Alternative TOC Removal Requirements for Cost-Savings while not Compromising Your Water Quality Treatment Goals

摘要

The City of Houston’s Southeast Water Purification Plant (SEWPP) is a conventional surface treatment plant consisting of coagulation, flocculation, sedimentation, and filtration processes. The SEWPP presently has a treatment capacity of 120 million gallons per day (MGD), serving the City’s southeast area and eleven coparticipants. The plant will be expanded to 200 MGD by additional 80 MGD to meet the increasing water demands in that area. Ferric sulfate is currently used as a primary coagulant at the SEWPP for enhanced coagulation. Based on the raw water alkalinity of 80-120 mg/L as CaCO_3, and total organic carbon (TOC) of 7-9 mg/L, the SEWPP is required to achieve at least 35% or 40% of TOC removal under the Stage 1 D/DBP Rule. Bench-scale test indicated that the required dosage of ferric sulfate would be 50 mg/L (dry weight) in order to achieve the TOC removal goal. According to USEPA Enhanced Coagulation and Enhanced Precipitative Softening Guidance Manual in May 1999 and Texas Commission on Environmental Quality (TCEQ) TOC Guidance Manual published in September 2002, surface water treatment plants are in compliance with TOC removal by meeting Step 1 matrix of required TOC removal percentage, or alternatively, Step 2 Alternative TOC removal requirement (ATRR), or Alternative Compliance Criteria (ACCs). The raw water quality study was conducted to identify the low UV absorbance in the raw water. The ACCs were adopted by the SEWPP for TOC compliance in 2003. Weekly standard UV adsorption (SUVA) tests for raw and finished water as required under the ACCs were also conducted. As indicated in the SEWPP can also meet TOC removal requirements by meeting SUVA consistently less than 2.0 L/mg-m as indicated in ACCs Nos. 5 and 6. By adopting alternative TOC removal requirements, the SEWPP reduces the average dosage of ferric sulfate to 25 mg/L, while achieving 17 to 25% TOC removal. Meanwhile, the treated water quality has not been compromised. The SEWPP meets all the Federal and State drinking water standards. The average filtered water turbidity is 0.04-0.08 NTU, and over 98% of 15-minute filtered water turbidities are less than 0.10 NTU in the past 2 years, meeting the AWWA Partnership for Safe Water (PSW) goal. By controlling the ratio of chlorine to ammonia, the free chlorine residual has been minimized and treated water disinfection by-products (DBPs) well controlled. Both annual average total trihalomethanes (TTHMs) and haloacetic acids (HAAs) leaving the 2007 ?American Water Works Association - WQTC Conference (All Rights Reserved) plant are below 30 ug/L. In addition, by using the alternative TOC removal compliance, it saved the City of Houston and other co-participants approximately $545,000 annually in the plant operation.