| Most of energy loss in sewage treatments can owe to aeration, therefore how to improve the efficiency of aeration system is a key point of management and operation of sewage plants. Nowadays common aeration methods can be classified such three types: blow aeration, mechanical aeration, the aeration integrate these two types of aeration. Due to its oxygenation performances and adaptability to varying oxygen requirements, blow aeration, especially fine bubble diffused aeration systems has been widely developed in application. However in fact, the values of its oxygen utilization rate and power efficiency are not high and its energy loss is really high. In addition, the research and application of the aeration systems has such following problems: (1) models integrating mass transfer between liquid-gas phase with turbulent diffusion of dissolved oxygen are not available; (2) classic thoery thinks the better effect of aeration will be gained if the interface aera between water and air bubble increase i.e. the size of air bubble is lessened, but this option doesn't match the measurement of much research. (3)the conventional aeration transfer models lay particular stress on the size of fine bubble, and ignore the effect of bubble distribution of the aeration. (4) classical mass transfer models focus more on theoretic and less on application, laying particular stress on microcosmic or sub-microcosmic but care little about macroscopic, conventional characteristic criteria of blow aeration are considerable complex and is not convenient for operation and manipulation of the aeration systems.According to the drawbacks of the fine, theory of Optimal Bubble Group for fine bubble aeration was raised. Its construction was composed of such component: basic viewpoints, basic model and main application index. Its basic viewpoints are: (1) mass transfer and turbulent diffusion are both important process in the aeration. Such two process both contributes the aeration, while mass transfer shift the oxygen in the gas bubble enter into the water and turbulent diffusion make the entering DO completely distributes the whole water body ;(2) In the given aeration condition and the number of gas bubbles, there existing optimal average gas bubble diameter; (3) when the gas volume (Qg), the size of the aeration tank and water depth is fix, there existing initial optimal bubble group. Based on these viewpoints, the follow corresponding was developed: (1) in the given aeration condition (the gas volume (Qg), the size of the aeration tank and water depth is fix), the model of optimal average gas bubble diameter; (2) equal effective energy curve space and aeration energy curve (3) the model of average gas bubble diameter (4) To solve the problem of application to engineering, based on the theory of Optimal Gas Bubble Group (OGBG) , the model of optimal gas-water ratio used as a characteristic criterion which is more convenient , easy and applicable for than other conventional index.To validate the model of optimal bubble group, such two part of experiments are done: model experiment and aeration. Fog aeration pipeline used as gas diffused equipment was firstly introduced into aeration of clean water and aeration engineering in this study. The experimental results in this study shows: (1) In the same experimental condition (same air flow rate and same pipeline length), with the average diameter of gas bubble increment, the usage rates of mass transfer between liquid-gas phase gradually decrease in the same time the usage rates of turbulent diffusion gradually increase, but the total usage rate will rise first but then descend, its maximum value is the highest usage rate in aeration (ηT M) . When the air flow rate is same and water height is different,ηT M is not same too, and the maximum is the most optimal value and the corresponding water height is named as optimal water height on that air flow rate. Similarly, optimal air flow rate exist in the same water height.(2) Energy spectral curve surface (ESCS) indicates that although the energy input into the aeration system is different, but the really useful energy may be same. Comprehending ESCS can make total useful energy of different initial energy aeration system move smoothly on ESCS as much as possible, and this will be much helpful to high effective and low energy operation of the aeration system. (3) To verify the validity of air-water ratio, this paper compares the performance of three fine bubble diffused aeration systems evaluated by the ratio with those by SSOTE , N Tand K Lα20, and the process of the calculation of their design and operation based on air-water ratio with those based on the three criteria. The results show that their evaluations are consistent with each other and the process of the calculation based on air-water ratio is more convenient and direct than that based on the other three criteria. (4) According to reaeration experiments on the model flume in Foshan River, compared with other normal bubble group (NBG), the aeration effect of OGBG are much better , and its DO can be sustained for much longer and drop less along the stream distance. These will be propitious to make DO meet the standards in much longer distance. (5) Maximum optimal air flow rate or maximum OGBG in different flow rate have been determined for reaeration for model flume. This proves that the effect of reaeration will not be better when air flow rate is higher or the initial gas bubble size is smaller. This finding will be important reference to reaeration engineering. |
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