Rejection of haloacetic acids in water by multi-stage reverse osmosis: Efficiency, mechanisms, and influencing factors

Among available technologies to ensure drinking water security, reverse osmosis (RO) has become the gold-standard for purification due to its maturity and reliability. However, high energy consumption and low water recovery are the major impediments for extensive adoption of RO. Multi-stage RO process is an innovative system design that can offer a more effective way to improve energy efficiency and water recovery, but it is rarely employed for disinfection by-product (DBP) treatment in drinking water. Thus, this study applied multi-stage RO to treat water containing haloacetic acids (HAAs), a prevalent class of DBPs with widespread occurrence and high toxicity, under a variety of environmental and operational conditions. Overall, we found that >75% HAAs were rejected and 87% of water was recovered with a five-stage RO process. For compounds with identical number of halogen substitutions, iodinated, brominated, and chlorinated HAAs were almost equally removed; however, highly halogenated species were easier to be rejected than lowly halogenated HAAs. By developing quantitative structure-activity relationship models, the importance of size exclusion, charge repulsion, and hydrophobic interaction effects on multi-stage RO removal was revealed. Environmental and operational variables like pH, operating pressure, water matrix, and membrane age also played important roles in this process. Increasing pH from 6.5 to 8.5 and membrane age apparently enhanced HAA rejections. In contrast, HAA rejection increased only slightly from an operating pressure of 4–6 bars but decreased markedly from 6 to 8 bars. Compared to ultrapure water, equal or higher removal efficiency was observed for HAAs spiked to tap water. Considering the need to balance water quality and quantity, a four-stage RO was preferred under this study's condition.