Providing a spot-free rinse cycle in a car wash typically involves the installation of a reverse osmosis system (RO). The RO is designed to minimize total dissolved solids (TDS) from the feed stream. Dissolved solids can come from a variety of sources and are present in virtually all water supplies. The amount of TDS present determines whether or not you can provide a spot-free solution in your wash. The industry standard for spot-free rinse water is less than 40 ppm (parts per million) TDS. The product water after the RO system is predicated on the amount of TDS in the feed water to be treated. Most RO membranes can remove on average 98 percent of the dissolved solids in the water. Typical TDS in feed water from most city water supplies are less than 500 ppm. Therefore, the expected results after the RO system is approximately 10 ppm.
There are several membrane manufacturers and types of membranes available. The most widely used membrane in a car wash application is a thin film composite membrane (TFC). There are several types available including high-rejection, high-flow, low-fouling, and low-pressure configurations that can be applied depending on the feed water source. The RO manufacturer can provide guidance via a projection that can be done on the source water.
TIME TO CLEAN
All RO systems eventually will require cleaning. Cleaning is recommended when the RO shows evidence of fouling. The time to clean the membranes is either just prior to a long-term shutdown, or when prompted by deterioration of performance. Fouling characteristics that signal cleaning is necessary are a 10 percent to 15 percent decrease in permeate flow, a 10 percent to 15 percent decrease in permeate quality, or a 10 percent to 15 percent increase in pressure drop as measured between the feed and concentrate fl
RO cleaning frequency due to fouling will vary by site. A rough rule of thumb as to an acceptable cleaning frequency is once every three to 12 months. Having to clean membranes more often indicates something in the feed water source that should be corrected through pretreatment. Under normal circumstances RO membranes can last several years, and with periodic cleaning it is possible to maintain the performance of the membranes. Fouling can vary from site to site, depending on the foulant. Often complicating the situation is that more than one foulant can be present. Typical foulants are:
• Calcium carbonate scale
• Sulfate scale of calcium, barium, or strontium
• Metal oxides of iron, manganese, aluminum, etc.
• Silica scale
• Colloidal deposits (inorganic or mixed inorganic/organic)
• Organic material of natural or man-made origin
• Biological (bioslime, mold, or fungi)
There are a number of factors involved in selecting a suitable cleaning chemical (or chemicals) and proper cleaning protocol. The first time you have to perform a cleaning, it is recommended to contact the manufacturer of the equipment, the RO element manufacturer, or a RO specialty chemical supplier. Once the suspected foulants are identified, one or more cleaning chemicals will be recommended. These chemicals can be generic and available from a number of suppliers.
Generally in a car wash environment, a low-pH cleaning is used first to remove foulants like mineral scale, followed by a high-pH cleaning to remove organic material. Some cleaning solutions have detergents added to aid in the removal of heavy biological and organic debris, while others have a chelating agent like EDTA added to aid
in the removal of colloidal material, organic and biological material, and sulfate scale. An important thing to remember is that the improper selection of a cleaning chemical or the sequence of chemical introduction can make the foulant worse and render the cleaning ineffective.
Membrane cleaning can be accomplished with the membranes left on the RO system inside their housings. Most car washes use an RO system with fewer than six membranes in a single array. For car washes with larger RO systems, the design might have multiple arrays and each array should be cleaned separately. As mentioned, it would be best to use the instructions available from each membrane manufacturer. However, a brief description of the process follows:
1. Mix (using RO permeate) the cleaning chemical in a tank per the chemical manufacturers instructions (mix cleaner to obtain 2-3 pH for low-pH cleaning and 11-12 pH for high-pH cleaning).
2. Disconnect the first membrane on the RO system from the high-pressure pump and connect a hose from a chemical recirculation pump that is fed from the chemical mixing tank.
3. Put the permeate and concentrate lines from the RO into the chemical mixing tank to allow the chemical solution to circulate the membranes and back to the mixing tank.
4. Turn on the re-circulation pump.
5. Adjust the concentrate control valve, until the concentrate pressure gauge reads 50 psi.
6. Re-circulate the cleaner for a minimum of 60 minutes.
7. Dispose of the solution and re-fill tank with clean RO water.
8. Disconnect the feed line from the tank to the RO, and run the station to drain. This will rinse the pump and the line. Do not run pump dry.
9. Re-plumb RO to normal state and run for 10 minutes, dumping the permeate and concentrate to drain. This will ensure that all of the cleaner is flushed from the machine.
10. Put machine back on line.
Note: you may have to perform a low- and a high-pH cleaning to unfoul the membranes. For best results, start with a low-pH cleaning, then a high-pH cleaning. Rinse with low pH for five minutes and flush with clean water for 10 minutes.
In conclusion, replacing RO membranes can be expensive. With the proper pretreatment and a good maintenance schedule there is no reason not to expect several years of service from the membranes. Cleaning your own membranes is relatively inexpensive and will help to keep the costs down on the spot-free rinse solution for your wash.
Dwight Royal is CEO, vice president of operations of Con-Serv Manufacturing. To learn more about membrane cleaning, cleaning kits/systems, or cleaning chemicals, he can be reached at firstname.lastname@example.org.