MEMBRANE CLEANING: FROM THEORY TO PRACTICE
Flexible cleaning regimes promote membrane permeability
Publication Date (Web): 15 May 2017
Low-pressure membranes (microfiltration and ultrafiltration) are increasingly replacing conventional water treatment processes as the result of more stringent regulations and greatly improved competitive pricing. It has been recognised that the most common operating issue of membrane plant operation is controlling membrane fouling. Although the impact of fouling may be mitigated via the modification of the pretreatment process, as inefficient cleaning can affect the extent of irreversible permeability loss, leading to the premature replacement of membranes.
Membrane fouling is fundamentally the result of interfacial interactions between membrane and fouling substances, and between different constituents of fouling materials. Those interfacial interactions primarily are electrostatic, hydrogen bonds, steric, van der Waals, and hydrophobic / hydrophilic in nature. The sum of those interactions would determine if attraction or repulsion forces dominate, which may be described by models such as extended Derjaguin–Landau–Verwey–Overbeek (DLVO) theory as feasible theoretical frameworks for characterising the membrane fouling phenomenon. Although research in this aspect has been limited, better understanding of the nature of fouling can help to improve cleaning efficiency and develop new cleaning techniques. Another area of interests relevant to membrane cleaning is the structure of the fouling layer, which is largely related to the time-dependent mass transport and kinetics of fouling layer formation. A stratified fouling layer structure may lead to sequential approach of membrane cleaning.
Membrane fouling is a complex phenomenon and typically results from multiple causes. As the fouling progresses, the interactions between membrane and foulants are soon replaced by foulant-foulant interactions and surface characteristics of the membrane would no longer play important roles. This is illustrated best in a comparison of fouling in a polymer – membrane system and a polymer – primary coagulant – membrane system. The fouling trend in the polymer – membrane system could be predicted qualitatively well based on electrostatic interaction. However, the addition of a primary coagulant into the system changed the dynamics of fouling by forming a cake layer and rendered the membrane charge characteristics no longer relevant.
Membrane cleaning study with various cleaning regimes provides a practical means to elucidate the nature of fouling and to devise effective cleaning regimes. By correlating the recovery in membrane permeability to specific cleaning steps, as well as the quantity of contaminants on the membranes’ surfaces, the cause for fouling could be elucidated and the most effective cleaning regimes could be determined. Depending on the temporal and spatial variations in the quantity and quality of fouling materials encountered by the membranes, and on the history of membrane cleaning, fouling can be dynamic and influenced by multiple factors. Reevaluation of cleaning regime is necessary as the nature of membrane fouling may change over time. For the fouling layers with a stratified structure, the cleaning sequence can affect the efficacy of the cleaning. Several case studies are selected to illustrate the complexity of membrane fouling in real world applications.
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