FilmTec Technical Manual - Chester Paul Company

Besides the above recommendations, general precautions must be taken during design and construction, such as:• Design with a minimum of crevices and dead ends.• Design the piping so that the flow velocity is above 5 ft/s (1.5 m/s). This promotes the forming and maintenance of thepassive film.• Use backing gas when welding in order to avoid the weld oxide film forming a base for crevice corrosion.• Pickle and passivate the pipe system as this gives the optimum safety against chloride attack.• Flush the plant with low TDS water before a shutdown period.3.15 System Design Considerations to Control Microbiological ActivityBiofouling is one of the most common and most severe problems in the operation of RO systems. It is particularly importantto control microbiological activity in plants using surface water or bacteria-contaminated water as a feed source. A properlydesigned system is a prerequisite for success:• If intermediate open basins or tanks are used, provisions should be made to allow for proper sanitization at that opensource and the part of the system downstream from it.• If intermediate sealed tanks are used, their air breathing or ventilation systems should be equipped with bacteriaretainingdevices (e.g. HEPA filters).• Blind, long pieces of piping should be avoided by design, and when unavoidable, should be periodically sanitized.• The components of the pretreatment system such as pipes, manifolds, filters and retention tanks should be opaque tosunlight to avoid enhancing the biological growth.• Stand-by devices with large surfaces, like sand or cartridge filters, should be avoided. If they are not avoidable, drainsshould be installed to allow discharge of the sanitization chemicals after the devices have been sanitized, and beforeconnecting them to the active system.• It should be made possible to physically isolate the RO/NF section from the pretreatment by using a flange. This allowsto use chlorine for sanitizing the pretreatment section while the membranes are protected from chlorine contact. A drainvalve should be installed at the lowest point close to the flange, to allow complete drainage of the chlorine solution.• Membrane selection: FilmTec offers membranes with a special surface that makes them more resistant againstbiofouling. These so-called BW30FR elements are typically selected for surface waters and tertiary effluent treatment.Other special element types are made with a full-fit configuration for applications requiring sanitary grade membraneelements. The full-fit configuration minimizes stagnant areas and complies with FDA standards. See also Section 1,Basics of Reverse Osmosis and Nanofiltration, and the relevant Product Information Sheets.3.16 System Design Suggestions for Troubleshooting SuccessWhen considering the design and purchase of large membrane systems, there are a number of possible equipment designsand additions that can be added to help with future troubleshooting. Depending on the size and complexity of the system,some or all of these suggestions could be discussed with your system supplier. None are required for successful operation,but all make day-to-day operation and troubleshooting easier, quicker and more effective.Access to Load and Troubleshoot the SystemEven though membrane systems are quite compact, there is often a temptation to save even more floor space by crowdingequipment, pipelines and supports so close together as to limit access to the membrane system. Ideally, one should haveunrestricted access to both the feed and brine end of each and every vessel for loading, unloading and troubleshooting ofthe membrane elements. When loading, at least the length of one element is necessary between the feed end of the vesselsand the nearest equipment or supports. When unloading, often more room could be used so that a large wooden board orsome other device can be used to push the element stack toward the brine end.Connections Allowing Probing, Profiling and SamplingWhen troubleshooting potential element problems, one of the first operations is always to attempt to localize any problem,either to a stage, a vessel, or even to an element. Having sample points on all the vessel permeates greatly facilitates theseoperations. Ideally, the permeate sample points can allow a probe tube to be passed through. Having additional samplepoints on the feed, the concentrate and any interstage headers, can help localize problems to a stage and even allow formass balance measurements to corroborate flow measurements and ultimately setting of the system recovery.Page 99 of 180 ® Trademark of The Dow Chemical Company ("Dow") or an affiliated company of Dow Form No. 609-00071