Year

1995

Degree Name

Doctor of Philosophy

Department

Department of Civil and Mining Engineering

Abstract

Aeration, or oxygen transfer, the means to increase the dissolved oxygen concentration water, is an old technology, but still plays a very important role in our environment. Atmospheric reaeration is the natural means by which oxygen gets transferred from air into streams and lakes. This replenishes the oxygen consumed in the biodegradation process and improves the water quality. Both metalimnetic and hypolimnetic aeration can also be used to increase oxygen content in deep lakes and reservoirs. In eutrophic lakes, aeration has long been employed in developed countries to prevent algal blooms from occurring.

Aeration is essential for shrimp and catfish farming in the aquaculture industry, and in fermentation for arious industries. In hydropower plants, aeration must be used to prevent cavitation in the spillway and reach the discharge standard for dissolved oxygen. Aeration is also the most essential process in the successful operation of an aerobic biological treatment system in wastewater treatment. Unless dissolved oxygen is available where and when the bacterial system requires it, the biological treatment process will not function.

The cost of Aeration system is considerably expansive in both capital and running expenditure. In the North American Continent about US$ 80 million per year is spent on replacement and on new aeration systems. If applications in Europe, Australia and Japan are added for consideration, the costs of operation for power would be approximately over one billion US dollars a year. The money involved shows how important aeration systems are to our environment. Nevertheless, available data suggests that the overall transfer efficiency of oxygen transfer in existing equipment is quite low

The objective of this research is to increase the efficiency of oxygen transfer. First a literature review has been conducted to examine the oxygen transfer efficiencies of the existing aeration systems. Then regression techniques have been developed to obtain a more precise value of the oxygen transfer coefficient. A new method is proposed which is simpler to use than the ASCE standard method. A new approach is suggested to use under varying temperature condition, and this new approach gives 25% less standard deviation than that of the ASCE standard method. The effect of Relative Humidity on the oxygen transfer coefficient has been investigated. It has been found that there exists linear relationship between relative humidity and the oxygen transfer coefficient. With every 3% rise in relative humidity, a 1% reduction of oxygen transfer coefficient was observed.

The promising membrane (bubbleless) aeration has been investigated by using a polypropylene hollow fibre microfiltration module. It has been found that 100% of oxygen can be transferred using hollow fibre membranes, and the Specific Aeration Efficiency SAE can be as high as 12.5 kg 02/kWh. This value is 67% higher compared with the SAE value of 7.5 kg 02/kWh for fine bubble aeration. A new functional relationship has been developed for bubbleless aeration. This functional relationship between relevant variables also indicates that the ratio of oxygen partial pressure fibres and water pressure within the fibre module is an important factor in addition flow Reynolds number. The effect of suspended solids SS has been examined to justify the practicability of the bubbleless aeration in the presence of sediments. With an oxygen pressure higher than 1.03 bar and a flow rate of 75.7 L/min, the SS though has an impact on oxygen transfer, and the effect could be less than 5% reduction of oxygen transfer coefficient under a high SS loading of 300 mg/L.

By the use of a two-component Laser Doppler Velocimeier, the velocities and Reynolds stresses in the vicinity of the hollow fibres have been examined. Results indicate when oxygen transfer takes place, the effect of buoyancy only extends to 1.0 mm from the fibres. Based on this finding, the optimised hollow fibre module has been suggested. The correlation equation of oxygen transfer between Sherwood number Sh, Reynolds number Re, the ratio of oxygen partial pressure to water pressure Pf and the Schmidt number Sc for bubbleless aeration after flow adjustment has also been presented.

Bubbleless aeration, though has the advantage of high value of SAE, suffers economic drawback. Economic analysis shown indicates that, with a capital cost of $2900 per module, the total cost per year of the suggested membrane aerator using oxygen would be 56% higher than the average value of other aeration systems for a 0.3785 MLD activated sludge treatment process. If the price of the module could be as low as one third of the original cost, then the total cost per year would be as competitive as average cost of the other aeration systems.

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