Unified fermentation and thickening process

Abstract

A method for treating wastewater that generates a supernatant having a concentration of volatile fatty acids (VFAs) that falls within a specified range and a sludge that has percent total solids. The method includes the steps of providing a fermentation tank and a gravity thickener, operatively connected to the fermentation tank. The wastewater is permitted to flow continually or intermittently into the fermentation tank and a portion of the wastewater overflows the fermentation tank, thereby defining an overflow rate. The fermentation tank is subjected to anaerobic conditions and the wastewater in the tank is permitted to stratify so that sludge forms at the bottom of the tank. In addition the sludge is continually pumped from the bottom of the fermentation tank so that each stratification level within the fermentation tank corresponds to a sludge residence time that in turn corresponds to a range of VFA concentrations. The overflow from the fermentation tank is combined with the sludge from the fermentation tank in the gravity thickener, permitting VFAs to be separated from the sludge and mixed into the overflow to form a supernatant rich in VFAs. The sludge is conditioned by elutriation to improve settling characteristics. The rate of the pumping of the sludge from the bottom of the gravity thickener tank to a proportion of the overflow rate, the proportion being set such that the sludge has the specified percent total solids independent of VFA production.