I am consulting with an engineering firm in the design of a return activated sludge sewage treatment plant. They have proposed the installation of an anaerobic zone at the headworks of an aeration basin. The hydraulic retention time in this zone would be about twenty minutes. The incoming sewage is high in sulfate, phosphorus and ammonia concentrations. Nitrate is not detectable. Their thinking is that bacteria in the return sludge will store soluble phosphates and soluble carbon compounds, which they will later use for respiration in the aerated zone. If they are right, this will greatly reduce the cost of aeration. But I am concerned that this zone will foster a community of sulfate reducing bacteria, leading to unacceptable odors.

I would welcome any feedback on this issue. In particular it would be helpful to know whether any other wastewater treatment plant is operating under similar conditions. But I’d also like to know what you think: Will the anaerobic zone select for a community of sulfate reducers?