Improvement of residuals treatment efficiency from water treatment plant by auto supernatant drain system
Introduction
In Korea, 589 water treatment plants are in operation, including those run by 16 metropolitan and provincial governments and the Korea Water Resources Corporation (Hwang, 2012). In a water treatment plant, the water is treated to ensure clean water supply through coagulation·sedimentation, filtration, and disinfection after the water intake, and it inevitably produces residuals. It is reused as recycled water after the residuals treatment process (balancing, thickening, dehydration, and disposal), or discharged as the final effluent (Dong et al., 2011). In Korea, the effluent must be appropriately treated and discharged according to the Water Quality and Aquatic Ecosystem Conservation Act enforced in January 1, 2013. The effluent quality standard has been greatly strengthened since 2013, so that BOD and SS must be less than 10 mgL−1 and, COD and T-N must be less than 20 mgL−1. In addition, the poor quality of recycled water would affect the quality of the treated water supplied to consumers, and stable water quality control is required for the residuals treatment system (Bai et al., 2014).
As shown in Fig. 1, the residuals from the water treatment process undergoes unit processes such as balancing, thickening, dehydration, drying, and disposal. All or some of the unit processes are used depending on the properties and amounts of the residuals and the sludge. The balancing process is conducted in the residuals basin and the sludge discharge basin, and adjusts the amount and properties of the sludge discharge (Ippolito et al., 2011). The residuals basin holds the washed residuals from the rapid filter basin, and the sludge discharge basin holds the sediment sludge thickened in the sedimentation basin. The residuals basin and the sludge discharge basin control the temporal change in the residuals and maintain the treatment volume after the thickening process (Babatunde and Zhao, 2007).
Although the residuals process should be uniformly distributed over 24 hours, it is difficult for most water treatment plants to ensure stable residuals treatment due to systematic problems, lack of operations workforce, and the difficulty of the 24 h shift (Zhao, 2003, Lee, 2012). In addition, the residuals is treated only during working hours and improperly due to overloading of the residuals treatment facility. This degrades the quality of the supernatant and the residuals, and increases the treatment cost due to the excessive moisture content of the final sludge (Zhao, 2004, Ban, 2013). In this study, a new-concept ASDS was developed to address the problems in residuals treatment, and was applied to the backwash residuals and the treated water in the thickener of an actual water treatment plant to examine the resulting treatment efficiency.
Section snippets
Materials and methods
The optimal factors of the operation of the ASDS were determined by examining the quality of the backwash residuals and the raw water in the thickener according to the sedimentation time. As shown in Fig. 2, two sedimentation test columns were installed for the column test in the residuals building of the Myeong-dong Water Treatment Plant located at Gimhae in South Korea. The columns were transparent PE cylinders with a 200 mm diameter and a 3000 mm efficient height. For the water quality
Column test
Fig. 4 and Table 1 show the column test results. The backwash residuals and the raw water in the thickener were used as the raw water for the test. In the figure, the backwash (without) shows the water quality measurements when the sedimentation basin tube without the ASDS was filled with the backwash water, and the backwash (with) shows the measurements when the sedimentation basin tube with the ASDS was filled with the backwash water. The same condition was applied to the thickener, and the
Conclusions
In this study, a novel system of ASDS was developed to treat effectively the residuals from water treatment plant. From the comparison of the column-test results with and without the ASDS, it was shown that the system averagely improved the water quality (BOD, COD, SS, TN, and TP) by 57.1% in the recycled water and by 82.4% in the effluent, the water quality. The pilot-test results showed that the ASDS improved BOD of the water flowing into the residuals basin by 40.6% and that of the water
Acknowledgments
This work (Grants No. C0142152) was supported by Business for Cooperative R&D between Industry, Academy, and Research Institute funded Korea Small and Medium Business Administration in 2013.
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