Adaptation of denitrifying bacteria to acetate and methanol in activated sludge

Adaptation of denitrifying bacteria to the external carbon sources acetate and methanol was studied in bench-scale sequencing batch reactors. The reactors were seeded with sludge from a full-scale pre-denitrifying activated sludge plant and operated in cycles consisting of a 23.5 h anoxic period followed by 23 h aeration. A control reactor received no additional carbon. Potential denitrification rate, measured with the acetylene inhibition technique, and the most probable number of bacteria denitrifying with the specific carbon source were followed for 52 days. Actual rates in the reactors were estimated from cumulative gas production during anoxic operation. A period of adaptation was necessary when methanol or acetate was the supplemental carbon source. Adaptation to acetate was also observed in the control reactor. The acetate sludge was, however, probably better adapted to acetate as evidenced by the greater increase in activity per bacterium. The increase continued even after the maximum denitrification rate was reached. During adaptation to methanol the methanol denitrifying capacity per bacterium was fairly constant while the potential rate increased after an initial lag-phase. Sewage water without external carbon did not cause an increased methanol activity that could be interpreted as adaptation. We suggest that the bacteria denitrifying with acetate could be the same as those using the electron donors in the sewage. The bacteria in the methanol reactor seemed to consist of one population denitrifying with methanol and another with compounds in the sewage.