Reverse Osmosis (RO) is aptly named. It actually reverses the natural osmotic process by using pressure to force pure water through a porous membrane. The membrane's pores are sized such that they allow pure water to pass through, while rejecting the contaminants in the water at up to 99% efficiency. In actual situations, the rates of rejection can vary from about 85% to 99% for various contaminants, based on the molecular weight or size of the contaminant and the operating conditions, which include pressure and temperature.
On the average, it is very safe to say that the RO pretreatment system removes 90-95% of the contaminants found in the incoming tap water. The net result is, that whatever water purification technology follows the RO pretreatment system (i.e., a Type I, Type II or Type III DI system), its operating cost for DI modules will be reduced by 90-95% when compared to not having an RO pretreatment system.
To justify the added cost of the RO system, calculate the total capital and operating costs for the DI lab water purification system, with and without the RO over 2-3 years. RO pretreatment should also be used when the DI system alone cannot provide the required water quality. RO should be considered whenever the incoming tap water contains more than 170 parts per million of total dissolved solids, and/or usage on a Type I DI system exceeds 20 liters per day, and/or when usage on a Type II DI system exceeds 40 liters per day.
