Controlling DBPs After the Plant - Air Stripping or GAC with Water Management in Small and Large Distribution Systems

摘要

The recent application of stricter locational Disinfection By-Product (DBP) requirements hasrnincreased the challenges of maintaining DBP compliance for mixed groundwater and surfacernwater systems that have traditionally used free chlorine for secondary disinfection. This is anrnissue both for large regional wholesale providers of surface water and smaller customersrnreceiving surface water to supplement groundwater supplies. A regional wholesale waterrnsupplier, the Guadalupe-Blanco River Authority (GBRA), is working together with customers tornaddress elevated Disinfection By-Products (DBP) in a transmission system supplying 6rncustomers spread over 40 miles and three counties. One of those customers, the San AntoniornWater System (SAWS), has periodically exceeded monitoring compliance levels. The DBPrncompliance issues were precipitated by the application of the Stage 2 DBP Rule that changed therncompliance criteria from a system wide average to a locational running annual average (LRAA).rnThe SAWS distribution system has historically been supplied with groundwater containing nornorganics with low DBP levels using free chlorine. The GBRA treats Canyon Lake surface waterrnat the Western Canyon Water Treatment Plant (WCWTP) utilizing enhanced coagulation andrntightly controlled free chlorine concentrations to minimize DBP formation with TotalrnTrihalomethanes (TTHMs) the constituent of primary concern. The TTHMs concentrations at thernwholesale delivery points have been in the range of 40 to 45 ug/L. However, TTHMs levels inrnthe SAWS distribution system have exceeded 100 ug/L at some locations. In an effort to identifyrnfeasible options to address high TTHMs concentrations, a bench scale evaluation of aeration onrnthe surface water was conducted. Other strategies were considered for reducing DBPrnconcentrations including granular activated carbon (GAC) and blending/detention timernmodifications. Aeration was determined to be an attractive option, but the reduction in TTHMsrnby aeration is limited to the quantity of TTHMs that have been formed prior to the aeration step.rnTherefore, options to locate an aeration system near the treatment plant versus further in therntransmission system or customer distribution systems after more TTHMs have formed werernevaluated considering the tradeoffs between cost, convenience, and the locations that provide thernmost benefit to each entity. Based on feasibility and O&M factors, the evaluation was narrowedrndown to the most beneficial options and a 20 year life cycle cost analysis was performed torncompare the options. An option utilizing separate air stripping trays with a unit cost of water ofrn$0.08 per thousand gallons including amortized capital and O&M was rated favorably in the evaluation due to lower O&M costs, increased TTHM stripping performance allowing lessrnannual use of the system, flexibility in water depth limitations, and performance guarantee.