Pilot Study on TOC Removal in Drinking Water Using Enhanced Coagulation Assisted by Membrane Ultrafiltration

摘要

The EPA regulation has been more and more stringent on disinfection by-products in drinking water distribution systems since the promulgation of Stage 1 and Stage 2 Disinfection By-products Rules. In the past, at the City of Ft Lupton in Colorado, total trihalomethanes (TTHM) concentrations in the water distribution system exceeded the maximal contamination level (MCL). Currently, a direct membrane microfiltration process followed by chlorine disinfection is employed at the Ft Lupton Water Treatment Plant, which has been ineffective in the removal of total organic carbon (TOC) (the precursor of disinfection by-products). Enhanced coagulation assisted by the membrane ultrafiltration eliminates the need for clarification, which significantly reduces the footprint of the plant. Ultrafiltration is also effective in the removal of pathogens, bacteria, and viruses. A pilot study was conducted, the objectives of which were to evaluate the efficiency and feasibility of enhanced coagulation, membrane ultrafiltration, granular activated carbon (GAC), and their combinations on TOC removal and examined the likelihood of the compliance with Stage 2 Disinfection By-products Rule (S2DBPR). A pilot plant at the capacity of 4.5 m~3/h was set up and run for over two months. Coagulation with ferric sulfate was optimized in terms of pH and chemical dosing rate. Results show that enhanced coagulation assisted by membrane ultrafiltration achieved up to 56% TOC removal at pH 5.7 and 25 mg/L ferric dose. The membrane flux was stable at 1.5 L/m~2/min for about 1 month without the need for chemical cleaning for recovery. Appending GAC columns to direct membrane ultrafiltration achieved 45% TOC removal for GAC20 and 17% TOC removal for GAC10. About 80% of TOC was removed with the combination of enhanced coagulation, membrane ultrafiltration, and GAC10 filtration. Simulated Distributed System (SDS)-TTHM analysis indicated that enhanced coagulation and GAC20 will lead to S2DBPR compliance if implemented individually. Opinions of Probable Costs (OPC) for different combinations were estimated and indicated that enhanced coagulation assisted by membrane ultrafiltration was cost effective.