THICKENING PRIMARY SLUDGE BY CENTRIFUGATION AND ITS EFFECTS ON THERMOPHILIC ANAEROBIC DIGESTION

The digestion process at the Hyperion Treatment Plant received Class A Exceptional Quality (EQ) Biosolids certification from the EPA, Region 9 on December 27, 2002. Currently, the feed to the digesters is a blend of Waste Activated Solids thickened by centrifuge to 6% and nonthickened Primary Solids. Investigations are underway to determine if the use of Thickened Primary Sludge can be employed to optimize the process. Potential benefits are twofold. Firstly, a reduction of the water content being sent to the digestion process will dramatically reduce the mass heated to thermophilic temperatures. This is significant as the energy needed to raise the temperature from Mesophilic to Thermophilic is approximately three times more at a cost of 300,000 per month. Secondly, the concentrated biosolids being digested effectively increases much needed digester volume necessary to achieve Class A Biosolids through Alternative 1-Time/Temperature. In effect this will increase operational reliability, as more units will be available for maintenance, standby duty or future treatment needs.To that end, the City engaged in large scale pilot testing to determine the viability of primary sludge thickening through the use of centrifugation. During the pilot study, centrifuges from two vendors were tested. Both centrifuges were able to thicken primary sludge to as high as thirteen (±13%) percent total solids concentration. Throughout the tests, both solids recovery and centrate concentrations were found to be satisfactory.Based on this preliminary information, it became apparent that a second set of pilot tests was necessary. These tests were aimed at determining the practical limit of primary solids dewatering relative to the operation of the thermophilic anaerobic digestion process. The investigation was aimed at the determination of two possible queries; determine the maximum (%TS) concentration of biosolids allowable to pump, and determine the maximum concentration of biosolids allowable prior to inhibiting the digestion process.