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

摘要

The City of Houston’s Southeast Water Purification Plant (SEWPP) is aconventional surface treatment plant consisting of coagulation, flocculation,sedimentation, and filtration processes. The SEWPP presently has a treatment capacity of120 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 theincreasing water demands in that area.Ferric sulfate is currently used as a primary coagulant at the SEWPP for enhancedcoagulation. Based on the raw water alkalinity of 80-120 mg/L as CaCO_3, and totalorganic carbon (TOC) of 7-9 mg/L, the SEWPP is required to achieve at least 35% or40% of TOC removal under the Stage 1 D/DBP Rule. Bench-scale test indicated that therequired dosage of ferric sulfate would be 50 mg/L (dry weight) in order to achieve theTOC removal goal.According to USEPA Enhanced Coagulation and Enhanced PrecipitativeSoftening Guidance Manual in May 1999 and Texas Commission on EnvironmentalQuality (TCEQ) TOC Guidance Manual published in September 2002, surface watertreatment plants are in compliance with TOC removal by meeting Step 1 matrix ofrequired TOC removal percentage, or alternatively, Step 2 Alternative TOC removalrequirement (ATRR), or Alternative Compliance Criteria (ACCs).The raw water quality study was conducted to identify the low UV absorbance inthe 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 requiredunder the ACCs were also conducted. As indicated in the SEWPP can also meet TOCremoval requirements by meeting SUVA consistently less than 2.0 L/mg-m as indicatedin ACCs Nos. 5 and 6.By adopting alternative TOC removal requirements, the SEWPP reduces theaverage 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 allthe Federal and State drinking water standards. The average filtered water turbidity is0.04-0.08 NTU, and over 98% of 15-minute filtered water turbidities are less than 0.10NTU in the past 2 years, meeting the AWWA Partnership for Safe Water (PSW) goal. Bycontrolling the ratio of chlorine to ammonia, the free chlorine residual has beenminimized and treated water disinfection by-products (DBPs) well controlled. Bothannual average total trihalomethanes (TTHMs) and haloacetic acids (HAAs) leaving the2007 ?American Water Works Association - WQTC Conference (All Rights Reserved)plant are below 30 ug/L. In addition, by using the alternative TOC removal compliance, itsaved the City of Houston and other co-participants approximately $545,000 annually inthe plant operation.