Aeration-Filtration is a process where air is injected into the water stream via a venturi valve or an air pump.  The air acts as an oxidant, speeding the precipitation of iron or hydrogen sulfide. The final precipitated material is then filtered.  The process must include an “off-gassing” step where excess air is allowed to leave the water stream.

Air Stripping involves the stripping out of dissolved gasses from water by forcing a large amount of air to contact the water stream.  This is used to remove radon and volatiles such as MtBE from water at atmospheric pressure.  After the air stripping the air that was passed through the water is vented out a specially installed vent pipe, and the water must be re-pressurized to provide water pressure for the facility.  Air stripping usually raises the pH of the water, as carbon dioxide is also removed from the water during this process, so it can be used as a step in corrosion control.

Backwashing Filters utilize a special media installed inside a fiberglass tank with a control valve on top.  The control valve will cause the filter to backwash the media on a periodic basis to remove accumulated sediments.  Depending on the media chosen, a backwashing filter can be used to remove solids or dissolved solids from the water, or to raise the pH or remove chlorine.  The hydraulics of the site must be studied because filter performance will depend on having a media bed that is properly sized.  It is also critical to have a pumping rate that is adequate to clean the filter.

Cartridge Filters are installed along the plumbing run and include a housing and some type of cartridge element.  The cartridge filter elements are available in a wide variety of sizes and filtration ratings.  The cartridge must be replaced periodically as it will trap particles larger than its rating, and will eventually stop water flow once its surface or depth is full to capacity.  A teaspoonful of bleach should be added to cartridge filters used on potable water to prevent the growth of bacteria.  Cartridge style filtration is not effective against dissolved solids such as ferrous iron.

Centrifugal Filters use the centrifugal action of the water to remove particles of sand, grit, sediments or mica.  The particles settle out and in some cases are further separated by the use of a screen or cartridge filter.  If a centrifugal filter plugs it may be that a backwashing filter would be a more appropriate solution, or the well should be evaluated.

Chlorination is a process where chlorine is injected into the water stream to oxidize iron, kill bacteria or solve for tastes and odors.  In addition, chlorine can cause arsenic to change its valence from the hard to remove arsenite to easier to remove arsenate.  The excess chlorine is usually removed after the proper retention time in a step called dechlorination which includes granular activated carbon in a filter.

Contacting Filters are designed to cause a chemical change in the water based on the water’s contact with the media.  An example is a “calcite contactor” where the water is passed through a bed of calcium carbonate which raises the pH of the water.  In order to obtain satisfactory results, contacting filters must be sized for the flow rate of the water and the water chemistry.

Demineralization, see deionization.

Deionization is a process similar to ion exchange where a special resin is used and where the regenerant is hydrochloric acid or sodium hydroxide.  All the cations and all the anions are replaced with hydrogen and hydroxyls, and the finished stream has only water in it.  This is used for industrial applications where high purity water is required.  Deionized water can only be sent through special grade, non-metallic plumbing as the water will be “aggressive” and will dissolve copper plumbing.

Disinfection.  When there is risk that illness-causing organisms could be present in the water, the water must be disinfected to assure a safe water supply.  This is most commonly done with the use of an ultra-violet light, a chlorination system, or ozone injection.  In all cases the other water quality parameters, as well as the household water use patterns must be carefully considered.  When chlorine is utilized, a dechlorination filter is usually installed to reduce the chlorine and improve the taste of the water prior to use.

Distillation involves heating the water to the boiling point.  The steam is collected and then cooled.  Contaminants are left behind in the boiling chamber, plus the heating process destroys bacteria.  The boiling chamber must be cleaned of the minerals that precipitate during this process to avoid shorting out the heating element.

Ion Exchange, Cation and Anion. With ion exchange dissolved ions are removed from the water by their attraction to a specially manufactured resin bead installed inside the fiberglass tank. The ions are then flushed off of the bead by a regeneration process involving sodium chloride. Cation exchange is used to remove the positively charged ions of iron, manganese and hardness, exchanging them for sodium. Anion exchange is used to remove alkalinity, arsenic and uranium, exchanging them for chloride.

Oxidation of dissolved metals (such as iron or arsenic) or bacteria can be achieved through the addition of chlorine, ozone or bromine.  These systems require specific contact time and removal of the oxidant prior to use of the water, so special attention must be paid to the configuration.  If oxidation is being used to remove dissolved metals, a filter must be included in the configuration to trap the oxidized particles.  Oxidation can also occur with a special catalytic media that causes a chemical change in the water to precipitate metals from dissolved to solid.  Some catalytic medias use a regenerant to refresh the media.  An example is a manganese greensand filter that regenerates with the powerful oxidant, potassium permanganate.

Neutralization refers to the process of raising the pH of acidic water to the point where it no longer presents a potential to damage the plumbing.  This is done either in a passive system where the water travels through a bed of calcium carbonate, or with an active system which injects a small amount of sodium carbonate or other anti-corrosive chemicals into the water stream. The system can be configured as a backwashing filter to also remove solids from the water.

Reverse Osmosis is a membrane technology where the water is forced against a semi-permeable membrane which is able to separate the water from contaminants dissolved in the water. The contaminants are flushed to drain and the cleaned water is stored for use.

Ultra Violet Light is used to disinfect water streams. The specific portion of the light spectrum that can disturb bacterial DNA is used. The water must be clear prior to the process in order for the light to work.

Water Softening, see Ion Exchange.