The purpose of this study is to clarify effects of temperature, volatile fatty acids (VFAs) and recirculation on nitrification-denitrification activity and biota such as nitrifying bacteria in a small-scale domestic wastewater treatment process. Effects of VFAs produced in anaerobic biofilm reactors under various flow-rates and recirculation ratios on nitrifying bacteria were also investigated with laboratory-scale plants on a long-term stable conditions. As a result, at a temperature of 10°C, nitrification activity could be increased about 65% by recirculation and nitrogen removal efficiency was also surely raised. Temperature coefficients of nitrification rate at recirculation ratios of 0 and 4.0 were 1.039 and 1.090, respectively, and that of denitrification was 1.065 at recirculation ratio of 4.0.

High concentrations of VFAs were found in anaerobically treated effluent, especially at 10°C without recirculation, and inhibition of nitrite oxidation and nitrite accumulation were observed in the aerobic biofilm reactor. From batch experiments, VFAs' inhibition constants γ in nitrification activity of aerobic biofilm was calculated, and consumption rates of each VFA was investigated on the condition that denitrification was progressed or not. It was clarified that each VFA did not inhibit ammonium oxidation in observed concentration, but slightly inhibited nitrite oxidation. Acetic acid was used as a carbon source of denitrification at the rate of 17.3mg/mg-SS/hr. On the other hand, propionic acid was not used effectively and denitrification did not occur. It was concluded that the recirculation was indispensable to promote nitrification-denitrification activity and biodegradation of VFAs in the small-scale anaerobic-aerobic biofilm process.