M Asaadi and P Marsh, Thames Water


Swindon STW is one of the few sewage treatment works in the UK, which benefits from having a two-stage anaerobic digestion plant. In 2003, Acid Phase Digestion (APD) was retrofitted to the existing conventional digestion at Swindon in order to provide more robust sludge compliance and improve volatile solids conversion. The APD process is a high rate short retention time digestion, which forms the first part of a two-stage anaerobic digestion process. The main process parameters are the organic volatile solids loading, typically up to 35 Kgm3 d-1, and the retention time, between 1.5 to 2.5 days. Under these conditions, sludge solids are converted to VFA resulting in low pH and partial hydrolysis of the sludge constituents and a degree of Ecoli kill. This pre-treated sludge is then fed to the second stage conventional mesophilic anaerobic digestion for conversion to biogas and stabilisation.

In this paper, Thames Water’s experience of implementation of this process is described. A number of engineering challenges were encountered at the outset of the operation, which led to changes to the original design of the APD mixing and heating systems. New HACCP and operating regimes had to be devised for the operation of the APD stage.

The plant has been extensively monitored in the last 10 months and data on performance of the process in particular, on the volatile solids reduction and gas production of the combined system, pathogen indicator reduction, VFA generation as a function of the sludge type and age will be presented and discussed.

KEY WORDS Acid phase, anaerobic digestion, sewage sludge

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