Assessing the Effects of RAS Fermentation on EBPR Performance and Associated Microbial Ecology

Enhanced biological phosphorus removal (EBPR) is an engineered water resource recovery facility (WRRF) process configuration that can produce effluent P < 0.5 mg/L. To consistently achieve low effluent P concentrations, EBPR requires volatile fatty acids (VFAs) to induce requisite biochemical reactions. Moreover, returned activated sludge (RAS) nitrate concentrations must be minimized. Returned activated sludge fermentation can potentially address process needs. However, research detailed herein highlights concerns with RAS fermentation integrated with EBPR. Under 2 and 4 hours of RAS fermentation periods, no consequential VFA production was observed; similar results were observed in batch tests with RAS from a full-scale EBPR WRRF. More critically, EBPR performance was poor, with average effluent concentrations of 1.0 to 2.4 mg/L. Furthermore, the glycogen accumulating organism (GAO) fraction under RAS fermentation was 4.3 to 8.7 times higher than in a conventional EBPR mixed microbial consortium (MMC). Integrated RAS fermentation-EBPR only performed well under "high" RAS nitrate; thus, should RAS fermentation be implemented, careful control to prevent anaerobic conditions in the fermentation zone is required.