaves blanket of granular sludge suspending in the container (see figure g). The resultant biogas is rich in methane gas (ICAEN, 2004). Operating the UASB at greater upflow velocity makes it the expanded granular sludge blanket (EGSB) reactor. Therefore, EGSB reactors work with greater effluent volume thereby producing larger amounts of biogas. A combined version of UASB and EGSB is the Internal circulation (IC) digesters with an enhanced mixing capacity and mass transfer abilities. In this, the sludge is recirculated, thereby rendering the solid granular sludge well-settleable. Anaerobic Baffled Reactors (ABR): ABRs use the Valorga process that operates on high solid operation in the range of 35% with operation at 37oC and residence time of 15 days. Chynoweth (1990) points that Valorga is the most commercially developed anaerobic digestion technology for municipal solid waste conversion (p.129). Anaerobic baffled reactors have been modified in various ways to be used for wastewater treatments. The latest includes a nine-chambered modified anaerobic baffled reactor (MABR) (Liu, Tian & Chen, 2010). Temperature phased reactors (TPAD): Initially developed by Iowa State University, TPAD utilizes two-stage anaerobic system consisting of two reactors operated at thermophilic temperature (55oC) and mesophilic (35oC) stages. Anaerobic digestion at elevated temperatures of upto 55oC has proven extremely effective in wastewater treatment. As a multi-stage digestion process, TPAD enhances pathogen destruction and increases the volatile substance residual at thermophilic temperatures; and reduces odor and water content at mesophilic temperatures (Yu, Morrison & Schanbacher, 2010). Anaerobic Sequencing Batch reactors (ASBR): The ASBR is regarded as a modified and simplified version of UASB eliminating processes of feed distribution, phase separation as well as upflow hydraulic process. As a high-rate anaerobic process, it produces greater granular biomass with longer solid retention time (SRT). However, results obtained with ASBR were far less favoured than UASB. Here, the influent is allowed to mix with existing anaerobic sludge to produce biogas. Subsequently, the sludge settles down and liquid is pumped out and the entire process is repeated (Yu, Morrison & Schanbacher, 2010). Anaerobic Filter and Hybrid Reactors: The Anaerobic filter reactors (AFR) have fixed growth biofilm for bacterial growth which aid in anaerobic digestion (see figure d). These reactors are used for low volume sludge and/or sludge with lesser solid contents. The two variants include anaerobic expanded bed reactor (AEBR) and anaerobic fluidized bed reactor (AFBR). Both these contain sand particles or materials to which the biofilm is attached. With increase in upward flow of influent and effluent, the bed of these particles gets expanded in AEBR and fluidized in AFBR (see figure e). Both require greater energy compared to simple AFR. In combination with sludge blanket reactors like the UASB, the AFR forms the anaerobic hybrid reactor (AHR) (see figure h) (Yu, Morrison & Schanbacher, 2010). Application and performance of these anaerobic digesters: As indicated by Williams (2005), the yield of gas from anaerobic digesters depends on the composition and biodegradability of waste. Yu et al (2010) assert that the methane biogas produced from these reactors is highly effective in producing electricity with minimal pollutants released; however, these gases need to be reformed to contain greater amounts of hydrogen in order to equal the conventional form of electricity. Inhibitors to anaerobic digestion include volatile acids and toxic chemicals. Digesters are designed to eliminate or counter these inhibitors and enhance the activity of biological properties through anaerobic microorganisms, which can help in biodegradation of waste as well as production of biogas. Anaerobic contact reactors: Most quotable example of CSTR is that of Karlsruhe, Germany, which produces 3800 cubic meters of biogas per day from 62-70% of methane (Yu & Schanbacher, 2010). Disadvantage of anaerobic CSTR is slow growth of methanogenic bacteria requiring longer retention time in the digester. Anaerobic contact reactors are mainly used in treatment of wastes from sugar, distilleries, citric acid and yeast production factories, production of canned vegetables, pectin, starch, meat products etc (Ramasamy & Doraisamy, 2009). A variety of mixed or plug flow contact reactors are extensively used in dairy manure. CSTR has been extensively used in production of large amounts of biogas, up to 4000 cu.m/day. Lemvig in Denmark and the City of Karlsruhe are major users of CSTR. Anaerobic contact reactors such as CMCR have been very effective in handling high-strength feedstock with large amount of suspended solid and are operable at high loading rates. Therefore, CMCR is considered as an advanced type of ACR, and is better than CSTR. These are reported to be extensively used by Dugremont Technologies, Switzerland and the ADI Group Inc, Canada (Yu, Morrison & Schanbacher, 2010; p.414). Sludge blanket reactors: The UASB are predominantly used to treat water containing floating solids; industrial waste water containing toxic and inhibitory chemicals; domestic wastewater as well. Show reported in his studies that UASB can be applied in treating wastewaters containing concentrated proteins and aromatic compounds. . The EGSB is an advanced version of UASB. The sludge blanket reactors are extensively used in industrial waste treatment because they are expensive and involve complex structures; however, require very little space compared to other digesters (Ramasamy & Doraisamy, 2009). Show (2006) reported that anaerobic treatment using sludge granulation has been very advantageous for industrial waste treatment mainly attributable to low operating costs, compact reactor construction, production of energy in the form of biogas and low surplus sludge production. UASB is the most predominant process used in most of the industrial settings. Performance of IC reactor has been much better than UASB or ESGB, with more than double biogas production although chemical removal efficiency remains same in all (Show, 2006). Anaerobic Baffled Reactors: The ABRs are employed when decomposing a wide range of materials, and are considered as holistic system for treatment of municipal solid waste. Most of the baffled reactors are made of many compartments and work by sorting out the solid retention time from the hydraulic retention time. Most of the baffled reactors are used as alternatives to aerobic treatment and primary treatment processes. The baffled reactors provide advantages of treating low solid wastes thereby reducing heavy load on secondary treatments; residual sludge is comparatively lesser; and are low-cost procedures. This process does not employ any mechanical mixing of the sludge. However, research conducted by Foxon et al (2004) indicated that effluents of ABR when used for on-site primary sanitation in low-income communities still contained unhygienic particles and were not reusable. Yet, these digesters are also used in distilleries and wastewater treatment with modifications to the conventional design (see figure i). Temperature phased anaerobic digester: Reports indicate that most of the wastewater treatment in the United States is based on TPAD systems. However, its applicability to treatment of MSW, livestock manure would be possible only with more improvisation (Sung & Santha, 2001). TPADs have resulted in substantial improvement of anaerobic digestion in the process of codigestion of food wastes or crop residues with dairy manure. TPAD is more efficient with dilute farm manure. In addition its applicability in treatment of food-processing wastes and sanitation of waste streams have also been reported as effective through TPAD. Lindorfer et al., (2006) reported that the multi-stage TPAD has aided in production of optimum biogas than single-stage process. Moreover, De Baere (2003) reported that the heat produced in theremophilic reactor was sufficient to run the digester in desired manner (Yu, Morrison & Schanbacher, 2010). Anaerobic Sequencing Batch reactors (ASBR): Researches on performance and efficiency of ASBR have indicated that ASBRs aid in handling higher organic loads with higher solids loading, which affects the settleability of the sludge blanket, thereby making this system more efficient for low medium strength liquids. Moreover, the maximum mechanization of wastewater achievable was around 41% (Li, Sun & Li, 2005) or methane yield of 0.46/lit of methane per g volatile solids (Angenent, Sung & Raskin, 2002). Evidences suggest that ASBR has been more successful in swine waste treatment with a constant rise in volumetric methane production rate through volumetric loading although requiring long start up time (Angenent & Scott, 2010). Anaerobic Filter and Hybrid Reactors: The filter reactors are of greater use in wastewater treatment; these could include septic tank treatment post primary treatment. In addition, they are also used for wastewater treatment in beverage industry, sugar and starch factories and paper mills. Studies indicate that the biogas produced through AFR contained higher percentage of methane than that produced through anaerobic contact reactors. AEBR and AFBR require greater energy to operate compared to AFR. AFRs are not suitable for digesting dairy wastes because the filters do not convert solid waste to gas and get clogged while digesting dairy manure slurries. However, they are useful to treat the liquid part of dairy wastes (Dennis & Burke, 2001). Energy consumption for AEBR AND AFBR is higher compared to AFR because of continuous recirculation of the effluents. Moreover, the efficiency of these reactors is lesser and it is also difficult to control the size and density of sludge accumulations. Yu et al (2010) describe this as a technical hurdle that can lead to loss of biomass due to changes in particle density, flow rates and biogas production (p.417). However, the improvised version, i.e, downflow or inverse AFBR is more efficient, requiring lesser energy than AFBR; however, its overall energy consumption still remains higher compared to other anaerobic digesters. In conclusion, anaerobic digestion of wastes from used water, manure, animal farms, agriculture etc has been recognized as a very efficient and necessary process not only to eliminate waste and depollute the environment, but also to produce useful energy resources in the form of biogas. For this purpose, equipments/digesters have been developed and have evolved with better performance and efficiency. Of the many different types of digesters, mostly used ones include ASBR, UASB, IC, ESGB, and ABRs. Although immense improvement and sophistication in design and usability of anaerobic digesters has been achieved, efforts are still being made to achieve greater efficiency in biogas production and waste management. References Angenent, L.T and Scott, N.R. 2010. Practical aspects of Methane Production from Agricultural Wastes. In Blaschek, H, Ezeji, T and Scheffran, J’s Biofuels from Agricultural Wastes and Byproducts. Iowa, US: John Wiley and Sons. (Ch.4, pp:39-66). Angenent, L.T, Sung, S and Raskin, L. 2002. Methane yield and methanogen levels of ASBR systems treating swine waste: effect of different inocula. Yucatan, Mexico: International Water Association. Chynoweth, D.P. 1990. Anaerobic Digestion Development. In Isaacson, R’s Methane from Community Wastes. OX: Taylor & Francis. (Ch.8, pp:113-132) Dennis, A and Burke, P.E. 2001. Options for Recovering Beneficial Products From Dairy Manure. In Dairy Waste Anaerobic Digestion Handbook. WA: Environmental Energy Company. Foxon, K.M et al. 2004. The anaerobic baffled reactor (ABR): An appropriate technology for on-site sanitation. South Africa: Water Institute of South Africa (WISA). Vol. 30 No. 5, pp:44-50. Accessed September 15 2010 from, www.bvsde.paho.org ICAEN, 2004. Solid Waste Management. In Sustainable Building: Design Manual, vol 2. Instituit Catala d’Energia. India: The Energy and Resources Institute. (Ch.5, pp: 47-56). Li, B, Sun, Y-I and Li, Y-Y. 2005. Pretreatment of coking wastewater using anaerobic sequencing batch reactor (ASBR). Journal of Zhejiang University Science. Vol.6, No 11, pp: 1115-1123. Published Nov 2005. Accessed September 17 from www.ncbi.nlm.nih.gov Liu, R, Tian, Q and Chen, J. 2010. The developments of anaerobic baffled reactor for wastewater treatment: A review. African Journal of Biotechnology. Vol. 9(11), pp. 1535-1542. Accessed on September 17 2010 from, www.academicjournals.org/ Polpraset, C. 2007. Organic Waste Recycling: Technology And Management. 3rd ed. London: IWA Publishing. (Ch.4; pp: 145-218). Ramasamy, K and Doraisamy, P. 2009. Industrial Effluents: Bioreactors for Treatment of Wastes. In Lal, B’s Wealth from Waste: Trends and Technologies. 2nd ed. Delhi: TERI Press. (Ch.10, pp: 411-444). Shammas, N.K, Liu, Y and Wang, L.K. 2009. Principles and Kinetics of Biological Processes. In Advanced biological treatment processes, Vol 9. NY: Springer. (Ch. 1, pp: 1-58). Accessed on September 17 2010 from, www.springerlink.com Show, K-Y. 2006. Application of Anaerobic Granulation. In Tay, J.H and Show, K-Y’s Biogranulation Technologies For Wastewater Treatment. OX, UK: Elsevier. (Ch.3, pp: 57-84). Yu, Z, Morrison, M and Schanbacher, F.L. 2010. Production Utilisation of Methane Biogas as Renewable Fuel. In Vertes, A, Qureshi, N & Yukawa, H’s Biomass to Biofuels: Strategies for Global Industries. West Sussex, UK: John Wiley and Sons (Ch. 20, pp: 403-434) Yu, Z and Schanbacher, F.L. 2010. Production of Methane Biogas as Fuel Through Anaerobic Digestion. In Singh, O.V and Steven, H.P’s Sustainable Biotechnology: Sources of Renewable Energy. PA, USA: Springer. (pp:105-128). Williams, P.T. 2005. Waste treatment and disposal. 2nd Ed. England: John Wiley and Sons. (Ch. 6, pp: 325-366). Appendix 1 The Dranco Anaerobic Digestion Plant for Biodegradable Waste. (Adopted from: Williams, P.T. 2005.p.363). Appendix 2 Figures Figure a) Continuous stirred-tank reactor b) Completely mixed contact reactor (CSTR) (CMCR) (Adopted from: Yu, Morrison & Schanbacher, 2010; p.414) Figure c) Sand bed filter reactor. Adopted from Yu et al. 2010, p.417. Figure d) Anaerobic Filter Reactor (AFR). Adopted from: Yu et al. (2010, p.417) Figure e) Anaerobic fluidized-bed reactor (AFBR). Adopted from: Yu et al. (2010, p.417) Figure f) Upflow Anaerobic Sludge Blanket (UASB). Adopted from: Yu et al. (2010, p.419) Figure g) Anaerobic Baffled Reactors (ABR). Adopted from: Yu et al. (2010, p.419) Figure h) Anaerobic Hybrid Reactor (AHR). Adopted from Yu et al. (2010, p.421) Figure i) Nine-chambered ABR. Adopted from: Liu, Tian & Chen (2010, p.1537). Read More