Reverse Osmosis, Part II: Importance of a pilot study in system design

This is the second part of a five-part series on the design, operation, and maintenance of reverse osmosis systems for powerplants compiled by Wes Byrne, U.S. Water’s consultant on membrane technologies. Parts I, III, IV, and V are identified below; the final three segments will appear in upcoming issues of CCJ ONsite.

An RO system and its pretreatment equipment designed solely on one water analysis may not be fully optimized for the fouling characteristics of the source. It might be oversized or, of greater concern, it might not be ideal for water that has a high membrane fouling potential. This can best be determined with a pilot study.

A well-designed pilot study will use components that have been scaled down but still offer the same type of media and use similar flow velocities and exposure times. The pilot RO should duplicate the permeate recovery, the permeate flux rate (that is, permeate flow per unit of membrane area) and concentrate stream vessel exit velocities, along with the scale inhibitor dosage and shutdown flush methods.

When the pretreatment methods are piloted along with the RO, the system operation can be adjusted to minimize the rate of RO membrane fouling, such as by modifying the permeate flux rate or the rate at which water passes across the membrane surface and through the membrane elements. With the right equipment choices and sizing, it might even be possible to eliminate membrane fouling, which could then dramatically reduce operating cost and maximize membrane life.

The choice of membrane might also be evaluated. With large systems, demonstrating that a low-fouling membrane element performs better than a standard element will help justify its higher cost. Low-energy elements might be evaluated for their potential to reduce pump size and associated power consumption.

The pilot study also offers an opportunity to learn more specifically about what will foul the RO system. A membrane element from the pilot study might be pulled and autopsied. Analysis of the solids then makes it possible to choose cleaning solutions that are best suited for removing the particular fouling materials. The effectiveness of the solutions and cleaning methodology might then be verified with the pilot unit.

The longer the pilot system can be operated, the more information will be gained. A minimum of several months is recommended.

Upstream equipment. The success of a new RO membrane system is often directly related to its pretreatment. Piloting the upstream processes can be challenging in sizing these components for the pilot’s low flow rate.

The most important role played by pretreatment is protecting the RO from incompatible substances. With the polyamide thin-film RO membrane commonly used today, the biggest concern is removal or destruction of any chlorine or other potentially oxidative compounds. This membrane has very little tolerance to free chlorine (present in many municipal water sources), and is only slightly tolerant of chloramines (in other municipal water sources).

The two most common methods for breaking down chlorine are reducing-agent injection and activated carbon filtration. The most common reducing agent is sodium bisulfite (NaHSO3) which will react preferentially with free chlorine in breaking it down to the innocuous chloride ion.

Sodium bisulfite/sulfite injection systems can fail in ways that will degrade RO membrane elements if not quickly remedied. Examples: The day tank could run out of solution, or the injection pump could lose power or pump-head prime. The injection pump setting might provide insufficient chemical to handle the full range of chlorine concentrations, or might be set for such a low pulse speed that the chemical does not sufficiently mix with the feedwater.

The proper sodium bisulfite dosage should be injected any time that the RO inlet valve opens, even if this opening is for filling the RO or for flushing it out before a shutdown. There cannot be significant pipe length distance between the point of injection and the RO inlet valve, since this length will become fully chlorinated during shutdown by chlorine diffusion. The point of sodium bisulfite injection should be immediately upstream of the inlet isolation valve.           

Activated carbon filtration may offer a more reliable means of breaking down chlorine. During manufacture, non-carbonaceous materials are burnt off, leaving porous granules with a substantial amount of pure carbon surface area. This has a high attraction for adsorbing almost any contaminant, including most organic materials and heavy metals, although there may be limited removal capacity for some contaminants that are shed into the effluent water.

The breakdown of chlorine by activated carbon involves an electrochemical reaction, which offers a high capacity for chlorine removal. The carbon gives up electrons to the chlorine atoms, forming innocuous chloride ions (Cl) that remain in the water. In this reaction, oxygen atoms previously bonded with the chlorine atoms as hypochlorous acid (HOCl) now attach to the carbon surface. Because the carbon will also react with dissolved oxygen in the water, the carbon surface can become fully oxygenated. It will then lose its ability to remove additional chlorine, but this typically takes a few years with inlet chlorine concentrations less than 1 mg/L.

When ammonia is present naturally or when added by a municipality, chlorine will chemically bond with the ammonia to form monochloramine (NH2Cl) or possibly dichloramine (NHCl2). The chloramines are not as chemically reactive so require more carbon volume for their breakdown. A catalyzed carbon media is available at an increased cost that improves the carbon reactivity with chloramines and reduces the need for oversizing the carbon filters.

Carbon system valves cannot leak or otherwise bypass. They should be normally closed and driven by sufficient air pressure to not allow chlorinated water to reach the RO system.

Maintenance is critical to the success of either reducing-agent injection or carbon filtration. It is recommended that the activated carbon media be replaced annually or based on an increase in its effluent concentration of TOC, to prevent the shedding of biological particles into the RO system.

Over-injection of sulfite will cause increased breakdown of dissolved oxygen in the water. This will increase the potential for heavy growth of slime-forming species of bacteria that can quickly foul an RO system if there is a sufficient concentration of organic food in the water source. This potential can be minimized by maintaining a residual sulfite concentration that is greater than zero but less than 2 mg/L as sodium sulfite, measured using a low-level test with sensitivity of 1 mg/L or less. As long as the sulfite concentration is greater than zero and it is well mixed into the feedwater, free chlorine will not be present.

 

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