Bench-scale study of oxygen transfer in coarse bubble diffused aeration

Abstract

A bench-scale experiment was conducted in a 701, tank of tap water to examine the effect of three design variables on oxygen transfer in a coarse bubble diffused aeration system. The experiment used non-steady state gas transfer methodology to examine the effect of orifice diameter, air flow rate and reduced tank surface area on the overall oxygen transfer coefficient ( K L a 20, h −1); standard oxygen transfer rate (OT s, g O 2 h −1); energy efficiency ( E p, g O 2 kW h −1) and oxygen transfer efficiency ( E o, %). The experiments demonstrated that K L a 20 and OT s increased 122% when air flow rate was doubled (9.4–18.81 min −1), while E o and E P increased approx. 12%. A reduction in orifice diameter from 3175 μm (3.2 mm) to 1588 μm (1.6 mm) to 794 μm (0.8 mm) to 397 μm (0.4 mm) significantly increased K L a 20, OT s, E P and E o. The presence of a floating surface cover had no effect on K L a 20 and OT g, and a marginal decrease in E P and E o. The mean bubble sizes produced by the 397, 794, 1588 and 3175 μm diffusers were 4.7, 6.6, 7.7 and 7.2 mm dia, respectively. There was no significant effect of air flow rate on bubble size within the range of air flow rates used in this experiment. A comparison of this study's coarse bubble results with a concurrent study on fine pore (fine bubble) aeration reveals that K L a, OT s, E P and E o continue to increase as orifice diameter is decreased to the 40–140 μm (i.e. fine pore) diameter range. The most efficient coarse bubble diffuser (i.e. 397 μm) was approx. 68% as efficient as the 140 μm fine pore diffuser. A classification system using existing EPA bubble size criterion for coarse and fine bubble diffusers, and the aeration systems' operational response of E P and E o to increased air flow rates is recommended for differentiating coarse and fine bubble diffused aeration systems.