Science Information

Reverse Osmosis and Deionized Water Filtration


Let's face it if you are in a business needing de-ionized water or reverse osmosis water you are always concerned about the quality of your water. I know as a mobile detailing business that when we have ultra clean water in our tanks we are a happy camper. We know it will make your job so much easier. We also like the idea and concept of spot free water. Have you ever considered how Reverse Osmosis really works? Let me describe the process from my industries perspective.

In R.O. water pressure forces water through a fine membrane that filters out between 85-95% of the dissolved solids in the water. Water lacking these solids won't water spot a car. It's important to have soft water filter inline the R.O. system prior to R.O. Some systems have a water softener/carbon filter/R.O. system in that order. This saves life on the R.O. membranes. R.O. units are measured in gallons per day (GPD) because there flow is relatively slow. Usually they run twenty-four hours a day and the water is put into a storage tank to be used later. Small R.O. units that cost between $2,500 and $5,000 put out 4 gpm. To go to 10 gpm, the speed at which you'd want to fill your tank, the cost of those units starts at $25,000.

Car washes need units that make between 500 to 1000 gallons a day for the rinse cyles. A small unit will work because 4 GPM (gallons per minute) X 60 minutes is 240 gallons. So, in four hours at night when the car wash isn't being used, they will have one thousand plus gallons. The only problem for them is that a thousand gallon tank is five feet wide and eight feet tall.

An R.O. machine works best when it's running clean water through it. Otherwise, the membranes let nature take over. Then you have not reverse osmosis but osmosis. It's better to have a small machine running twenty-four hours a day than a big machine running two hours and then shutting off. R.O. membranes should have a constant positive flow through them. Some industry leaders believe twelve to thirteen hours is the maximum you should run an R.O. unit a day. They think the machine will last longer.

Silica, iron, aluminum and bacteria cause irreversible damage to membranes and that's why pre-filters are a must to keep membranes from fouling. Fouling results when particles suspended in the feed water are deposited within the R.O. unit. Fouling reduces permeate flow rate. Bacteria fouling usually doesn't cause the permeate conductivity to increase until your system is extremely plugged. Other particles may cause permeate conductivity to increase.

Most biological and particulate silica foulants can be removed with the proper chemicals. Bio-fouling is a problem. Pre-treatment may have little impact on R.O. unit fouling. Simply killing the bacteria won't work because the same number of fouling particles (living or dead) still enter the R.O. unit. The best way to handle bio-fouling is cleaning the equipment and the upstream pipes and filters. Chlorinated water will be bad and should be un-chlorinated before entering the R.O. unit. Fouling by non-living particles is handled by your pre-filters, or additional sediment filters or cartridge filters with a nominal filtration rating of five microns or less. This can be complicated stuff and you can call your franchsior for a telephone number of a fair and honest local or regional water purification consultant.

Chlorine is the most common agent to chemically attack and destroy the polyamide thin film membranes that are commonly used in car washes. Activated carbon blocks are used generally in filters upstream from R.O. units. Larger units use granulated carbon filters or other methods. Active chlorine consuming sites on the activated carbon material is depleted over time and eventually dechlorination will diminish and finally cease. This will allow chlorine to damage downstream chlorine sensitive membranes in your R.O. unit. Carbon filters must be replaced as frequently as their manufacturers specify.

Most commercial R.O. units are too small to justify installing instrumentation to determine when to clean them. Cleaning periodically can remove most scalants and foulants. If you wait too long permanent damage will be done and proper cures will not work. If you wait until you have low flow problems, the elements may be plugged significantly and chemical cleaning may be ineffective or may take many hours to get the elements back to near original performance.

If you want, you can have the R.O. dealer do periodic cleaning on a service contract. It may be worth your while. Expect to pay between $80 to $160 per quarter. Watch for parts price gouging. Some companies are like sharks. If well taken care of, the membranes will last years. You may want to have a technician also check upstream filters. That way, if anything does go wrong, it's their responsibility and you can make them pay. Make sure your contract is tied to their lifetime warrantee. Regular Maintenance Of Reverse Osmosis Units is ultra important and you should change your prefilters often. Water softeners every month, carbon filters quarterly. Be sure to check the pressures weekly and membranes will need changing every so often too. Think on this.

"Lance Winslow" - If you have innovative thoughts and unique perspectives, come think with Lance; www.WorldThinkTank.net/wttbbs


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