Generally cleaning is done once or twice a year, but more often if the feedwater is problematic. The specific cleaning procedure is a function of the feedwater chemistry, the type of membrane used, and the type of fouling encountered. A typical cleaning regimen consists of flushing the membrane modules byre-circulating the cleaning solution at high speed through the module, followed by a soaking period, and a second flux, and so on. The chemical cleaning agents commonly used are acids, alkalis, chelatants, detergents, formulated products and sterilizers.
Acid cleaning agents such as hydrochloric, phosphoric or citric acids effectively remove common scaling compounds. With cellulose acetate membranes, the pH of the solution should not go below 2.0, or else hydrolysis of the membrane will occur. Oxalic acid is particularly effective for removing iron deposits. Citric acid is not very effective with calcium, magnesium or barium sulfate; in this case a chelatant such as ethylenediamine tetracetic acid (EDTA) may be used.
To remove bacteria, silt, or precipitates from the membrane, alkalis combined with surfactant cleaners are often used. Detergents containing enzyme additives are useful for removing biofoulants and some organic foulants.
Sterilization of a membrane system is also required to control bacterial growth. For cellulose acetate membranes, chlorination of the feedwater is sufficient to control bacteria. Polyamide and interfacial composite membranes do not normally require sterilization, as they are usually fairly resistant to biological attack. Periodic shock disinfection using formaldehyde or peroxide is usually enough to prevent biofouling.
Repeated cleaning gradually degrades reverse osmosis membranes. As membranes approach the end of their useful life, the water flux will normally have dropped by at least 20% and the salt rejection will have begun to fall.
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