A reverse osmosis membrane is semi-permeable or selectively-permeable, allowing some molecules or ions to pass through, while others are filtered out. As it relates to water treatment, the size of the pores determines that particulates that will and will not be removed.
Particle filters are commonly used in wells to reduce naturally occurring deposits, such as clay or silt. The pores of the filters come in various sizes, 5-10 microns being the most common.
A simple paper filter, comparable to what is used in a coffeemaker, will remove particles larger than 10 microns. A filter made of cotton twine or a similar material will remove particles larger than 5 microns. When you are buying a filter, you can check the label to determine the size of particulates removed; the higher the number, the more particles that will pass through.
Paper filters have the shortest lifespan and as they wear out, more particles pass through and the flow-rate is reduced. The same is true of all particle filters. If you are just moving into a new home with a well, talk to your neighbors about how long their filters typically last and what kind they use.
The pores of reverse osmosis membranes are smaller than a micron in size. Assuming no tears are present, anything larger than one micron will be removed. Filters that are certified to filter down to one micron or below are suitable for water that is contaminated with cryptosporidium or giardia cysts.
Cysts can be an issue for people with a private well or for those serviced by a public treatment facility. Well-owners, particularly those that live in agricultural areas, should have testing conducted. Deep wells that are properly covered may be protected from cysts.
A reverse osmosis membrane is not the only option for cyst removal. Any sub-micron sized filter will work. Some are more efficient and less expensive.
In farming communities, it is necessary to test for chemical contaminants, such as herbicides and pesticides. The chemicals make their way through the soil and into the groundwater at a slow pace. Some of the most dangerous ones have been banned, but they may still be present in the groundwater, because of that slow passage.
Chemicals are much smaller than a micron in size. They cannot be removed by reverse osmosis membranes or particle filters. They can be trapped through a process called adsorption.
A common adsorption filter is one composed of granular activated carbon. Water is passes over the carbon granules. Chemicals, such as chlorine, chloramines, chlorine-byproducts, herbicides and pesticides, are trapped on the surface of the granules.
Typically, a second stage, sub-micron sized filter is used to prevent the carbon granules from passing through the tap. Other stages, such as ion exchange, may also be used to reduce other contaminants, such as lead.
Lead is a problem in older homes. It is no longer used in plumbing, but it was for many years.
A reverse osmosis membrane will remove traces of lead, but it does cost more than other options. It’s a good idea to do a little comparative shopping, before you buy.