Alternative Configurations of Anaerobic Digestion and Thermal Hydrolysis to Enhance Performance

With growing emphasis on energy sustainability for water and wastewater treatment, a great deal of interest has been placed on improving anaerobic digestion. This has involved the development of numerous projects based on combining pre-treatment processes, especially thermal hydrolysis, with anaerobic digestion with the intent of performance enhancement and concomitant renewable energy generation. In North America, DC Water has become the first utility to employ this system at its Blue Plains site, and a number of others in the continent are following suit. The system at DC Water is based on processing both primary and waste activated sludge through thermal hydrolysis prior to parallel-fed anaerobic digestion at 20 days followed by recycling of Class A (pathogen-free) biosolids cake to various agricultural outlets. The plant is expected to generate approximately 10 MW of electricity through turbines at average throughput, as well as generate savings of $20 million annually and reduce carbon footprint. However, it is well understood that thermal hydrolysis has a parasitic demand for energy in the form of steam which may be minimized by high grade heat recovery from engine exhaust or by, in the case of DC Water, using turbines although at the expense of electricity output. Only one site currently, Davyhulme in the UK with an installed capacity of 370 US tons/d dry solids, is almost free of parasitic demand despite using engines (Edgington, et al., 2014). Although only recently introduced in North America, thermal hydrolysis has been used in Europe for over 20 years, with the UK leading its use and development with approximately a third of all sludge produced being processed using the technology (Panter, 2015). In addition to pre-treatment of sludge prior to digestion to make more biogas, another initiative gaining interest is co-digestion whereby waste organics are added to anaerobic digesters, also to increase biogas (Koch, et al., 2015; Kangle et al., 2012). Although the literature regarding the impacts of doing this is varied, there appear to be a number of examples where technical and financial benefits of co-digestion can be realized (EPA, 2014).

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