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Treatment and disposal of municipal sludge continues to attract the attention of City Officers and Engineers. Anaerobic digestion is one of the most widely used method of sludge treatment. Anaerobic digestion results in stabilization of sludge with the reduction in the quantity of solids in addition to a reduction in bacteria levels and odors. The products of anaerobic digestion are environmentally friendly and have economic value – the biogas is used for heating or energy generation while the solids are used for plant nutrition.

Anaerobic digestion is a multistage biochemical process that involves two physiologically different types of bacteria – acidogenic bacteria, which converts the organic matter into soluble compounds and volatile fatty acids and methanogenic bacteria, which converts the fatty acids into methane and carbon dioxide. Traditionally, all four stages – hydrolysis, acidogenesis, acetogenesis, and methanogenesis – occurred in a single digester tank. And because both types of bacteria are in the same vessel, growth of either type is not optimized, making the conventional digestion process inherently unstable and inefficient.

Two-phase acid/gas anaerobic digestion is a relatively new process in which the various stages of digestion are performed in different vessels or digesters. This process optimizes the growth of both types of bacterial resulting in a stable and efficient process. However, the installed base and collective experience in the operation of two-phase digestion is very limited. The design of acid digesters are mostly based on that of conventional single phase digesters. In addition, most of the existing two-phase digesters are being operated as conventional digesters due to operational problems.

Southside Wastewater Treatment Plant employs two-phase digestion for its solids treatment. This paper explores the problems encountered during process startup and the various ways to maintain operations. It identifies conditions that impact process stability. It also confirms known advantages of higher specific gas production, higher reduction in bacteria levels and higher volatile solids destruction.
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Document Type: Research Article

Publication date: 2007-01-01

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