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EVALUATION OF THERMOPHILIC ANAEROBIC DIGESTION TO MEET CLASS A BIOSOLIDS CRITERIA

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The removal of pathogens and indicator organisms was evaluated at laboratory scale (20 L) in a completely-mixed, continuous-flow thermophilic anaerobic digester operated at 55 °C and a four-day hydraulic residence time. Feed sludge consisted of a blend of primary and waste activated sludges from the South Columbus (GA) Water Reclamation Facility and was spiked with Ascaris suum and vaccine-strain poliovirus. Digester feed and effluent samples were analyzed for these two species as well as Salmonella spp., fecal coliform, Clostridium perfringens spores, and somatic and male-specific coliphages. The effluent did not contain detectable concentrations of Ascaris, poliovirus or Salmonella, with removals across the continuous digester corresponding to greater than two-log10, greater than three-log10 and nearly four-log10, respectively. Removal of fecal coliform exceeded four-log10 but the effluent biosolids contained slightly greater than 1000 MPN/g total solids. Batch treatment of the effluent for an additional two hours resulted in an order of magnitude further reduction in fecal coliform. C. perfringens spores were not removed in the continuous digester nor in batch treatment of the effluent for eight hours, suggesting that it would not be a good indicator of pathogen destruction in thermophilic anaerobic digestion at temperatures of 55 °C or lower. Volatile solids destruction did not reach the level required for vector attraction reduction in land-applied biosolids, indicating that either a longer residence time or a second-stage process would be required. This study was part of a comprehensive demonstration project in support of a planned request for national Class A biosolids equivalency based on thermophilic anaerobic digestion in a mixed, continuous-flow reactor followed by a batch or plug-flow reactor (the Biosolids Flow-through Thermophilic Treatment – BFT3 – process).
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Document Type: Research Article

Publication date: 2003-01-01

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