This article is an overview of common Point-of-Use
and Point-of-Entry water treatments.
It's ironic that many areas of the world face critical shortages of drinking water on a planet whose surface is 3/4 covered with water. Most of the water, of course, is seawater, which is far too saline for human consumption. And of the little "fresh" water that remains, most is trapped in polar ice caps where it is difficult to harness for use by the world's population.
Much of the natural supply of potable water that is accessible faces stress from a growing world population, which increases the basic demand for this natural resource, while reducing the supply further through biological and industrial contamination.
Major population centers in developing nations without established waste treatment or water treatment infrastructures often suffer from epidemics of waterborne disease. In these areas, raw sewage often directly contaminates the rivers and streams used for drinking, washing, and cooking. In other cases, unchecked industrialization leads to water contamination through improperly disposed-of chemical and nuclear wastes.
Some good news about this problem is that individuals can take control of their own water quality, and treat their water for nearly all biological and chemical contaminants that may be encountered. These technologies also treat for "aesthetic" contaminants that cause potable water to have unpleasant tastes, colors, and odors.
Point of Use (POU) and Point of Entry (POE) water treatment equipment can effectively treat the water used by a small community, home, or business.
POU equipment treats the water that is used at a single tap, while the rest of the water in the building remains untreated. POU equipment is primarily used to treat health contaminants like lead, and aesthetic contaminants like sulfur. These contaminants are a concern in water used for drinking and cooking.
POE equipment treats most or all of the water before it is distributed, either throughout a small community or at a single building. POE equipment treats for health contaminants like volatile organic compounds (VOC's) that can be absorbed through the skin, or contaminants like radon which exist as a harmful vapor suspended in the water that can be inhaled during showering. POE is also used to describe water softening, which inhibits scale formation in plumbing while increasing the efficiency and longevity of water-related appliances like water heaters.
There are many effective technologies used to provide POU/POE treatment solutions, and no single technology is effective for treating all of the possible contaminants. A specific technology or combination of technologies is usually applied to treat the specific problem at hand.
It should be noted that different levels of
performance can be found between products using each technology. If a product
is to be used to treat a health contaminant, it is important that the specific
product be tested successfully for the reduction of that contaminant. Offered
below is a brief description of the main technologies, and what they are
typically used to treat.
Because of its molecular makeup, activated carbon can adsorb well, meaning that it can take in or collect many organic molecules on its surface. Granular activated carbon filters are typically inexpensive, and maintenance involves replacing six to twelve cartridges a year, depending on the quality of the raw water and the filter media.
Specially designed solid block and precoat
activated carbon filters are also available, which are effective at reducing
heavy metals such as lead and mercury. Solid block filters with a pore
size smaller than 0.2 microns are often effective against biological contaminants
The anions or cations on the resin are eventually exhausted, and replaced by the anions or cations of the contaminant being removed. When this occurs, the bed must be backwashed using a concentrated solution of the base cation or anion, which recharges the bed and flushes the built-up contaminant.
Anion exchange typically uses chloride or
hydroxide anions, and can be used to treat for mercury, nitrates, arsenic,
and various staining agents. Cation exchange typically uses sodium or potassium
chloride, and can also treat for some forms of lead and radium. It is also
commonly used to soften water.
Chlorine is typically fed directly into a well, or into a retention tank where concentration and contact time can be controlled. Chlorination is effective for treating pathogens like coliform bacteria and legionella, though it is ineffective against hard-shelled cysts like Cryptosoridium and Giardia lamblia. Other chemicals like bromine and iodine can also be used to disinfect water through much the same process as chlorination, though they are not as frequently used.
Carbon block media usually has to be disposed of after each use. This media, however, provides additional treatment for a variety of other health and aesthetic contaminants (see activated carbon section). Microfiltration is effective for treating the full range of biological contaminants, including hard-shelled cysts like Cryptosporidium.
Ozone treatment oxidizes organic contaminants in much the same way that chlorine does. An ozone generator converts the oxygen found in air to O3, or ozone. As with chlorination, proper concentrations and contact time is essential for disinfection. Ozone usually requires the use of a retention tank to accomplish this, and can be used to provide partial treatment in pools. Ozone is effective for treating pathogens like coliform bacteria and legionella, but it is not effective against hard-shelled cysts like Cryptosporidium or Giardia lamblia without using high contact times and concentrations.
Distillation typically provides a high degree of effectiveness against a broad range of health contaminants.
Distillation is typically not effective
for treating contaminants such as benzene and radon, which give off harmful
vapors that can move through the system with the steam. The energy requirement
of distillation and a relatively long production time typically limits
its use to POU drinking water applications in home and commercial markets.
Some distillation untis are also tested and approved for the reduction
of biological pathogens.
Typical contaminant rejection rates range from 85% to 95%, and a gallon of highly treated water can usually be produced from two to four gallons of raw water, depending on the initial quality of the water. Maintenance involves the replacement of the RO membrane cartridge every two or three years, and the carbon filter cartridges six to twelve times per year.
RO is effective for the reduction of a broad
range of health and aesthetic contaminants, though it is typically not
used for the reduction of biological pathogens. RO also incorporates an
activated carbon filter, which can provide added treatment for the volatile
organic compounds (VOC's) not treated by the membrane itself.
(iron, manganese, copper oxide,
(cloudy or dirty look)
(bath tub ring, dingy
fabrics, irritated skin)
(plumbing, water heaters)
(rotten eggs, chlorine,
other odors )
sediment, iron stains)
With properly installed and maintained treatment systems, most water can be made safe and pleasant to drink. Treatment systems should be checked routinely to detect possible problems. The following paragraphs review specific methods of water treatment and what they are used for. Before getting into the individual treatment processes it will be important to know the general order in which these treatment steps should occur. Multiple treatments are common but if initiated in the wrong sequence, one treatment may negate another.