Water. desalination + reuse

February-March 2014 | Desalination & Water Reuse | 41 | TECHNOLOGY _________ Lee Durham, Dan Gibson and Dan Freeman, Avista Technologies Inc, USA ___ Editor's Note: While the ultimate goal must be self-cleaning membrane systems, until that golden age is reached for systems large and small, the questions will continue to arise about what the best chemical cleaners are to do the job. With today's sophisticated systems, few top engineers or chemists believe that generic chemicals will suffice. But how to choose exactly the formulation required to do the job? MEMBRANE CLEANING technologies and chemistries have evolved well beyond the use of generic chemical cleaning. The use of generic chemicals has technical pitfalls that can complicate membrane system cleaning and in some cases create new foulants that did not exist before the generic chemicals were applied. RO membrane systems concentrate soluble minerals, microbes and organic matter that will eventually lead to membrane fouling. Cleaning fouled membranes is necessary to restore permeate flow effectively and reduce differential pressures so that treatment facilities continue to produce the desired water quantity and quality. The use of formulated cleaning chemicals combined with fundamental guidelines and foulant knowledge can reduce the time required to clean a system, and extend membrane life, thereby saving monies over the life of a membrane system. This article will identify important considerations and focus on proven fundamentals of membrane system cleaning. Generic cLeaners vs FormuLateD cLeaners Today many formulated cleaners and their application techniques are developed and tested using proven science to ensure high performance and compatibility with membranes. When formulated and applied correctly, these products rely on a team of ingredients that work together synergistically, including designated chemical components that address a specific foulant and serve a specific purpose. An attribute of properly formulated cleaners is a strong buffering capacity to help avoid pH changes during cleaning. Adverse co-precipitates will not form when pH is stable. tarGet chemistries For FouLants As a result of the early generic cleaners, system users became accustomed to using only one high pH and one low pH cleaner, primarily because this was all that was available. This one-size-fits-all approach endured when early specialty cleaners were developed and, even today, some suppliers have a very limited cleaner selection. When deciding how to begin the task of identifying the source and cause of any membrane fouling problem, the simple high pH/low pH approach with generic cleaning chemicals will not be sufficient in most cases. It is better to start with experienced laboratory results to identify the foulant. A membrane autopsy would be the best method of identifying membrane surface fouling. In lieu of a membrane autopsy, a detailed system profile could be performed and combined with a feedwater and cartridge filter analysis. This up-front work will help narrow down the selection of specialty cleaners. Today, specialty cleaners are specifically formulated for a variety of targeted foulants and scales. The results of this targeted approach are more effective cleaners and reduced cleaning frequency. These cleaners focus on specific foulants such as organics, metals, silts, sulfate and silica scales. Because of the complexity of foulants found in today's applications, the most successful cleaners contain several components that act synergistically. Including buffers, surfactants, builders and dispersants in one single cleaner is not uncommon and is actually preferred for the more challenging membrane system problems. BuFFereD memBrane cLeaners A low pH chemical like citric acid tends to dissolve metals like iron, aluminum and calcium carbonates. Membrane Cleaning Facts, Considerations and Guidelines

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