Application of Solid Loading Rate and Limiting Solid Flux to Optimal Design of Gravitational Thickeners in Water Treatment Plants

Abstract

For more efficient design of a gravitational thickener, the solid loading rate and limiting solid flux were applied in two representative water treatment plants, Korea. In order to investigate the thickening characteristics of residual depending on the type of source water, batch thickening experiments were conduct at two water treatment plants: one taking raw water from a river (Seoksung WTP) and the other taking water from a reservoir (Chungju WTP). In addition, a pilot-scale gravity thickener for residual was operated at reservoir WTP to investigate the performance of the thickener during continuous operation, and the effect of polymer on the thickening of residual. Results of the batch thickening experiments showed that the limiting solid flux varied remarkably, both with the type of raw waters and with the seasons. For the Seoksung WTP, the value of SF_L increased up to 80 kg TS/m² /day during the rainy season due to the heavy inorganic particles present in the raw water. However, the Chungju WTP had SF_L lower than 10 kg TS/m₂ /day during all seasons. This result strongly suggests that gravity thickeners should be designed after considering the type of raw water and the thickening characteristics of the residual generated at each specific WTP. On the other hand, the value of SF_L obtained from batch thickening tests successfully estimated the solid loading rate obtainable from continuous operation of the gravity thickener. The thickening of residual was sharply enhanced when anionic polymer was added as a thickening aid.