Eradication of E.coli from operational full-scale anaerobic digestors by thermal treatment using in-situ heat-exchangers: PAPER & PRESENTATION

Devlin, D., Parry, I., Pick, M., Gerado, M., Dwr Cymru Welsh Water, UK


E coli levels in biosolids are routinely tested for compliance in accordance with the biosolids assurance scheme. During routine sampling of biosolids it was found that E. coli levels were elevated at >1,000 cfu/g DS. Following this result E. coli was also found in all four anaerobic digesters at the AAD facility.

It was concluded that the source of the E. coli was from using disinfected final effluent (DFE) in a recently installed foam mitigation system which sprays DFE into the anaerobic digesters. High levels of E. coli substantially increased operational costs due to re-treating of biosolids, increased haulage charges and limiting throughput.

We sought to eradicate the E. coli by carrying out in-situ heat treatment. The chosen option involved raising the operational temperature of the digesters up to c. 44 °C to slow E. coli growth, then recirculate digester contents with hot digester feed sludge (from a thermal hydrolysis plant) through heat exchangers that were set to c.54 °C. This temperature high enough to achieve an E. coli reduction of 2-3 log. This was a unique approach that used existing plant equipment in a novel way which saved significant costs and plant downtime.

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