Engineering aspects of wastewater treatment in aerated ring-shaped channels

Abstract

Ring-shaped aerated channels may be used for wastewater treatment as oxidation ditches, aerated lagoons, or high rate ponds. The aerators utilized in these systems provide the necessary oxygen, and generate a flow velocity that will keep particulate matter in suspension. When mechanical surface aerators are used, the flow velocity and the oxygen supply rate depend on channel geometry, and the aerator rotation speed and submergence. A design procedure that will simultaneously satisfy oxygen and flow requirements is presented. It involves process kinetics, open channel flow hydraulics, and aerator performance aspects. Experiments in a pilot aeration channel, operated as a high rate pond, indicate that the minimum flow velocity for such systems is between 3·5 and 6·5 cm sec −1 when the suspended solids concentration is bout 500 mg 1 −1. The pumping efficiency of a 70-cm cage rotor aerator was found to be on the order of 4 per cent when the submergence depth is 12–17 cm. Calculations show that atmospheric reaeration, induced by high flow velocity, is not economical. On the other hand, a combination of a pump and an aerator may prove feasible in cases where flow velocity rather than oxygen supply is the limiting factor.