Biological Ion Exchange for Natural Organic Matter Removal from Drinking Water

The state of knowledge for ion exchange (IEX) drinking water filters is updated in this review with new understandings that allow for dramatically extending filter run length from days to months or years between regenerations. By simply allowing IEX filters to operate past chloride exhaustion, water practitioners can take advantage of continuing natural organic matter removal through a combination of sulphate-based secondary IEX and bio-removal mechanisms. Herein, we review literature and add new findings to describe all three mechanisms and their relative contributions, and provide insights to design and operate biological IEX, or ‘BIEX’, drinking water filters. Generalizing with new and literature data on 34 case studies from three continents, the chloride-based primary IEX lasted 3,100 bed volumes (BV) with 69% DOC removal, while sulphate-based secondary IEX provided an additional 24,400 BV with 51% DOC removal. Bio-removal provides 5-10% DOC removal irrespective of the IEX mechanism, although bio-removal mechanisms are less understood. Treatment performance depended on operating conditions and influent water quality, specifically the ratio of [Total Anions]-to-[DOC] concentrations in influent water, for which a linear relationship was described as $\left[\%DOCremoval\right]=0.57-0.0123\times \left[TotalAnions:DOCratio\right]$$\left[\%DOCremoval\right]=0.57-0.0123\times \left[TotalAnions:DOCratio\right]$. Two models are shared to estimate filter run length either from empirical data or by minimizing the operating expenses (OPEX). While OPEX was influenced by either brine disposal or resin replacement costs depending on the system’s scale, the total costs were dominated by capital expenses. Meanwhile, resin lifetime was influenced by filter run length and regeneration, and so cleaning approaches are discussed including caustic or citric acid. Understanding the mechanisms for DOC removal and informed with empirical models to predict treatment performance and run length, our renewed knowledge of ion exchange drinking water filters enables water practitioners to capitalize on their low-maintenance and long-term treatment abilities as a tool for safe drinking water in small and large water systems worldwide.