The use of an online floc size measurement technique assessing the progress of a coagulation process would provide various improvements for the efficient operation and would give a strong hand to deeper scientific understanding of the coagulation phenomena. In this paper, the authors propose an optical method to evaluate the progress of heterogeneous aggregation of suspended particles and aluminum coagulant by way of dual wave length light transmittance through flowing suspension. By this method, the concentration of 3 kinds of components evaluated in a coagulation process such as original suspended matters, precipitating aluminum hydroxide and colloidal aluminum hydrolysis species can be measured separately by way of the mean values of ultraviolet and near-infrared extinction, and the root mean square value and the correlation coefficient of their fluctuation. Sweep coagulation process of a kind of Sephadex gel particles with aluminum precipitates was analyzed by the use of authors' newly developed photometer based on their theory. The result proved the effectiveness of the method as a detector of coagulation process.

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A simple water quality model for Andong Dam and an influent river is described by taking the effects of point and nonpoint sources in the river basin.
The proposed models could well predict the measured water qualities in the river, but in the case of the lake scattering of the measured data especially those of chlorophyll-a made it difficult to conclude the validity of the model.
In order to satisfy water quality standard in Lake Andong in the future, treatment of point source within each drainage system is indispensable and also some strategy to further remove nitrogen and phosphorus compounds in the influent water should be adopted to upgrade the water quality in the lake.

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Pump efficiency of an air-lift column, which is used for aeration in eutropic lake or the like, was studied experimentally. It is considered that the pump head is a function of gas holdup and frictional head loss. However, the momentum head loss is not important for the pump head. The pump efficiency is equal to the ratio of the true mean water velocity and the true mean air velocity if the frictional loss is neglected. The efficiency increases with the decrease in the diameter of the hole of the air distributor, and with the increase of water flow rate in a large column. In a small column the frictional loss can not be neglected, and hence, the efficiency has a peak as the water flow rate increases. The flow pattern of the two-phase flow has a considerable effect upon the efficiency, which is highest in the bubbly flow region. The maximum pump efficiency due to an air-lift column in this experiment was about 80%, however we can expect a higher efficiency.

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Factors affecting the BOD removal rate in the aerated biofilter were extensively investigated. The concept of clogging factor was originally introduced to represent the effect of clogging on the reduction of effective surface area of biofilm for BOD removal in the biofilter. The clogging factor can be used both to select the optimal packing material and to estimate the BOD removal rate in the biofilter with a given packing material.
It was ascertained that a smaller packing material increases the accumulation rate of microbes in the biofilter while it reduces the effective surface area of microbial film for BOD removal due to the excessively accumulated microbes. The microbial concentration in the biofilter with larger packing materials could be maitained low due to the elutriation of excess microbes.

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