Water. desalination + reuse

TECHNOLOGY | 44 | Desalination & Water Reuse | February-March 2014 cleaning process. With the possibility of membrane damage and highly questionable results, the use of air remains a significant liability for the end-user and chemical supplier. HYdrauLiCs Of rO mEmbraNE sYsTEm CLEaNiNG In the early days of membrane technology, the mechanisms of fouling and scaling of membranes were not well understood. At that time, it was not uncommon for equipment manufacturers and end-users to conduct "maintenance cleans" where all of the stages in the system were cleaned at once. In fact, some systems were designed so that it was difficult or impossible to do otherwise. We now understand that scaling and fouling can occur in a specific order and in specific stages of the system. Since the nature of the foulants is often different between stages, cleaning each stage independently allows the use of different cleaners for different foulants, providing better overall results. In addition, recirculation rates of cleaning solutions should be high and the pressure low to avoid re-deposition of foulants through the membranes. If operators attempt to clean all of the stages at once, the pressure drop will exceed the maximum cleaning pressure guidelines and ensure foulant re-deposition. Hydraulic conditions reference the flow rate through elements in order to create a quality clean. As a standard, an 8-in diameter element cleaning flow should be 9 m 3 /h and a 4-in element should be 2.4 m 3 /h. An optimal cleaning system design should allow these flow rates. When cleaning spiral-wound membranes, it is best to maintain flow rates at the lowest possible pressure to minimize permeation and reduce the convective force that holds foulants onto the membrane surface. In general, it is important not to exceed 4 bar. ruLEs TO maximizE mEmbraNE CLEaNiNG • Use proper chemical dilutions - when diluting cleaners, take into consideration the total volume of water in the system including: tanks, pipes, hoses and pressure-vessel water volumes. • Always clean membranes in parallel – avoid cleaning multiple stages during cleaning. The pressures and cleaning flow rates recommended by the manufacturer are for single-stage cleaning. Cleaning multiple stages with one cleaning circulation can also push foulant from one stage to another resulting in a shift of fouling that will result in an ineffective cleaning. • Heat the cleaning solution to the maximum temperature allowed by the membrane manufacturer. • Ensure that the cleaning solution return line is below the level of cleaning solution tank volume to prevent air being entrained into the cleaning solution, which would cause excessive foaming and reduce the effectiveness of the clean. • When cleaning with low and high pH follow the order of pH which is best suited for the identified foulant. • Re circulate the cleaning solution for a minimum of 45 minutes. CONCLusiONs By identifying the foulant present and using targeted chemistries, an RO membrane system has the best chance of achieving optimum cleaning results when these factors are combined with the recommended guidelines for cleaning. Combining time proven basics of membrane cleaning, and state of the art membrane cleaning chemistries will give each end-user improved success in cleaning effectiveness.l Oxidized manganese (green) above a layer of calcium sulfate scale (orange) fouling a membrane surface

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