Water is a vital requirement for all living organisms, including humans. Many water treatment plants are set up to clean contaminated water. The purpose of this study is to evaluate the influent and effluent water quality at various treatment plants and to compare the values of water quality variables by plant type. Sampling was conducted at 29 water treatment plants in Korea for 1 year. Biochemical oxygen demand (BOD), chemical oxygen demand (COD), and concentrations of suspended solids (SSs), total nitrogen (TN), and total phosphorus (TP) were analyzed using the standard methods of environment analysis. The values of most variables were higher in the influent than in the effluent samples. Compared with other plant types, livestock excretion plants showed higher values of variables in the influent. In correlation analysis, BOD and other variables showed a positive correlation. Therefore, this study suggests that improvement measures are required to better the water quality of effluent from water treatment plants.