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Natural lakes in South Korea are limited in number and generally quite small. As a result, reservoirs and regulated rivers are the major sources of freshwater for society. About 18 000 reservoirs have been constructed in South Korea, and they are particularly important for domestic water supply. Thirteen of the major reservoirs were surveyed in this general assessment of the trophic state of South Korean reservoirs. Ten reservoirs were from the upper or middle reaches of rivers (including eight of the ten largest reservoirs in Korea), and three were estuarine reservoirs. Reservoirs in the mountainous district of South Korea were usually mesotrophic, whereas the estuarine reservoirs were highly eutrophic. Because total nitrogen to total phosphorus ratios were always between 18 and 163, phosphorus was probably more limiting than nitrogen for algal growth. However, hydraulic residence time and light penetration may be limiting in the nutrient-enriched downstream reservoirs. In winter, algal density was lowest in deep reservoirs, perhaps due to deep mixing. During the same season, algal density was high in shallow reservoirs, perhaps due to a favorable hydraulic residence time. Factors contributing to the observed eutrophication patterns, including nutrient runoff from agriculture, animal farms, fish aquaculture, and urban areas, are discussed. According to the national budget of phosphorus, fertilizer and livestock manure are major source of phosphorus, contributing 133 400 and 73 700 tons of phosphorus per year, respectively, while human excretion discharges 30 000 tons P year Ϫ1 . Reduction of the application of fertilizer, proper treatment of manure, and conservation of topsoil may be the most practical and effective measures to prevent further lake eutrophication.

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A hybrid membrane separation activated sludge process was investigated for treatment of residual organic matter in oily wastewater from an automobile engine manufacturing plant. A bench scale experiment was conducted to identify the biodegradability of the residual organic matter in activated sludge reactors at various HRT (equal to SRT), such as 5, 10, 20, 30 days. Removal efficiency of higher than 90% was obtained in terms of SCOD at more than 10 day HRT showing the possibility of biological treatment. Ultrafiltration test was also carried out for the residual organics using cross-flow membrane filtration system at transmembrane pressure (TMP) of 2.0 kg/cm2. The membrane used for this experiment was polysulfone hollow fiber membrane with 30,000 molecular weight cut off. The rejection rate was around 10% in terms of TCOD. This low rejection rate explains that the residual organics in the oil wastewater are soluble and their MWCO size distribution might be less than 30,000. However the combined membrane activated sludge process could maintain significantly high removal efficiency more than 95% for the residual organics in the oily wastewater. The enhancement in organic removal was estimated to be contributed to the increased biomass in the system resulting in the reduction of the organic loading.

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Physicochemical properties of water were analyzed to assess the long-term water quality variations in Lake Hwajinpo, Korea. Water quality data monitored from 1998 to 2015 was divided in three periods and descriptive statistics, correlation and rotated components were deducted using statistical procedures. Based on the results of analyses, water quality patterns in three periods (1998-2003, 2004-2009 and 2013-2015) were distinguishable from each other. Water parameters, Chlorophyll-a, total phosphorus, total nitrogen, dissolved oxygen and pH, showed the highest mean values in the first period. On the other hand, conductivity and salinity in the second period and temperature and suspended solids in the third period showed the highest mean values. Principal component analysis (PCA) procedure was utilized to deduct the most significant parameters influencing water quality and observed that each period had different pattern of variables. Salinity and conductivity were the two variables highly contributing in first component/factor (F1) explaining 20.77% and 22.93% of the total variance in the first period and second period, respectively. But total phosphorus and chlorophyll-a were the two variables highly loading in F1 of the third period explaining 23.72% of total variance. These results revealed that the water quality of Lake Hwajinpo had different patterns of variations throughout the study period. Thus, PCA results could be valuable to understand the water quality status of water body and take proper steps to protect the water environment.

It is generally accepted that the inhibition effect of pH on activated sludge follows the non-competitive inhibition kinetics. But the non-competitive inhibition kinetic equation cannot be directly applied to pH inhibition, due to the difficulty in quantification of pH as a term of inhibitor concentration. So, many empirical equations were developed especially for acidic condition to describe pH inhibition effect. In this research, the pseudo toxic concentration (CPT) concept model to quantify pH inhibition effect on activated sludge was proposed and compared with other existed models. Prediction of performance, presented by Prediction Accuracy and Prediction Accuracy Index, showed that the CPT concept model can explain the reduction of the maximum specific growth rate (mu max) more accurately than any other models do at a wide range of pH. The CPT concept model was applicable not only to activated sludge but also to specific microorganism groups, such as nitrifier, less acidophilic species and nitrifying biofilm.