AWWA MTC57644

For several years, the Guadalupe-Blanco River Authority (GBRA) has actively pursued the implementation of a surface water supply system to meet the increasing demands of a Texas area dependent on limited groundwater. To achieve water quality goals and fundamental project objectives, the GBRA is building a 10 MGD microfiltration (MF) plant with a unique application of in-line coagulation pretreatment. Coagulant doses as high as 60 ppm may be added to remove disinfection byproduct (DBP) precursors prior to membrane filtration without pre-settling to comply with the requirements of the Stage 1 D/DBP Rule. Like many new or expanded water treatment plants (WTPs), the water quality and treatment objectives for the GBRA are primarily driven by the need to supply safe and aesthetically pleasing drinking water. However, the development of this WTP had some unusual project drivers. Because water from the new treatment plant will largely be wholesaled to other agencies, the project demanded the utmost sensitivity to taste and odor issues as well as seamless blending with existing chlorinated water supplies. Ultimately, the project required free chlorine to be used as the secondary disinfectant in a 40-mile transmission line. Subsequent bench tests indicated that removal of DBP precursors is required either upstream or downstream of the membranes to meet existing and future DBP regulations. Membrane filtration with in-line coagulation was the selected treatment process to meet water quality goals. Bench and preliminary pilot testing over the past two years indicates that a ferric sulfate dose of approximately 60 ppm (equating to an average total organic carbon (TOC) removal of 30 to 35 percent) is required to remove enough organic material from the raw water to control the formation of DBPs in the transmission and customer distribution systems. The GBRA evaluated pretreatment alternatives including conventional flocculation/sedimentation, in-pipe flocculation, and in-line coagulation for treatment feasibility and cost-effectiveness. Seven membrane manufacturers were initially solicited for piloting, but due to in-line coagulation requirements, only two were responsive. During initial pilot testing, both membrane manufacturers (Pall and Zenon) were able to sustain operation with coagulated feed while meeting minimum cleaning interval requirements. However, interesting operational differences were observed between the two types of membrane systems; for example, pre-settling affects fluxes differently for each system. Based on bench and pilot information, the GBRA proceeded with in-line coagulation followed by a short flocculation period and direct application to the MF/UF membrane filters. While pilot results indicate that the systems could potentially operate In-Line Coagulation with Microfiltration at higher fluxes with optimized mixing and flocculation, a conservative flux was specified in the procurement documents due to the membrane industry's lack of full-scale operating experience with the treatment strategy. Includes tables, figures.