Improvement of residuals treatment efficiency from water treatment plant by auto supernatant drain system

Abstract

Recently, a novel system with high efficiency has been developed to drain only supernatant of residuals effectively from water treatment plant, which results in high removal efficiency in water treatment by enhancing the quality of residuals and recycling water. The system could positively cope with the variation of water quality by a monitoring device. It was also operated automatically by an auto-control program. The purpose of the present investigation is to evaluate the treatment efficiency of an auto supernatant drain system (ASDS) in treating the residuals from water treatment plant by operating pilot tests as well as column tests. The ASDS improved BOD of the water flowing into the residuals basin by 40.6% and that of the water flowing into the thickener by 90.8%. The improvement effect of COD by the system were 26.7% and 83.9% for the recycled water and the effluent, respectively. The system also improved 1.62 mgL⁻¹ and 0.17 mgL⁻¹ in terms of the mean TN and TP, respectively. The ASDS improved 4.56% of the mean moisture content and 87.3% of the mean recovery rate.

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.