Theory of Serial Anaerobic Digestion

Considering that anaerobic digestion is a complex multi-step progression of series and parallel reactions, it is interesting to note that most designs for anaerobic digesters are based on single stage vessels often fed in parallel with typical retention time of 16 days. In the UK, digesters are generally followed by a secondary digestion stage where digested sludge is kept under batch conditions for a further 14 days to ensure various legislative requirements are met. Data collected within United Utilities shows that 15 - 25% additional reduction in volatile solids may occur in secondary digestion phase. Changing drivers and legislative requirements have highlighted the shortcomings of suboptimal design of typical anaerobic digestion plants. This has led to the development of a sludge pre-treatment industry where numerous technologies are used to alter the
characteristics and/or potential digestibility of sludge prior to typical digestion. This paper discusses some of the theory behind series anaerobic digestion to determine if process improvements can be made using existing digester assets.

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