by Bill Andresen, Water Resources Technician, Sask Water, Outlook, Saskatchewan
A home reverse osmosis system As one of the newest methods of water purification for improving the quality of drinking water. It is used principally for groundwater supplies and for municipality treated water supplies. The water supply must be potable. This method of treatment is not recommended for dugout water or other surface supplies which have not been municipally treated. It can substantially reduce dissolved minerals such as calcium, magnesium, sodium sulphates, nitrates, and many other materials. The result is cleaner, healthier drinking water.
Thus, the water moves through more rapidly resulting in a separation. The driving force is furnished by both the pressure and the concentration differentials across the thin layer. For water, the pressure effect is the most important, and for dissolved mineral ions the concentration difference is most important. Therefore, increases in pressure increase the water flux without a corresponding increase in salt flux. Water pressure must be maintained at 40 to 70 pounds per square inch to keep a driving force across the membrane. In some installations, a small booster pump may be required.
The central part of the reverse osmosis system is the module which is a pressurized container that houses the semi-permeable membrane. Several square feet of this membrane are arranged to form the module.
There are two types of membranes, one for chlorinated supplies and another for non-chlorinated supplies. It is important that the proper membrane is used. Chlorinated water can damage the membrane used for non-chlorinated water.
There are filters on either side of the module. One filter pretreats the feed water (5 micron) by removing tiny particles of suspended dirt and sediment and the other post treats the treated water (carbon) by removing taste and odour to give water a final "polish" prior to delivery. It is important that this filter is changed regularly. The water is then passed into a small pressure tank for storage until used at the tap.
Most reverse osmosis systems operate in the 75 to 80 percent recovery efficiency range. For example, this means that a unit operating at 75 percent recovery efficiency will produce 75 gallons of pure "product" water and 25 gallons of wastewater from 100 gallons of raw feed water. The wastewater cleans the membrane of dissolved solids. Optimum operation is obtained when the raw water temperature is 25¯C.
The water source to reverse osmosis units must not contain more than 10 grains of hardness (0.2 mg/L of iron and less than 0.25 mg/L of manganese). Pretreatment may be required. Do not use where water is microbiologically unsafe.
When choosing a reverse osmosis unit, you should look at what pretreatment is required and what the upper limit for total dissolved solids are for the unit to operate properly. Most reverse osmosis units require pretreatment to remove all manganese, iron and hardness minerals and generally have an upper total dissolved solids limit of approximately 1500 mg/L. Fine grain filtration may be required if suspended solids are a problem.
It is important to select a unit which is capable of producing a sufficient amount of treated water for your home. Residences using groundwater could benefit from the installation of a convenient, economical, reverse osmosis system to improve their drinking water quality.
