Evaluating primary sludge fermentation at the Central Valley Water Reclamation Facility

Bench scale and pilot scale investigations of primary sludge fermentation were conducted at the Central Valley Water Reclamation Facility (CVWRF) in Salt Lake City, UT. The purpose of these investigations was to assess the costs and benefits of implementing primary sludge fermentation as the Plant shifts from BOD removal to biological phosphorus removal in response to new regulations. Bench scale tests conducted in the summer of 2016 estimated a volatile fatty acid (VFA) yield of 0.08 g VFA as COD / g TS for batch systems, and 0.13 g VFA COD / g TS for semi-batch systems. The Unified Fermenter and Thickener (UFAT) configuration demonstrated a higher yield than complete mix or activated primary clarifier configurations. The tests also evaluated fermentation of food waste, where yields ranged from zero to 5.7 g VFA COD / g TS, depending on substrate. A 2,800-L pilot reactor was constructed in the summer of 2017, and operated in semi-batch mode for 117 days in complete mix configuration. This system demonstrate an average yield of 0.25 g VFA COD / g TS at an average hydraulic retention time of 72 hours. The yield was temperature-dependent, and a relationship was developed to express this dependence.