Reverse osmosis is becoming a common home treatment method for contaminated drinking water. Best known for its use in desalination—the process of turning sea water into drinking water—it can also be effective for treating water quality problems in the home.
Reverse osmosis can dramatically reduce the amounts of organic and inorganic particulates in drinking water. The efficiency of removing various contaminants can vary, so home owners should evaluate the pros and cons when considering using reverse osmosis for home treatment. Other home treatment methods may be better for eliminating a specific contaminant in an area's water supply.
Reverse Osmosis Process
Based on the process of osmosis, these filtration systems make use of the selective movement of water from one side of a membrane (a plastic film that looks similar to cellophane) to the other. Pressure applied to the contaminated water forces it through the membrane like a sieve. Since contaminants don't move with the water through the membrane, purer water collects on the other side, where it becomes available for storage or immediate use.
The specific amount of pressure required depends 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 amount of a particular contaminant that doesn't cross the membrane. However, rejection percentages don't tell the whole story. It's also important to know the incoming pollutant concentrations to effectively reduce contaminant concentrations in the drinking water to safe levels.
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, but that still leave a dangerous amount in the water if the nitrates were present in very high concentration. An incoming nitrate concentration of 110 milligrams per liter (an unlikely amount), would still leave 11 mg/l of the nitrate in the purified water after 90% was removed. That would be greater than the 10 mg/l maximum contaminant level for nitrate most governments recommend for drinking water supplies.
The basic elements of a 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.
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.