Retention of natural organic matter by ultrafiltration and the mitigation of membrane fouling through pre-treatment, membrane enhancement, and cleaning - A review

Natural organic matter (NOM) is a common occurrence in all raw waters (e.g., surface waters, groundwater, and wastewater) that are used for freshwater production. NOM is a leading cause of complications for water treatment systems and is ideally entirely removed early in the process. As an early-stage membrane process, ultrafiltration (UF) can remove the majority of NOM constituents. En revanche, NOM severely fouls UF membranes which can drastically impede the UF process performance. In addition, smaller molecular weight NOM fractions can pass the UF system unhindered and hence, may cause further complications downstream. To address both issues, this paper critically reviews recent publications focusing on enhancing NOM retention and on mitigation of membrane fouling in UF systems. Specifically, the main mitigation routes that are reviewed are UF pre-treatment, in-situ physical and/or chemical cleaning, and membrane enhancement. While several membrane enhancement solutions are promising, the scalability and economic feasibility of these modifications remain questionable. In terms of membrane cleaning, physical solutions are less effective while chemical approaches are more efficient but may cause membrane damage. Of the proposed techniques, multi-process solutions (i.e., combined pre-treatment and UF) are the most promising for NOM retention and prevention of UF membrane fouling. However, optimal operation of multi-process systems is challenging, especially due to the spatio-temporal variability of NOM. Hence, specific focus is given to in-situ, on-line NOM fouling monitoring techniques that can be integrated for dynamic plant operation.