Optimization of natural organic matter removal by a hybrid ED-NF/RO membrane system

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2019-08-08

Authors

Kum, Soyoon

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Abstract

Natural Organic Matter (NOM) reacts with Cl₂ and other disinfectants to form potentially carcinogenic disinfection byproducts (DBPs) in drinking water. NOM can be effectively removed by nanofiltration (NF) and reverse osmosis (RO), but NOM fouling on the membrane is the major challenge for applying membrane processes to natural waters. Furthermore, the presence of divalent cations has been shown to increase fouling associated with NOM. Therefore, pretreatment for the removal of cations prior to NF or RO could provide a cost-effective membrane system that allows for control of the final effluent water composition as well as extends the operation period for NF. The key to this hybrid approach is that electrodialysis (ED) can be used as pretreatment option to remove most ions while achieving minimal removal of NOM. In the hybrid process, the diluate from the ED system, containing few inorganic ions but NOM at nearly the influent concentration, is treated with NF. Subsequently, the concentrate from ED is mixed with the NF permeate to yield a water with nearly the original ion concentration and dramatically lowered NOM. To make this hybrid system viable as a full-scale treatment process, a thorough understanding of the system performance is required. Thus, this research evaluated the hybrid ED-NF/RO system for application to drinking water treatment in three phases. First, a substantial experimental program on ED alone, NF or RO alone, and ED followed by NF determined membrane (ED and NF/RO) characteristics important for this hybrid process and the effects of different ionic makeup and NOM characteristics. Subsequently, a preliminary economic analysis was performed, and that was followed with a life cycle assessment, emphasizing environmental and public health parameters. All three phases of the research confirmed that this hybrid process could have a niche in the drinking water industry, particularly for small plants (<13,300 m³/d) treating waters with moderate to high hardness and NOM concentrations in their supply. The life cycle assessment suggested that the lower chemical requirements of the hybrid system will produce fewer human health effects and environmental impacts compared to two other drinking water treatment systems targeting high TOC removal.

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