74 r!�RTICLE With worker safety a top priority, RO further benefits from the elimination of the need far strong acid and alkali materials (e.g. sulphuric acid and caustic soda) that are staple consumables in ion-exchange plants. t Other technologies such as nanofiltration (NF) and electrodialysis reversal (EDR) should be carefully considered. Both can improve project economics especially when dealing with more saline feed waters, as they act as "roughing" demineralisers. Nanofiltration can comfartably handle high TOS concentrations but at much lower operating pressures, saving on operating costs, and allowing the downstream RO system to assume a more polishing role. EDR plants alsa operate successfully in Power plant around the world in various roles, that include the treatment of cooling tower blowdown and RO concentrate, and can play an important role in zero discharge and other water reuse applications. Reverse osmosis is undoubtedly the technology of choice far Figııre J. Coınpetiııg Technologies (lon-Exchange, /eji; RO, righı). new Power plants. lf you select RO, here is some very important advice far owners: Have a bid specification far all OEMs that covers the fallowing points, so that bids can be compared on an "apples to apples" basis: ❖ Ensure that the RO system is designed with a permeate flux of 12-16 GFD (gallons/ft2/day) far raw waters that undergo the standard pre-treatment sequence of clarification/sand filtration, and generally have < 5 NTU turbidity. Some unethical equipment vendors, in a competitive bid situation, will design a unit at much higher values, far example at 20-24 GFD. This greatly reduces the size of the plan!, making it cheaper and likely to win the bid. What the owner gets is a RO system that is highly prone to all types of fauling, risking costs, and operator frustration. ❖ lf at all possible, avoid 400 ft2 and higher RO membrane elements (far 8 inch diameter elements). lnsist on either 365 or even 350 ft2. Since the diameter is fixed, higher membrane surface area simply means a tighter wrap, or a smaller brine channel spacer, and more fauling. ❖ Most RO systems are designed far a standard 25° C ENERJi DÜNYASI EKiM 2006 feed water temperature. Microbiological activity increases rapidly above 15-18° C. Ensure your bid specification asks far 15° C (no higher than 18° C). This will result in a slightly larger plant (~ 25%) but it will be money well spent. in colder climates, vendors will include a heat exchanger to preheat the feed to 25° C. Reject this and rather spend the money on more RO membrane capacity. ❖ Ensure provision is made in the design far cleaning each stage individually. This will require isolating valves between stages. This is critical to the effective maintenance of the units. ❖ Adequate instrumentation is critical far monitoring a RO plant. Flow indicators, pressure gauges (including an often missing inter-stage gauge), analyzers, ete, should be specified in the tender document. ❖ One common problem is the inadequacy of many Clean-ln-Place (CiP) systems. These are generally skid-mounted and are used in the cleaning of the RO plant. Must-haves on a CiP system: adequately sized cleaning solution tank and heating element (capable of heating solution to 35 degrees C), a flow indicator, sufficient pressure gauges (befare/after CiP cartridge filter, after CiP pump) and a properly sized pump. ❖ Specify variable frequency drives far the main RO high-pressure feed pumps. These help avoid water hammer that is detrimental to the plant, and causes poor quality permeate. ❖ Ensure plant layout avoids equipment crowding that may limit access to the RO system. ldeally, unrestricted access to both feed and concentrate end of each and ever vessel should be available far loadin and troubleshooting of the RO elements. ❖ Request a PLC feature that allows far periodic permeate flushing of the system. This is of critical importance in systems that will see significant periods of downtime. Permeate flushing is done by recycling permeate through the entire system at.a high rate to flush debris. A 5-minute flush every couple of idle hours will suffice.
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