PRELIMINARY INVESTIGATION OF RECUPERATIVE THICKENING FOR ANAEROBIC DIGESTION

Laboratory and full-scale investigations of recuperative thickening for anaerobic digestion were conducted ) in Spokane, Washington at the Advanced Wastewater Treatment Plant (SAWTP). Recuperative thickening is defined as removing digesting solids from the anaerobic digestion process, thickening the solids and returning the thickened solids to the anaerobic digestion process. Recuperative thickening provides a function analogous to secondary clarification in the activated sludge process. Recuperative thickening allows operation of anaerobic digestion at independent hydraulic and solids residence times.A simple laboratory test was developed for measuring gas production as a surrogate for methane forming bacteria activity. Laboratory tests showed gas production was not significantly reduced by exposure to 15 minutes of intense aeration. Laboratory tests also showed significant gas production from a mixture of digested and waste activated sludge (WAS) thickened using conventional dissolved air flotation thickening (DAFT). In one test there was immediate gas production and a second test observed no gas production for a 12 to 24 hour period followed by gas production.A full scale test of recuperative thickening was conducted from September 2000 to May 2001. The following benefits of recuperative thickening were observed:Use of existing DAFT capacity allowed implementation with essentially no capital cost.Co-thickening with WAS allowed no increase in thickening labor or power costs.Polymer use increased for thickening and decreased for dewatering. The net result was a net reduction of polymer requirements by 15-percent with an annual savings of 28,000.Biosolids production was reduced 22-percent with a resultant annual savings of 85,000.Recuperative thickening of 25-percent of the digesting solids increased solids retention time in the anaerobic digesters from 15.7 days to 24.0 days.Anaerobic digestion volatile solids reduction increased from 50-percent to 64-percent.Recuperative thickening did not affect effluent quality.