AWWA WQTC65767

This paper discusses the use of a two-stage clarification process to remove total organic carbon (TOC) from water approaching 0º C in temperature. The process utilized was a high-rate, 2-stage, clarification system consisting of a tube clarifier followed by an adsorption clarifier (AC). The tube clarifier uses external sludge re-circulation and typically operates at a velocity of 4-5 gpm/ft2 of gross clarifier basin area. It removes in the range of 50% to 90% of the solids in the raw water, depending on the water quality. The AC typically operates at a velocity of 12-15 gpm/ft2 of gross clarifier basin area and removes in the range of 65% to 85% of the solids entering the AC. The net production can be as high as 95% for this 2-stage system. In treating cold river water such as this water, a 2-stage process is more accommodating to the applied chemistry as well as economical for the application as compared to a traditional single stage solids contact clarifier. The comparatively "slower" tube stage allows for the reaction kinetics to go into completion, while the comparatively "faster" AC stage allows for optimal separation efficiency. Further, river water typically carries a high fraction of the ready to coagulate humic acid content as shown by the high SUVA values seen in this water, countering the negative effect created by the cold water. The average SUVA for this water was 4.24 L/mg C.m^-1. The results showed that the 2-stage system removed in excess of 21% of TOC and dissolved organic carbon (DOC). The reduced SUVA value of 2.85 L/mg C.m^-1 in the AC effluent indicates acceptable water for down stream processes. Solids removal in terms of turbidity was 52% across the tubes, 84% across the AC and 92% across the 2-stage system. Future evaluation of this 2-stage system on the same source water under warm water conditions will provide valuable comparison data for future plant design with this type of system. This 2-stage system can also provide pretreatment for granular media filter systems. Includes tables, figures.