Water Journal : Water Journal May 2011
refereed paper resource recovery water MAY 2011 97 the greatest benefits, as previously shown. As sludge type is intrinsically linked to type of wastewater treatment, it may be time to revisit the wastewater process treatment train to encourage production of primary over secondary or chemical (e.g. ferric) sludges. Enhancing primary sludge production also has benefits of reducing load to the downstream wastewater treatment, which concomitantly reduces aeration, which accounts for a large majority of a water company's electricity consumption. The data suggest that wastewater treatment and sludge treatment can no longer be viewed in separate silos but should be viewed jointly, as choices made to meet a wastewater driver can have fundamental and long-lasting impacts on sludge processing. After sludge type, conversion to electricity with new engines, and installation of pre-treatment to enhance biogas production (Hunt and Wilson, 2008) are the most influential. However, highly efficient engines are limited to larger sizes and may not be viable on smaller sites, and pre-treatment plants may have parasitic demands which need to be accounted for in a full energy balance (Bungay, 2009). If there is sufficient existing capacity and sludge thickness is above the minimum threshold required to provide digester heat, then further thickening, cleaning digesters to increase retention time, slight increases in CHP availability and volatile solids have little influence. Nevertheless, any one or combination of these parameters may prove beneficial, dependent on site-specific conditions. Sludges fed to digesters below the minimum dry solids required for heat self- sufficiency should be thickened to levels above those shown in Figure 3, to avoid lost biogas production (from a drop away from ideal temperature conditions) and/or requirement of supplementary fuel. Finally, a number of technologies, some of which are common to other industries, can be employed to further optimise municipal sludge anaerobic digestion. These include: plug-flow configurations; high-rate digestion such as anaerobic filters or recirculation processes; or addition of nutrients, enzymes or biogas precursors. Conclusions In the water industry, anaerobic digestion infrastructure was not originally built to meet modern drivers for renewable energy generation and carbon footprint reduction. As it is impracticable to completely replace this infrastructure, it needs to be optimised and used in novel ways to assist with these targets. A number of parameters are influential in controlling the renewable energy potential from municipal sludge digestion. The most important of these is sludge type. Primary sludge produces over twice the energy from an equivalent content of secondary sludge. However, environmental drivers may encourage production of more secondary sludge. It is important that wastewater and sludge treatment be viewed jointly, as wastewater technology controls the amount and type of sludge produced, which in turn has downstream impacts. Separate treatment of primary and secondary sludge may generate benefits with respect to digestion performance. After sludge type, the most influential parameters are electricity-efficient CHP engines and pre-treatment technologies to enhance biogas production. If the sludge digested has reached a minimal dry solids required to provide digester heating, and the digestion plant is not hydraulically overloaded, there is little benefit in further thickening the sludge. As typical digestion configurations are inherently sub-optimal with respect to the biological processes taking place, it is possible to retrofit a number of high rate and advanced digestion processes to further enhance biogas production. The Author Dr William (Bill) Barber (email: Bill. Barber@aecom.com) is Biosolids and Wastewater Technical Director for AECOM and is based in their Sydney office. He was previously Biosolids and Sustainability Technical Specialist for United Utilities in the UK where he was involved in developing biosolids strategies, and worked on a large number of different biosolids technologies including the world's largest thermal hydrolysis facility. He is internationally regarded as an expert in biosolids treatment. References Barber WP, 2005a: Modelling the influence of municipal sludge anaerobic digestion and co-digestion on downstream unit operations. Proceedings of the Aqua Enviro/CIWEM 10th Biosolids and Organic Residuals Conference, Wakefield, UK. Barber WP, 2005b: The effects of ultrasound on anaerobic digestion. CIWEM Journal for Water and Environmental Management, 19 (1), pp. 2--7. Barber WPF, 2007: Observing the effects of digestion and chemical dosing on the calorific value of sewage sludge. IWA Specialist Conference: Moving forward -- Wastewater Biosolids Sustainability: Technical, managerial, and public synergy, June 24-27, 2007, Moncton, New Brunswick, Canada, pp. 351--358. Bungay S, 2009: Operational experience of advanced anaerobic digestion. Proceedings of the Aqua Enviro/CIWEM 14th Biosolids and Organic Residuals Conference, Leeds, UK. Chae KJ, Am Jang, Yim SK, and Kim IS, 2008: The effects of digestion temperature and temperature shock on the biogas yields from the mesophilic anaerobic digestion of swine manure. Bioresource Technology, Volume 99, Issue 1, pp. 1--6. Dawson MK, Heywood NI, Alderman NJ, Papachristou C & Loannou P, 2009: Sludge system pumping losses: UK Industry Standard is Flawed. Proceedings of the Aqua Enviro/ CIWEM 14th Biosolids and Organic Residuals Conference, Wakefield, UK. Gujer W & Zehnder AJB, 1983: Conversion processes in anaerobic digestion. Water, Science and Technology, 15, Copenhagen, pp. 127--167. Hunt P & Wilson T, 2008: Overview of recent developments in anaerobic digestion Technologies. Aqua Enviro/CIWEM 13th Biosolids and Organic Residuals Conference, Wakefield, UK. McCarty PL, 1966: Kinetics of waste assimilation in anaerobic treatment. Developments in Industrial Microbial Sciences, 7. American Institute of Biological Sciences, Washington D.C. McCarty PL, 1982: One hundred years of anaerobic treatment, In: Hughes et al. (eds). Anaerobic Digestion 1981, Elsevier Biomedical Press B.V., pp. 3--21. Panter K & Kleiven H, 2005: Ten years' experience of full scale thermal hydrolysis projects. 10th European Biosolids & Biowastes Conference, Wakefield, UK. Speece RE, 2008: Anaerobic Biotechnology and Odor/Corrosion Control for Municipalities and Industries. Archae Press, ISBN:1-57843-052-9. Tong X & McCarty PL, 1991: Microbial hydrolysis of lignocellulosic Materials, In: L R. Isaacson, Ed. Methane from Community Wastes, London: Elsevier Applied Science, pp. 61--100. Wilson CA, Novak JT & Murthy SN, 2009: Thermal Hydrolysis of the Lipid and Protein Fractions of Wastewater Sludge: Implications for Digester Performance and Operational Considerations. 82nd Annual Water Environment Federation Technical Exhibition and Conference, WEFTEC, New Orleans, LA, USA, pp. 3918--3922. Winter P & Pearce P, 2010: Parallel digestion of secondary and primary sludge, 15th European Biosolids & Biowastes Conference, Leeds, UK. Zehnder AJB, Ingvorsen K & Marti T, 1982: Microbiology of methane bacteria, In Hughes et al. (eds), Anaerobic Digestion 1981. Elsevier Biomedical Press B.V., pp. 45-68.
Water Journal April 2011
Water Journal July 2011