The role of HRT and low temperature on the acid-phase anaerobic digestion of municipal and industrial wastewaters

Two identical 3l completely mixed reactors with solids recycling capabilities were used to investigate the effects of hydraulic retention time (HRT) and low temperatures on volatile fatty acid (VFA) production. One reactor was fed with a 1:1 ratio of diluted primary sludge and a starch-rich industrial wastewater, while the other was fed with diluted primary sludge alone. The VFA and soluble COD concentrations and specific production rates reached their highest values at 30 h HRT and at 25°C. Further increase in HRT (at 25°C) or decrease in temperature (at an HRT of 30 h) resulted in lower amounts of VFA and COD produced. All parameters related to VFA and COD production were significantly higher in the industrial–municipal reactor than in the municipal-only reactor. The VFA:COD ratios were very high, with values ranging from about 0.8 to 1.0 indicating that hydrolysis was the rate-limiting step. Degradation of proteins (measured by ammonia production) was inhibited by the starch-rich wastewater in the industrial–municipal reactor, while no evidence of inhibition was found in the municipal-only reactor. This study revealed that VFA production was feasible at low temperatures (down to 8°C), particularly in the presence of the industrial waste. Ultimately, the amount VFA produced was adequate, in most cases, to support subsequent biological nutrient removal (BNR) processes.