Membrane cleaning

An important technique for membrane regeneration is chemical cleaning of the fouled membrane. Membrane fouling can be prevented by feedwater pretreatment but it cannot be completely avoided. Fouling at the membrane surface is often controlled by employing periodic cleaning processes. The objective of these cleaning processes is to restore the original pore size distribution. This cannot always be achieved if cleaning cycles are delayed or in cases where fouling is severe. However, periodic cleaning of the desalination membranes is critical to the performance of wastewater treatment plants. Cost associated with the membrane cleaning constitutes 5-20% of the operating cost of the RO process [17]. Efficiency of cleaning generally depends on the type of cleaning agent and its concentration. It also depends on the understanding of specific interactions occurring between cleaning agents and membrane foulants. The basis for choosing a cleaning agent is the type of the foulant deposited on the membrane surface. Various kinds of agents such as acid, alkaline, surfactants such as sodium dodecyl sulfate (SDS), and commercial detergents are used for RO membrane cleaning depending on the foulant nature. In the case of fouling caused by calcium phosphate and calcium silicate, acids were the weakest cleaning agents. However, alkali had a moderate effect and the combination of chelating agents and surfactants with alkali (EDTA+SDS+NaOH) provided the best cleaning efficiency [18]. Similar results were reported in the cleaning of organic-fouled RO membranes. Fouling caused by organic feed solutions containing alginate and natural organic matter (NOM) can be effectively cleaned by either EDTA or SDS by optimizing chemical (concentration and pH) and physical (time, CFV, and temperature) conditions during cleaning [19]. Salt cleaning can be used for the cleaning of RO membranes fouled by gel forming hydrophilic organics [20]. It is assumed that cleaning agents diffuse into the deposited fouling cake layer on the membrane surface [18]. Diffusion rate depends on different factors including turbulence, shear factor, and concentration of the cleaning agent. Various studies indicated the occurrence of a reaction between cleaning agent and foulants present in the cake layer leads to the removal of fouling matter from the membrane surface [18,20]. These reactions could be hydrolysis, dissolution, or dispersion depending on the nature of foulants.

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