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Comparative analysis of oxygen transfer in full lift and downflow bubble contact hypolimnetic aerators Ashley, Kenneth Ian

Abstract

This research conducted a detailed analysis of design factors influencing the oxygen transfer capabilities of two types of hypolimnetic aerators: full lift and Speece Cone, utilising non steady-state gas transfer methodology in a laboratory scale system. Full lift hypolimnetic aerators use the airlift pump principle to circulate water from the hypolimnion to the surface and return oxygenated water to the hypolimnion. Speece Cones use the Downflow Bubble Contact Aeration (DBCA) principle to oxygenate hypolimnetic water at depth using mechanical pumps to circulate water through the aeration chamber. Four standard units of measure were used to quantify oxygen transfer: (a) K[sub L]a₂₀ (hr⁻¹), the oxygen transfer coefficient at 20 °C; (b) SOTR (g O₂ hr⁻¹), the Standard Oxygen Transfer Rate; (c) SAE (g O₂ kWhr⁻¹), the Standard Aeration Efficiency and (d) SOTE (%), the Standard Oxygen Transfer Efficiency. The design factors examined in the full lift experiments were: the effect of diffuser orifice pore diameter (140 μ, 400 μ and 800 μ), diffuser submergence (1.5 and 2.9 m), gas flow rate (10, 20, 30 and 40 1 min⁻¹), oxygen partial pressure (air and Pressure Swing Adsorption generated oxygen) and four in situ modifications (floating surface cover in the separator box, mid-depth inlet tube bubble screens, counter-rotating blades and DBCA) in the outlet tube. In the full lift experiments, diffuser submergence positively influenced K[sub L]a₂₀, SOTR, SAE and SOTE and water velocity. Higher gas flow rates increased water velocity, K[sub L]a₂₀ and SOTR but decreased SAE and SOTE. Oxygen partial pressure treatment exerted a significant positive effect on KIAJO, SOTR and SOTE, whereas SAE response decreased on PSA oxygen. Smaller orifice diameters exerted a significant positive effect on K[sub L]a₂₀, SOTR, SAE and SOTE. The in situ modifications exerted a significant negative effect on K[sub L]a₂₀, SOTR, SAE and SOTE. The DBCA treatment generated the smallest bubble sizes. The design factors examined in the Speece Cone experiments were the effect of oxygen flow rate (1,2 and 3 1 min⁻¹) and outlet port discharge water velocity (20, 30, 40, 50, 60 and 70 cm sec⁻¹) on K[sub L]a₂₀, SOTR, SAE and SOTE. The inlet water velocity was insufficient to create an efficient gas transfer environment in the Speece Cone experiments. Oxygen flow rate and discharge velocity treatments exerted a significant positive effect on K[sub L]a₂₀, and SOTR. SAE increased with oxygen flow rates but discharge velocity had no effect on SAE. SOTE increased with water discharge velocity but responded negatively to increasing oxygen flow, suggesting that oxygen flow rates > 11 min⁻¹ were not being fully dissolved inside the Speece Cone. A precipitous decline in SOTE at low oxygen flow rates and discharge velocities > 50 cm sec⁻¹was due to a destabilisation of the bubble swarm in the Speece Cone. The Speece Cone, at oxygen flow rates between 1 and 31 min⁻¹, delivered K[sub L]a₂₀, and SOTR performance comparable to the full lift system at 101 min⁻¹ on oxygen, and outstanding SOTE performance. The DBCA counter-current flow principle of the Speece Cone provides a superior gas transfer environment to the full lift design by positively influencing the fundamental aspects of gas transfer (i.e., a, K[sub L] andC[sub i] –C[sub L]). The efficiency of the Pressure Swing Adsorption (PSA) generating units increases with unit capacity, as smaller PSA units are not designed solely for optimum oxygen recovery efficiency; improvements in SAE could be achieved by using lower pressure PSA generators. The relative magnitude of the effects from this study should be transferable to full-scale application, within reasonable limits. Further lab testing, comparative field studies, development of predictive models, reporting in engineering journals and case study reviews of specific installations will assist in resolving the engineering aspects of system selection. The Speece Cone and PSA oxygen generating systems represent the most significant advances in hypolimnetic aeration since their inception over half a century ago. Civil engineers should consider hypolimnetic aeration as a proven technique, which can significantly improve raw water quality, and serve as an integral component of an overall watershed management program.

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