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Evaluation of Class A vs. Class B Processes for Beneficial Use of Biosolids

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The City of Janesville Wisconsin currently uses landfilling of cake biosolids and limited agricultural land application of liquid biosolids (anaerobically digested sludge). The City's liquid application equipment has reached its useful life and the decision was made to phase out liquid biosolids because of the cost of transportation to the farms. The biosolids meet all criteria for Exceptional Quality (EQ) with the exception of the Class A pathogen reduction. With the elimination of liquid hauling the City wished to investigate a beneficial use program using cake biosolids. The sludge stabilization process at Janesville is thermophilic-mesophilic (temperature-phased) anaerobic digestion (TPAD) operated in a continuous flow feeding regime. The City has documented results on the liquid biosolids meeting the Class A pathogen levels. Reactivation of fecal coliform indicator organisms in the cake sludge was also observed. The digested sludge is dewatered using centrifuges.

The City converted to TPAD as part of a digester rehab project in order to increase gas production and reduce volatile solids. The TPAD process has met the process improvement goals. The project was conceived with a future move to Class A biosolids. As part of the facility planning process, Class A and Class B alternatives were compared.
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Keywords: Class A; Class B; TPAD; anaerobic digestion; land application; pathogen

Document Type: Research Article

Publication date: 01 January 2009

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