Water. desalination + reuse

TECHNOLOGY | 42 | Desalination & Water Reuse | February-March 2014 A high pH chemical like caustic soda is generally used to remove organic and colloidal materials, clay, bacteria and fungi. Years ago, it was common practice to spike the pH of generic cleaners in the hopes of making them more effective, believing that "more was better". However we now understand that a pH too low or too high can damage the membrane. Spiking cleaners has also been applied unsuccessfully to specialty cleaners and, in these cases the user may unintentionally sabotage themselves. For example, the hydroxide ion added by caustic soda competes with dispersants already present in many formulated cleaners. Since dispersants are much more effective, competition with the hydroxide ion for sites present in the foulant can reduce overall cleaning performance. The membrane manufacturers' limits for cleaning pH and temperature should always be confirmed and maintained, as operating outside of these ranges can cause irreparable membrane damage and severely reduce productive membrane life. It is also important to remember that measuring Sulfuric acid was used to clean calcium carbonate from a membrane surface which co-precipitated calcium sulfate. The prismatic crystals in the center and forefront of the image are calcium sulfate crystals. the pH at extreme low and high levels can be very difficult and the accuracy will be questionable. CHELaTiNG aNd CONTaCT TimE – SHOrTCuTS CaN BE HarmfuL In the 1980s, air was introduced into the cleaning process to generate turbulence. The laboratory theory at the time was that air bubbles could help dislodge and carry away the foulants, especially within the feed spacer in cases of heavy organic fouling. This method was soon abandoned after extensive testing on full sized systems determined that using air during cleaning simply didn't work as previously thought and could even damage the membrane. The best cleaners work through wetting, chelating, contact time and sequestration of metals that prevent precipitation. However, the presence of air will cause metals such as iron and manganese to convert from +2 valence to +3 valence and thus precipitate them as metal oxides. It is well known that today's most effective membrane cleaners are formulated specifically to wet the foulant and to disperse particles. Modern cleaning chemical formulations include chelants. A chelant is a specialized molecule designed to bind to positively charged metal ions, most commonly calcium and magnesium, in solution. Chelants prevent these minerals from forming insoluble precipitates on the membrane surface. The presence of air bubbles can decrease contact time and chelant effectiveness. Additional problems that may arise with the use of air during cleaning include: • Excessive foaming • Insufficient fluid flow to carry foulants from subsequent elements in a system • Problems with air lock from system configuration • Mechanical damage from unintentional water hammer • Balancing air and water pressure during cleaning • Creating on-site bacterial aerosols Air can also physically redistribute particles from the membrane surface, causing feed spacer fouling during the

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