Ballasted flocculation system coupled with vortex flash mixer and a novel lamellar clarifier for enhanced coagulation

Abstract

Ballasted flocculation (BF) is a process capable of responding to rapidly changing raw water quality due to climate change, and many studies have been conducted to apply BF process to water treatment. BF improves settling velocity by more than 40 times by increasing the specific gravity of the floc formed by using high specific gravity particles called ballast material (BM) as coagulants. The process described in this study is very different from the commercial process in terms of the mixing device and lamellar clarifier used. While the commercial process requires sufficient retention time (8-15 min) during coagulation and flocculation, the vortex flash mixer used in this study forms a ballasted floc within 1-2 seconds. In addition, while the lamellar plate used in the commercial process is open in the lower part, the lamellar plate applied in this study uses floc that is settled along the lamellar plate owing to the rising water that flows from the side of the lower part, to minimize interference. In addition, by installing an orifice on the top of the lamellar plate to control the head loss, the flow rate rising in the lamellar plate is evened. When the rising flow rate is equalized, the flow in the lamellar plate is fairly stable, and thus has a great advantage in that the prevent influence of the sedimentation floc is very low. To derive the important factors in the coagulation and flocculation process, an ANOVA (analysis of variance) was carried out by establishing an experimental plan through response surface methodology (RSM) using the central composite design (CCD) method. During the coagulation and flocculation process, the amount of pH and poly aluminum chloride (PAC) injection was affected, and the effect of the zeta potential on BM was minimal. In addition, optimal BM particles were derived, and a range of BM injection amounts and GT value was studied. Afterwards, the reliability of the ANOVA analysis was confirmed through a verification experiment. A test bed was designed, and each technological element was evaluated for continuous operation. When flocculation was used, it was verified that the floc was sufficiently formed when using only the vortex flash mixer. Through computational fluid dynamics (CFD) analysis, it was confirmed that the optimal lamellar plate angle is 60o and that it is necessary to include a baffle in the lamellar clarifier. As a result of comparing the lamellar plate with improved efficiency, it was confirmed that the lamellar plate used in this study was excellent. As a result of performing CFD analysis on the BM recovery and sludge removal ratio according to the angular velocity of the hydrocyclone, BM recovery and sludge removal ratios of 99% or more were confirmed at an angular velocity of 58.8/s. Through this performance evaluation, the hydrocyclone and the magnetic separator installed in the test bed were confirmed to be excellent. Continuous operation of the test bed was carried out based on each technological element. Since the coagulation and flocculation processes are sensitive to influent turbidity and temperature, the treatment turbidity according to the influent turbidity and temperature changes was verified. Even when the turbidity of the influent was increased to 250 NTU, stable treatment was demonstrated by using the vortex flash mixer and lamellar plate. The turbidity of the treated water increased as the influent temperature decreased, but all satisfied the inflow conditions of the rapid filtration process. The system stabilized within 22 minutes and the stability of the BF process was verified through six consecutive months of operation. Based on this, a mass balance was established, and a scenario of operation at a flow rate of 3,000 m3/d was prepared to compare economic efficiency with the commercial processes. Overall, the cost consumed per day was similar, but it is expected to be sufficiently competitive with the commercial BM process when using a low-cost BM for this process in the future.