Membrane fouling is a complicated phenomenon and typically results from multiple causes. In spite of its complexity, electrostatic and hydrophobic/hydrophilic interactions that involve both membrane and fouling materials are recognized to have significant bearing, especially for the more-difficult-to-clean membrane fouling dominated by natural organic matter (NOM) and microbial activities. Electrostatic interactions occur among functional groups of membranes, fouling materials, and water primarily through dissociation, which strongly depend on the pH, ionic strength, and concentrations of multivalent cations in the solution. Hydrophobic/ hydrophilic interactions are functions of structure similarities between membranes and fouling materials, the types and density of functional groups on both membrane surfaces and fouling materials, and solubility of molecules of fouling materials. As analytical techniques and knowledge of structural details of NOM progress, the structure-solubility correlations for synthetic compounds can be extended to natural organic matter with modifications to provide a rough assessment on the hydrophobic nature of NOM.
Membrane chemical cleaning: from art to science
Membrane chemical cleaning is an integral part of operation for micro- and ultrafiltration systems in the water industry and has significant impact on process operations. However, this issue has not been adequately addressed primarily for two reasons: 1) cleaning protocols are typically recommended from membrane manufacturers and some cleaners are proprietary; 2) the issues of membrane fouling are poorly understood and related to site-specific water quality issues. As advances in aquatic chemistries and the analytical tools of membrane fouling occur, a broad understanding of the interactions between fouling materials and membranes, among fouling materials, and between fouling materials and cleaning chemicals becomes possible, although there still are many gaps in the understanding of the basic chemistry of fouling. This paper will present the basic chemistry of membrane fouling and cleaning, physical and chemical conditions that have significant impacts on cleaning efficiency, and the compatibility of membrane materials with different cleaning alternatives. Three case studies will be used as examples.