Reverse Osmosis Water Treatment

Read our Home Water Treatment Instructions First

Reverse osmosis is becoming a common home treatment method for contaminated drinking water. Reverse osmosis is best known for its use in desalination, the process of turning sea water into drinking water. However, it is also effective for treating water quality problems in the home. Reverse osmosis can reduce the amounts of organics, inorganics, bacteria and particulates that can be found in contaminated drinking water. Because the efficiency of removal of various contaminants can vary, home owners should evaluate this when considering using reverse osmosis for home treatment Other home treatment methods may be better for specific contaminant.

Reverse osmosis is based on the process of osmosis. Osmosis involves the selective movement of water from one side of a membrane (a plastic film that looks similar to cellophane) to the other. To make the process work, pressure is applied to the contaminated water, forcing water through the membrane. Since contaminants do not move with the water as it moves across the membrane, purer water collects on the other side of the membrane. The purified water that accumulates on one side of the membrane can then be used or stored. A specific amount of pressure is necessary to separate purified water and contaminants. This required pressure is based on the type and concentration of contaminants in the water. Supplying even more pressure to the contaminated water than is required provides better separation and a higher production rate.

The levels of most dissolved compounds and suspended matter present in water can be reduced by reverse osmosis treatment. However, not all compounds can be efficiently removed by this process. The efficiency with which membranes reject the contaminant molecules depends on the pollutant concentration and chemical properties of the pollutant. Membrane type and operating conditions will also affect the degree of pollutant removal.

Efficiency of removal is often described using the term "rejection percentage," which is the percent of a particular contaminant that doesn't cross the membrane, i.e., is rejected by the membrane however, rejection percentages do not tell the whole story. For example, the rejection percentage for nitrate can be as high as 90 percent with some systems, indicating the membrane is highly efficient in rejecting nitrate. However, for an incoming nitrate concentration of 110 milligrams per liter (mg/l), an unrealistically high level, 90 percent removal would still leave 10 percent of the nitrate in the purified water, or 11 mg/l. This is greater than the 10 mg/l maximum contaminant level for nitrate allowed in drinking water supplies. It is important to know not only rejection percentages, but also incoming pollutant concentrations to effectively reduce contaminant concentrations in the drinking water to safe levels. Basic components of an reverse osmosis system should include a pre-filter to remove fouling agents such as rust and lime; a reverse osmosis module containing the membrane; an activated carbon post-filter to remove residual taste, odor and some compounds from the purified water; a storage tank; and various valves, including a shut-off valve that stops the water flow when the storage tank is full. The system must also provide for waste flow to drains. Pre-filters containing activated carbon are commonly used to protect chlorine-sensitive membranes. All of these components can be purchased from your dealer.

To continually perform well, reverse osmosis systems, like all other home water treatment devices, require regular maintenance and replacement of various components. Pre-filters and post-filters need to be re-placed on a regular basis. The length of time between changing pre-filters will depend on the water quality, especially the concentration of solids. The contaminant concentration, membrane rejection percentages, and efficiency of activated carbon removal determine when post-filters should be replaced. reverse osmosis membranes should typically last for one to three years, depending on operating conditions, membrane type and pre-filter performance.

A particularly major disadvantage of reverse osmosis is the large amount of contaminated wastewater generated. This can be as much as 50 to 90 percent of the incoming water. This amount depends largely on the pressure difference across the membrane. The larger the pressure difference, the smaller the wastage rate.

This article has been contributed in part by Michigan State University Extension