SULFATE-REDUCING AND METHANE PRODUCING BACTERIA IN AEROBIC WASTE-WATER TREATMENT SYSTEMS

A selection of aerobic biofilm reactors and activated sludge plants were investigated for the presence of methane producing bacteria (MPB) and sulfate reducing bacteria (SRB). Detection tests showed that acetotrophic and hydrogenotrophic MPB as well as lactate, acetate and propionate oxidizing SRB were present in all reactor types investigated, except in an activated sludge reactor aerated with pure oxygen. Methane production rates from acetate by biomass samples of aerobic reactors were less than 1% of the rates measured in anaerobic UASB sludge. The presence of SRB was independent of the reactor configuration, the organic loading and the influent sulfate concentration. Aerobic biofilms growing in a trickling filter packed with plastic carrier and in a rotating biological contactor contained between 5.7 x 10(7) to 1.1 x 10(8) CFU/g VS of lactate oxidizing SRB and showed a potential sulfate reduction rate using lactate as electron donor of about 10 mg SO42-/g VS.h. The latter is comparable to the values found in sludges from typical anaerobic wastewater treatment reactors. In activated sludges, SRB densities were some 10(3) time lower and the maximum sulfate reduction rate was ca 3 times lower compared to the aerobic biofilm reactors. In biofilms, high sulfate reduction rates, in combination with the high sulfide removal rates (11.6 to 131.7 mg HS-/g VS.h) suggest that in situ reoxidation of sulfide might sustain the SRB population. The SRB enumeration, together with activity and oxygen microprofile measurements showed that the biomass of aerobic wastewater treatment systems can serve as inoculum or as site for a wide spectrum of redox related biotransformation processes.