Identification

• In the United States, the standards for water purity are set by the National Standards Institute and the Environmental Protection Agency (EPA). Any water purification system should have a certification from one of the three independent organizations accredited by the EPA prominently displayed on its packaging. As of 2010, those organizations are NSF International, Water Quality Association or Underwriters Laboratories, Inc. These groups test, monitor and approve the manufacturing processes of water purification material and the performance of the final products.
  
  

Considerations

• Federal guidelines dictate the required purity of water used in food and drug manufacturing. If immune deficiency or other health concerns are an issue in the home, a large point of entry unit that purifies the home's entire water supply may be necessary. For most homes, a smaller point of use system such as a pitcher water purifier or a unit installed in-line with a kitchen faucet will suffice.
  
  

Gas Contaminants

• There are certain gaseous contaminants that don't present a serious health risk but can greatly effect the quality of the water. Bad odors and an unpleasant taste is an indication of gas contaminants such as sulfur. A purifier with an activated carbon filter can be very effective in removing these types of contaminants. Activated carbon filters can be found in water filter pitchers or can be installed in-line to a specific faucet. Some bacteria thrive in the carbon material so these filters should be changed regularly per the manufacturer's recommendation.
  
  

Chemical Contaminants

• Chemical contaminants include metal ions such as lead, and aqueous salts like sodium and chlorine. Distillation water purifiers will clear all chemical contaminants from the water supply but are not always appropriate for home use. Reverse osmosis is highly effective in clearing chemical contaminants although according to the Centers for Disease Control and Prevention, arsenic may not be cleared by a reverse osmosis filter. Nanofilters are considered moderately effective in filtering out chemical contaminants. Ultrafilters are only somewhat effective and microfilters are not effective at all.
  
  

Biological Contaminants

• You will need a specific kind of purifier if the water contains biological contaminants such as bacteria, protozoa or viruses. Reverse osmosis pushes water through a semi permeable membrane with a pore size of approximately 0.0001 micron and will clear all biological contaminants, even the smallest of viruses. Nanofilters are the second most effective with a pore size of about 0.001 micron and can clear almost any biological contaminant as well. Ultrafiltration units have a larger pore size (0.01 micron) which can be used if only bacterial or protozoa contamination is a problem as it is ineffective against viruses. Microfilters will suffice in the case of protozoa contaminants but their large pore size of 0.1 micron will not clear every type of bacteria nor any viruses.