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Factors influencing oxygen transfer in diffused aeration systems and their application to hypolimnetic aeration

University of British Columbia

A series of laboratory and field experiments were conducted to examine the effect of several design variables on the oxygenation capacity of hypolimnetic aeration systems. The laboratory experiments used non-steady state gas transfer methodology to examine the effect of air flow rate, air flow rate per diffuser, orifice size and reduced tank surface area on the overall oxygen transfer coefficient (K[sub L]a₂₀, hr⁻¹); standard oxygen transfer rate (OT₅, g O₂/hr); energy efficiency (E[sub p], g O₂/kW-hr) and transfer efficiency (E[sub o], %). The field experiments examined the effect of diffuser depth, orifice size and reduced separator box surface area on the oxygen input per cycle (mg/L), daily oxygen load (kg O₂/day), transfer efficiency (E[sub o], %), energy efficiency (E[sub p], kg O₂/kW-hr) and water velocity (m/sec) in a full lift hypolimnetic aerator. The laboratory experiments demonstrated that K[sub L]a₂₀, OT₅, E[sub p] and E[sub o] increased with air flow rate in the orifice range of 397 μ to 3175 μ diameter. In the 40 μ and 140 μ diameter orifice range, K[sub L]a₂₀ and OT₅ increased with air flow rate; however, E[sub o] and E[sub p] were not affected. A decrease in orifice size from 3175 μ to 140 μ diameter increased K[sub L]a₂₀, OT₅, E[sub p] and E[sub o]; however, there was no significant difference between the 140 μ and 40 μ diameter silica glass diffusers. Reducing the air flow rate per silica glass diffuser (40 μ and 140μ diameter) significantly increased K[sub L]a₂₀, OT₅, E[sub p] and E[sub o]. A reduction in tank surface area had a minimal effect on K[sub L]a₂₀, OT₅, E[sub p] and E[sub o] in two tank configurations with different surface area to volume ratios (0.94 and 2.2 m⁻¹). The field experiments demonstrated that increased depth of air release increased the oxygen input per cycle and water velocity, which, in turn increased the daily oxygen load, E[sub p] and E[sub o]. Orifice size in the 140 μ range significantly increased oxygen input per cycle, daily O₂ load, E[sub p] and E[sub o]; however, the size range from 794 μ to 3175 μ exhibited similar but reduced gas transfer characteristics. A reduction in surface area in the separator box had no effect on the oxygenation capacity of the hypolimnetic aerator. Design criteria for hypolimnetic aerators are discussed including several modifications which should increase the oxygenation capacity of full lift hypolimnetic aeration systems.

Text

2010-08-25

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http://hdl.handle.net/2429/27757

Civil Engineering

University of British Columbia

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