Staging of anaerobic processes for reduction of chronically high concentrations of propionic acid

The objective of this research was the reduction of chronic elevated propionate from a bench-scale, anaerobic completely stirred tank reactor (CSTR) without biomass recycle treating synthetic molasses wastewater. Pretreatment and posttreatment alternatives were specifically examined using a high-food-to-microorganism (F/M) contact chamber as the preliminary operation unit in one case and an upflow anaerobic sludge blanket (UASB) reactor as the posttreatment unit in the second case. The high-F/M contact reactor unit, operated before the main reactor, was a two-stage (nor a two-phase) configuration. After the feed contacted biomass that was recycled from the main reactor for a period of 1 day in a high-F/M contact reactor, the entire contents of the high-F/M contact reactor were transferred back to the main reactor. This brief contact operation resulted in a dramatic decrease in propionate concentrations in the effluent of the main reactor, which had been producing consistently high concentrations for approximately 4 months. After incorporation of the high-F/M contact reactor in the preliminary treatment step, the propionic acid concentration dropped significantly, from 1 050 mg/L to 300 mg/L. The results of this investigation showed that the contact time and F/M ratio in the high-F/M reactor were critical parameters in determining the efficacy of the two-stage configuration. In the second part of the study, the effluent from the main reactor (not incorporating a precontact reactor) was subjected to posttreatment by a UASB reactor filled with granular biomass. After an acclimation period of approximately 1 month, which provided a hydraulic retention time of 1 day in the UASB unit, the concentration of approximately 1 050 mg/L of propionic acid in the main reactor effluent fell to less than the quantification limit of 20 mg/L in the effluent of the UASB posttreatment unit. The overall success of both portions of the study seem to demonstrate the inappropriateness of using a one-stage CSTR process configuration for anaerobic metabolism of propionate derived from carbohydrate wastewaters.