Water. desalination + reuse

technology Soaking ���dead��� membranes brings surprising results _________ Umar Farooq, technical manager, Water Regime (Pvt) Ltd, and Sarwar Shaikh, production manager, Al-Abbs Suger Mills Ltd, Pakistan ___ editor���s note: Restoring the flux to reverseosmosis membranes damaged by clay deposition is a tricky procedure. the authors provide a case study of various different remedies that were tried before they found the right one. ReVeRSe-oSMoSIS (Ro) desalination membranes in a sugar plant in Pakistan had been badly damaged by the deposition of clay, which is very difficult to remove, leaving the problem of how to restore the flux of the ���dead��� membranes. Various attempts were made to establish effective removal of clay by chemical cleaning procedures using conventional and non-conventional cleaning chemicals. Membranes are characterized by their unique properties of high water-permeation, very low saltpassage and dimensional and chemical stability. In spite of their small sizes and high solubility in water, salts, even those with small molecular weights, do not pass through the membrane at a significant rate. Their passage is held to a very low level, eg, less than 1% using thin-film composite (TFC) membranes. Passage of the larger size colloidal and other suspended solid particles is not permitted at all through the membrane���s closed structure. This is also true of microorganisms such as bacteria, which, when present in the feed, will be trapped on the membrane surface causing it to foul. Membrane fouling is also introduced by the presence in the feed of scaling and corrosion products. The presence of any of the above matters in the feed could cause membrane fouling and the lowering of the RO plant���s efficiency. Raw wateR qUality For this reason, in water desalination by RO, process feed water pretreatment is essential in order to remove all the potential membrane foulants from the feed. The degree of pretreatment, however, is dependent on the raw water quality, particularly its content of suspended and biological matter as well as on the membrane | 34 | Desalination & Water Reuse | February-March 2012 configuration. Water quality tends to be site-dependent. Use of highly turbid canal water as raw water for RO membranes is a big problem in water treatment. It is very difficult to handle this aggressive water through conventional pretreatment. Surface waters (river, sea, canal and lake) are enriched with high turbidity, clay (alumino-silicates), inorganic and organic silt. All these parameters dangerously affect RO membranes. Because of its particle size <5 microns, it is very difficult to prevent clay passing using simple filtration, which is the basic cause of fouling the membranes at this site and decreasing the flux. Conventional prefiltration does not have the ability to behave as a barrier for those parameters. Many plants cease operation because of this problem. Turbidity (measured through SDI) and organics cause membrane blockage, so that membranes required a high cleaning frequency monthly or sometimes on a weekly basis. MeMbRane cleaning: aRt oR Science? The cleaning of membranes is not a defined formula, not a rule of thumb and not 2+2 = 4. Chemical cleaning of membranes is an art, which results in different, sometimes amazing, behavior and results, in different styles, in different places and at different times. The more you practice the better you get. Cleaning chemicals and the cleaning process play the major role in restoring the flux of membranes. If the cleaning chemical is not appropriate, then the membrane can be damaged or not work as desired. The cleaning procedure also has a very effective and prominent role in cleaning the membrane and getting optimal results. Consideration must be given to: l Sufficient quantity of solution l Proper ratio of solvent and solute l Selection of cleaning chemical l Maintenance of parameters like pH, pressure, temperature, flow etc l Proper designing of CIP system (pump, filtration, piping).

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