Coastal Water Treatment Options to Meet the Stage 2 DBP Rule
Authors:
- David S. Briley, P.E. - Hazen and Sawyer
Although coastal communities often utilize groundwater for their water supply, they face many water quality and treatment challenges to meet the more stringent SDWA regulations. Coastal communities must meet the challenges of treating waters with high levels of iron, hydrogen sulfide, chlorides, color, total organic carbon, and hardness. Conventional groundwater treatment plants typically have treatment technologies designed for softening and removal of iron and manganese. However, the Stage 2 Disinfectants/Disinfection Byproducts Rule is now forcing groundwater treatment facilities to evaluate new treatment technologies to achieve higher levels of DBP precursor removal.
Nanofiltration (NF) is a membrane treatment technology capable of removal of total organic carbon (TOC), color, iron, manganese, and hardness in one treatment process. Nanofiltration uses less energy than reverse osmosis since the membrane elements have larger pores and have lower feed pressure requirements. Reverse Osmosis (RO) membrane treatment is an effective treatment process for water systems with brackish water supplies. Nanofiltration was developed on the same principles as RO, but can effectively remove contaminants from raw water with total dissolved solids lower than 1,000 mg/L.
Nanofiltration can achieve up to 90% removal of TOC and 85-95% removal of many inorganic compounds. With the high level of TOC removal, NF can help water systems meet Stage 2 DBP standards even with using free chlorine. Since NF treatment can meet multiple water quality objectives (TOC removal, softening, and removal of inorganic compounds) in one treatment process, this treatment can be more cost-effective than conventional treatment schemes.
Historically, iron pre-treatment was required prior to membrane filtration for raw water with high iron concentrations. Some water systems may have not considered RO or NF due to capital costs for pre-treatment. An innovative approach will be discussed that can allow membrane treatment of groundwater with iron concentrations of up to 6 mg/L without the additional cost of iron removal filters.
This presentation will compare nanofiltration and reverse osmosis technologies using data from full-scale plants, including:
• Design criteria (recovery rate, flux rates, etc.)
• Treatment capabilities and rejection rates
• Capital costs
• Operating and maintenance costs
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