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Anaerobic sludge digestion has been in use continuously for more than 100 years and remains a valued process for sludge stabilisation. Its ability to covert foul and putrescible organic waste to beneficial biosolids and energy with simplicity and reliability of operation and low operating costs is unrivalled by more modern and complex processes. There is renewed interest worldwide to develop the process to meet even more stringent product quality standards as defined in the USEPA503 Class A and UK sludge matrix and the revised ‘Sludge (Use in Agriculture) Regulations. There is also much interest in enhancing the process to achieve improved conversion of solids to energy.

The initial hydrolysis step which always precedes the conversion of organic particulate matter to organic acids and eventually methane has always been taken very much for granted and has not been considered as a route to improving anaerobic digester process performance. Initial laboratory scale work by the authors indicated opportunities to optimise the process conditions and at the same time improve the pathogen kill occurring in the digestion process. The possibility of using this to guarantee a Treated Sludge quality (as defined in the revised ‘Sludge (Use in Agriculture) Regulations) was recognised and a full scale demonstration plant was built and commissioned in January 2002 (United Utilities have patent applications filed).

The performance of the full scale plant has been monitored with results confirming the levels of pathogen kill levels achieved at bench scale. The full scale plant is currently being modified to overcome some sludge handling problems which resulted from changes in the sludge properties during hydrolysis and to increase robustness as the plant is now the final solution for pathogen control at the works rather than a research development plant.

The performance of the full scale plant and the continuing bench scale work has shown potential to produce a higher quality product likely to comply with US EPA class A and UK Enhanced Treated sludge classification and further modifications have been incorporated into the full scale plant to allow this to be investigated further.

This paper reports on the progress to date of this novel process; its potential to enhance the anaerobic digestion process not only in terms of sludge solids destruction and energy production but also as a route to ensuring production a sludge product guaranteed to meet the treated sludge classification and its potential to meet the enhanced sludge standard.
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Document Type: Research Article

Publication date: 01 January 2003

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