Wastewater treatment mechanisms in anoxic stabilization ponds

Abstract

Studies on the performance of wastewater stabilization ponds operating in the “grey” area of organic loading between fully anaerobic and facultative conditions show that organic matter removal is still satisfactory. The results obtained revealed that wastewater treatment mechanisms are mainly based on biochemical reactions in the anoxic system. Although physico-chemical mechanisms were not negligible, algal activities in the presence of light proved to be important in the surface layer of the pond and facultative bacteria were working mutually with the algae. Motile flagellate algae (Euglena and Chlamydomonas) were the only species found to exist under anoxic conditions. It was also determined that sulphate-reducing bacteria predominated in the lower volume of the anoxic ponds, rather than acidogenic bacteria, and this caused sulphide and hydrogen sulphide build-up in the pond's contents. The bottom volume of the ponds and the benthic sludge in laboratory-scale anoxic ponds contained acid producers and methanogenic bacteria causing the release of biogas. It is hypothesized that phototrophic bacteria (purple sulphur-, green sulphur- and purple non-sulphur bacteria) act as a biological filter to oxidize sulphide and hydrogen sulphide in the euphotic zone of the anoxic ponds. As a consequence of the latter biological activities, anoxic ponds are likely to prove viable alternatives to anaerobic and facultative ponds, where odour nuisance and high land requirements, respectively, are to be avoided.