Modeling the effects of serial off-gas reuse on the performance of a hooded surface oxygen absorption system

A numerical model was used to evaluate the performance of a surface agitation system designed to contact commercial oxygen with water. The modeled system was unique in that oxygen-rich off-gas, normally discharged to the atmosphere, was directed in serial reuse through additional contact stages receiving untreated water. A correlation between the agitator mass-transfer coefficient and power demand, needed to calibrate the model, was established using a single-stage (37 W) contactor of 1.18 m3 capacity. Additional tests, conducted with both single and three-stage equipment, verified model assumptions and performance predictions. Simulation runs indicated oxygen flow or power input required to meet a given effluent dissolved gas criterion can be substantially reduced by the off-gas reuse step; for example, to achieve an effluent dissolved oxygen of 24.1 mg\litre with a single stage agitator the oxygen feed rate needed was 61.5% greater than that required by a six-stage system receiving the same total power input (standard aeration efficiency, 0.5 kg\kW h; water flow rate, 100 litre\min; influent dissolved oxygen, 9.08 mg\litre at 15-degrees-C). The savings achieved increased with (1) greater target effluent dissolved oxygen concentrations, (2) lower oxygen feed rates, (3) higher input power levels, and (4) number of contact stages.