The effects of temperature, pH and retention time on volatile fatty acids production from primary sludge

A typical biological phosphorus removal process consists of alternate anaerobic and aerobic zones. Recent research at the University of British Columbia (UBC) has indicated that the addition of volatile fatty acids (VFA's) to the anaerobic zone of Bio-P processes improves the overall phosphorus removal efficiency, at least when treating weak sewage. At the full-scale biological phosphorus removal plant at Kelowna, B.C., and at a pilot-scale facility at UBC, anaerobic fermentation of primary sludge has been used to produce VFA's to add to the anaerobic zone of these processes. The primary objective of this research was to improve the knowledge of the acid phase of anaerobic digestion (the production of VFA's) to help achieve optimum design and operation of the fermenter. The effects of temperature, retention time and pH on the production of short-chain VFA's from primary sewage sludge were studied at lab-scale. Oxidation reduction potential (ORP) was also monitored throughout the study to explore the relationship between ORP and the production of VFA's. A completely random 3X3X2 factorial experimental design was used to determine the experimental sequence. Three litre anaerobic reactors were run on a fill and draw basis in a temperature-controlled room. Reactors were fed once a day with primary sludge brought from the UBC pilot plant. Results showed that the net VFA production consistently improved with increase in temperature in the range of 10°C to 30°C. At low temperatures (10°C and 20°C) the net VFA production improved with the increase in retention time; however extension of the retention time to 9 days at 30°C appeared detrimental to VFA production. The effect of pH control, at a value of 7.0, was not consistent with both retention time and temperature and had a confounding effect. At 10°C and 20°C temperatures, a relationship was observed between ORP and VFA production. No definite relationship between the net VFA production and ORP was found at 30°C.