FACTORS INFLUENCING OXYGEN-TRANSFER IN FINE PORE DIFFUSED AERATION

A bench-scale experiment was conducted in a 70 1. tank of tap water to examine the effect of four design variables on oxygen transfer in a fine pore diffused aeration system. The experiment used non-steady state gas transfer methodology to examine the effect of air flow rate, air flow rate per diffuser, orifice diameter and reduced tank surface area on the overall oxygen transfer coefficient (K(L)a20 h-1); standard oxygen transfer rate (OT(s), g O2 h-1); energy efficiency (E(p), g O2 kWh-1) and oxygen transfer efficiency (E(o), %). The experiments demonstrated that K(L)a20 and OT(s) increased with air flow rate (9.4-18.8 1 min-1) in the 40 and 140-mu diameter orifice range; however, E(p) and E0 were not affected. Reducing the air flow rate per fine pore diffuser (40 and 140-mu-diameter pore size) significantly increased K(L)a20, OT(s), E(p) and E(o). A decrease in orifice diameter from 140 to 40-mu had no effect on K(L)a20, OT(s), E(p) an E0. A reduction in tank surface area had a marginally significant inverse effect on K(L)a20 and OT(s), and no effect on E(p) and E(o). The mean bubble size produced by the 40 and 140-mu diffusers was 4.0 and 4.2 mm, respectively. There was no consistent effect of air flow rate on bubble size within the range of air flow rates used in this experiment. In clean water aeration applications, the optimum system efficiency will be obtained using the largest number of fine pore diffusers operated at low air flow rates per diffuser. In wastewater treatment plants, higher air flow rates per diffuser should be used to prevent diffuser biofouling and keep biological solids in suspension. Wastewater systems are purposely operated at less than optimum transfer efficiencies in exchange for reduced diffuser maintenance and improved mixing. In either situation, changes in tank surface area and diffuser pore size (provided that pore diameter remains between 40 and 140-mu) are unlikely to have any significant effect on aeration system efficiency.