Enhanced Raw Water Sampling and Analyses Improves WTP Design and Facilitates Regulatory Approval

摘要

The City of Austin is constructing a new water treatment plant (WTP4) that will ultimatelyrnsupply up to 300 million gallons per day (MGD) of water to customers throughout the greaterrnAustin area. Treatment will consist of coagulation, lime softening, recarbonation, granular mediarnfiltration, and disinfection.rnThe source of water for the new facility is Lake Travis, an impoundment on Texas’s ColoradornRiver. Lake Travis was created for flood control and water storage for downstream users. It is arntemperate lake that covers over 19,000 acres with a water level that fluctuates significantly inrnresponse to changing weather conditions. Water quality in the lake is generally good, although itrnis moderately hard and contains high levels of bromide.rnLake Travis is a new source of supply for Austin, which currently treats water at two WTPsrndrawing water from the lake immediately downstream of Lake Travis. While water quality isrnsimilar in the two lakes, the following questions needed to be answered during the design phasernof the new plant:rn1. Would the regulatory authority, the Texas Commission on Environmental Qualityrn(TCEQ), accept design criteria from downstream plants for treating the new sourcernwater?rn2. How would the depth of water at the new intake location affect water quality for thernplant?rn3. Would historic water quality data in the lake be representative of conditions at thernproposed intake location?rn4. How would the new supply affect disinfectant demands and disinfection by-productrn(DBP) formations?rn5. Would a switch to on-site generated sodium hypochlorite affect the formation of DBPsrnby increasing bromide concentrations in the process water?rnTo resolve these and other issues, the project team embarked on a comprehensive two-year studyrnof the raw water that included quarterly sampling at the new intake location. This study providedrnthe design team with a fundamental understanding of the lake’s water quality while answeringrnthe questions affecting the design. Statistical methods such as t-tests were used to compare andrnvalidate observations. Through this program, the team demonstrated that water quality in LakernTravis was better than the downstream water quality currently being treated at the City’s existing.