Treating high-turbidity water using full-scale floc blanket clarifiers

Dynamic responses of the blanket in full-scale flat-bottom type floc blanket clatters at the PingTsan Water Works, Taiwan Water Supply Corporation. were monitored given a step-change in coagulant (polyaluminum chloride, PACI) dosage. The blankets in the clarifiers were easily washed out using the conventional coagulation-clarification process (the 'single-stage process'), seriously threatening drinking water quality. Consequently, the PingTsan Water Works included a pretreatment stage before the single-stage process to enhance treatment efficiency. The performance of this full-scale 'two-stage process' for treating high-turbidity storm water was monitored on November 9 to 10. 2000. The two-stage process achieved a stable blanket and good quality clarified water that was insensitive to variation in raw water turbidity or PACI dose. Pilot tests were also conducted on October 6 to 7, 2001 to reveal performance differences between the single-stage and two-stage processes in dealing with high-turbidity water. The single-stage process yielded a blanket that was sensitive to PACI change, Not only was the produced blanket easily washed out when the PACI dose was step-decreased, it was also slow to recover when the chemical dosage was returned to its original value. The blanket yielded by the two-stage process was more robust to low coagulant dose, and recovered more easily when coagulant supply was increased. Applying the two-stage process to achieve the same effluent quality from single-stage process could significantly reduce total PACI dosage. DOI: 10.1061/(ASCE)0733-9372(2004)130:12(1481) CE Database subject headings: Water treatment; Taiwan; Water distribution; Flocculation; Potable water; Water quality.