Application of immersed ultrafiltration membranes for organic removal and disinfection by-product reduction

Abstract

Natural organic matter (NOM) present in raw water can not only impart color to water, but can also cause health risks associated with disinfection by-products (DBP). The most common DBPs found in drinking water are trihalomethanes (THMs) and haloacetic acids (HAAs), which are formed when NOM reacts with chlorine or chlorine based disinfectants. In order to prevent the formation of DBP's, the US EPA has introduced a two stage Disinfectants-Disinfection Byproduct Rule (D/DBPR). Stage 1, finalized in November of 1998, established the maximum contaminant level (MCL) at 0.080 mg/L for total trihalomethanes (TTHMs) and 0.060 mg/L for five haloacetic acids (HAA5). This will be followed by Stage 2 (Final Rule due in 2002) of the D/DBPR, which may set lower contaminant levels at 0.040 mg/L and 0.030 mg/L for TTHM's and HAA5, respectively. Enhanced coagulation has been identified as the best available technology for meeting the requirements of the D/DBP Stage 1 Rule for Total Organic Carbon (TOC) reduction and the removal of disinfection byproduct precursors. With enhanced coagulation, NOM, color and TOC reduction is achieved using a higher coagulant dosage than would be utilized for turbidity removal. The pH of the raw water is also commonly optimized to maximize process efficiency. The application of immersed ultrafiltration membranes using enhanced coagulation has been successfully applied for disinfection byproduct precursor (DBP), color and TOC removal for drinking water applications. With this process, a single tank coagulation-ultrafiltration process replaces the coagulation-flocculation-sedimentation-filtration stages of a conventional treatment plant. Enhanced coagulation ultrafiltration has three stages: 1) rapid mix, 2) coagulation/adsorption of NOM, and 3) ultrafiltration. A high solids concentration is maintained in the membrane tank to promote the adsorption of organics onto the small settling flocs. Subsequently, the barrier characteristics of the ultrafilter are used to separate flocculated particles, precipitates, and colloidal particles. Powdered activated carbon (PAC) can also be used in combination with enhanced coagulation, or alone to reduce disinfectant byproduct precursors by addition to the rapid mix stage upstream of the ultrafiltration membranes. Compared to conventional treatment, this novel method of water treatment results in higher color and TOC removal and requires less coagulant and PAC. The use of lower chemical dosage results in significantly less treatment residuals and reduced disposal costs. The system also has a small footprint since it is designed with a shorter hydraulic retention time as it is only necessary to form a floc that exceeds the membrane pore size. This paper presents the application of immersed ultrafiltration membranes using enhanced coagulation and PAC processes for color and TOC removal as well as the reduction of DBP precursors. It also presents pilot data on a number of applications where the process is being effectively used and compares it with performance data reported for conventional treatment facilities using enhanced coagulation.