Effect of temperature on nitrogen removal and microbial community composition in nitrifying biofilm reactors

To investigate how the temperature affects the microbial community and efficiency of biological nitrogen removal process, we operated four nitrifying biofilm reactors with synthetic wastewater of 40 mg/L ammonium and 100 mg/L chemical oxygen demand (COD) concentrations. In an operation of 225 days, the four reactors were controlled at different temperatures 10, 15, 20 and 25°C, while hydraulic retention time (HRT) and dissolved oxygen (DO) were maintained as 12 hr and 4.0 mg/L, respectively. On average, the removal efficiency of ammonium was more than 80% in a reactor of which temperature was controlled at 25°C, but the efficiency was below 30% at reactor temperatures of 15°C and lower. On average, the nitrate concentrations were higher as the reactor temperatures increased. In a 20°C reactor, the nitrite concentration was higher than nitrate, indicating partial inhibition of nitrite-oxidizers. Unlike the ammonium removal, the efficiency of COD removal was similar regardless of the reactor temperatures. When community structures of biofilms in the reactors were analyzed through denaturing gradient gel electrophoresis of 16S rRNA and compared between different temperatures, the banding patterns were similar and sequences of most bands corresponded to the members of β-proteobacteria.