This article – from the American Ground Water Trust – briefly describes the treatment methods commonly used in the residential market to improve water quality. The treatment methods are divided under six categories – filtration, oxidation, ion exchange, ultraviolet irradiation, aeration and pH neutralization.
Filtration MethodsFiltration, simply stated, removes suspended matter from water by mechanical “screening” (note that, sometimes, the word “filtration” is used incorrectly to refer to all types of water treatment). Basic filters usually are porous beds of insoluble material. Other examples include cast forms, plates of sheet material, synthetic membranes, finely perforated plastic or specially sized beds of inert particles. Suspended silt, clay, colloids and some microorganisms are removed by the filtration process. Simple cartridge filters may be effective for low levels of turbidity.
The ability of a filter to efficiently screen out particles depends on the size of the filter area, the quality of the water to be filtered, the required flow rate of the water, the design capacity of the filter, and its porosity. Filters generally are used for particles less than 0.0029 inches in diameter.
Filtration, by itself, is inadequate to remove biological contaminants from water. Fine filtration can be a very effective means of particulate removal. It strains out large organisms like protozoan cysts and worm eggs, but should be followed with a chemical disinfection method, because some bacterial and viral pathogens may pass through.
Cartridge filters are available in two common types – pleated (sheet-like) fiber or solid (fill or particle-type). Common filter size ratings are 50, 20, 10, 5 and 1 microns. The solid types generally will provide more filtering capacity than pleated varieties for a given cartridge volume and filter size.
Activated carbon filtration systems involve the adsorption (adhesion) of one material on the surface of a second solid substance based on opposing electrical charges of each material. These systems are used to eliminate certain hazardous compounds related to industrial wastes, chemicals and pesticides. This treatment method also can remove unpleasant tastes and odors caused by decaying organic matter, dissolved gases, and residual chlorine. Activated carbon is placed on a filter medium or installed in treatment tanks, where it adsorbs the taste and odor impurities in water, leaving the water taste- and odor-free. When required to eliminate hazardous compounds, the system should be designed by a professional competent to assess the effectiveness of the treatment with regard to the specific hazardous compounds detected in the water. Specific system maintenance plans may be necessary to ensure on-going effective removal of the compounds of concern.
Adsorption filtration does not treat microorganisms and should also include a method of chemical disinfection. It is recommended that water be chlorinated before passing through an activated carbon filter. The purpose of the chlorination is to assist in the removal of substances causing taste and odor, and more important, to prevent bacteriological growth on the filter.
Reverse osmosis methods employ a unit divided into two chambers by a semi-permeable membrane. One of the chambers contains “raw” water with undesirable constituent(s) (e.g., salt). Reverse osmosis involves the application of pressure to the side of the chamber containing the “raw” water. This forces the water to leave the contaminated chamber and flow through the treatment membrane into the “treated” water chamber, leaving the unwanted minerals behind, which then are rinsed to the drain. The membrane filters the water on a molecular scale. Reverse osmosis provides partially de-mineralized water.
The process is effective for removing many substances, including sulfate and chloride, and it generally leaves the water 90 percent free of mineral and biological foulants. However, pre-filtration or other treatment may be needed for the system to work properly. The removed substances are disposed of in approximately 1 gallon to 3 gallons of water, which are wasted for every 1 gallon that is produced.
Backwashing media beds are used in larger filter tank systems. The tank is filled with an inert (non-reactive), relatively dense material such as sand or ceramic granules. As the untreated water passes through the bed, unwanted particles are trapped in the bed. The bed periodically is backwashed to flush out the unwanted particles to regenerate filter space in the bed. In ion-exchange systems, the backwashing process also may regenerate chemicals in the bed that have been used in the treatment process.
Oxidation MethodsChlorination is used primarily for disinfection. It probably is the most popular oxidizing technique that changes taste- and odor-causing substances into innocuous forms. Because chlorine controls the growth of algae and microorganisms, it is able to reduce the quantity of the taste- and odor-causing organisms in a water system. Chlorine also has a residual germicidal action that provides continuing antibacterial protection.
Chlorine is available for domestic water treatment use in solid and liquid forms. Liquid sodium hypochlorite is sold in grocery stores as household bleach. Calcium hypochlorite is the solid form of chlorine and can be obtained as a soluble powder or tablet.
Chlorination equipment is available in three types of units:
- Positive displacement feeders, the most common type, are electrically powered and operate by using a piston or diaphragm pump to inject the chlorine solution.
- Eductor-type chlorinators use the natural vacuum created by the flow of water in a pipe to draw the chlorine solution from the disinfectant reservoir.
- Tablet or granule-type feeders allow solid disinfectants to contact the flowing water to be treated. As the disinfectant dissolves, more tablets are added to the dissolving chamber by gravity.
Because of the variability of the chlorine demand for domestic water systems, chlorine dosages usually are larger than required; thus, the treated water usually has a noticeable chlorine taste and odor. To eliminate the chlorine taste and odor, an activated carbon filter can be placed after the chlorination system to remove excess chlorine.
Chlorine is the most widely used method in the United States for disinfecting municipal and individual water supplies. Nevertheless, chlorine has some drawbacks. Chlorinated organics (i.e., certain trihalomethanes) are produced when organic chemicals combine with chlorine in water. Some of these chlorinated organic chemicals are suspected of being carcinogenic. However, these substances occur more often in surface water than in ground water supplies because surface waters have higher concentrations of organic materials. Chlorine’s effectiveness can be hampered by turbidity in water. Chlorine will probably continue to be the dominant disinfection method. Homeowners who select a different procedure should first check with state and local health officials to see if such treatment conflicts with any regulations.
Iodine is chemically more stable than chlorine, but more expensive. Iodine disinfection units are not common. They have been used in lunar modules to protect the drinking water of astronauts, and for disinfection in remote areas and emergency situations.
Iodination equipment, as with chlorination equipment, is installed between the pump and holding or pressure tank, and a continuous flow of concentrated iodine is fed into the mainstream of water. This equipment is simple to operate and requires little maintenance. It may, however, impart a slight taste to the water.
Potassium permanganate is an oxidizing agent that destroys tastes and odors resulting from dissolved hydrogen sulfide gas. Dissolved metallic ions that cause taste problems also may be oxidized. Since chlorine and potassium permanganate oxidize soluble metallic ions into insoluble oxides, some filtration method should follow this treatment to remove chemical precipitates.
Ozonation uses ozone as an oxidizing agent. Ozone is an unstable form of oxygen, having three atoms per molecule rather than the two atoms typical of atmospheric oxygen. As such, ozone is more reactive than oxygen, and is, therefore, a powerful oxidizing agent. The ozonation system involves passing dry, clean air through a special form of high-voltage electric discharge. The mixture of air leaving the ozone generator may contain about 1 percent ozone, which is passed through the water to be treated.
In the ozonation process, gases and volatile chemicals in water may be stripped by aeration, a process that mixes air and water. Ozonation can strip water of iron, manganese and sulfur by oxidizing them into insoluble compounds that can be removed by filtration. Ozone also can destroy odor- and taste-producing bacteria. Organic constituents may be oxidized. While this process is used widely in Europe and in industrial applications, it is not commonly used in U.S. residential applications. This method has a greater germicidal effect against bacteria and viruses than does chlorine. Also, ozonation adds no chemicals to water because it purifies naturally with a form of oxygen. While ozonation does produce residual germicidal power, it is not easily measured. Ozonation equipment and operating costs are higher than other treatment procedures.
Catalytic oxidizing filters can be used when the type or amount of iron exceeds the treatment limits of a water softener. The catalytic oxidizing filter employs a medium that has been impregnated with various oxides of manganese. As ferrous iron-bearing water passes through this filter, the medium oxidizes the iron in the water to form insoluble ferric iron. The resulting rust particles then are trapped in the filter bed. As the rust accumulates, the filter must be cleaned.
This procedure usually removes 75 percent to 90 percent of the iron, but is only effective at pH 6.8 or above. A water softener should be installed following the filter to remove the remaining iron and any hardness that may be present. Substantial quantities or different forms of iron and iron bacteria can be removed by a water softener, or, for more severe conditions, by a catalytic oxidizing filter (oxidation followed by filtration).
Oxidation-filtration may be necessary for adequate water treatment when the iron level in water exceeds 25 mg/L or when high amounts of iron bacteria are present. This process usually involves preoxidizing the iron, and removing the precipitated particles with a filter. Preoxidation usually is accomplished by injecting air or chlorine into the inlet supply line ahead of the pressure or storage tank (potassium permanganate also can be used as a preoxidation method). The iron oxidizes and precipitates in the tank, and is removed by a filter. An activated carbon filter often is used because it removes the excess chlorine – as well as the iron particles – leaving the water odorless and tasteless. Contact time, filter sizes and backwash rates all are critical variables for effective treatment.
Oxidation-filtration is widely used to control iron bacteria. When these bacteria are first detected, shock chlorination (an injection of chlorine about 10 times larger than the dose used in regular chlorination) is recommended prior to the installation of water conditioning equipment. When extremely high iron levels are present, some equipment may need to be doubled (i.e., repeated in the treatment sequence to increase contact time or treatment time) for thorough treatment.
Ion-exchangeWater softening is based on the ion-exchange process and employs a tank containing a bed of insoluble material. This material (a resin) has a negative charge with positively charged sodium ions attached to it. With most water supplies, the resin has a stronger affinity for calcium and magnesium ions than for sodium ions. Thus, when water containing calcium and magnesium passes through the resin, the hardness ions are attracted to the resin and the sodium ions are released in an equivalent quantity to the water supply. In essence, the water softener trades sodium ions for calcium and magnesium ions; hence the term ion exchange. The total ionic content of the water does not change.
When all sodium ions are displaced, the resin becomes exhausted and must be regenerated by passing a strong sodium chloride solution (brine) through the resin during a backwash process. Sodium ions are placed on the resin while hardness ions are washed to the drain with the spent brine. This reversal of the sodium/hardness preference is caused by the strength of the regenerative brine.
Persons on low-salt or low-sodium diets should consult a physician before regularly drinking softened water. In normal situations, the added salt from drinking softened water is a small fraction of salt that is consumed from foods.
Although a water softener has some filtering ability, water with heavy turbidity or particulate matter should be filtered prior to softening. A water softener can remove limited quantities of certain forms of iron, but it should never be used alone when the water is red or rusty (indicating precipitated iron) or when iron bacteria are present.
A water softener is not the only means of combating hardness. Where a water softener is impractical, certain polyphosphate compounds can be added to the water supply with a chemical feeder to alleviate some hard water problems. While such treatment in no way provides all of the advantages of soft water, and does not inhibit the formation of a soap curd, it can help curb scale formation within the hot water system.
Dealkalization is very similar to water softening except that a different ion exchanger is used that can exchange chloride ions for sulfate ions, leaving the water free of sulfate. Dealkalization will also reduce the alkalinity level of a water supply. A 70 percent to 90 percent reduction in both sulfate and alkalinity can be expected from this system – if properly used. It should be noted, though, that the resultant chloride content of the water might exceed the 250 mg/L EPA-recommended limit.
Deionization, also known as demineralization, involves the removal of all ionized minerals and salts from a solution by a two-phase, ion-exchange procedure. Positively charged ions are exchanged for a chemically equivalent amount of hydrogen ions, and negatively charged ions are exchanged for a chemically equivalent amount of hydroxide ions. The hydrogen and hydroxide ions then unite to form water molecules, leaving the treated water free of all ionized contaminants. This treatment normally is only used for commercial or industrial applications.
Ultraviolet IrradiationUltraviolet light provides bacterial-killing action much the same way sunlight helps kill bacteria. The ultraviolet unit consists of one or more ultraviolet lamps – usually enclosed in a quartz sleeve – around which the water flows. The lamps are similar to fluorescent lamps, and a quartz sleeve surrounding each lamp protects the lamp from the cooling action of water. The killing effect of the lamp is reduced when the lamp temperature is lowered.
Water passes in a relatively thin layer around the lamp, as the germicidal action of ultraviolet irradiation depends on the intensity of the light, depth of exposure and contact time. Water flow must be regulated to ensure that all organisms receive adequate exposure. Turbidity and minute traces of iron compounds reduce the light’s transmission; therefore, the water should be pre-filtered so that untreated organisms do not slip by. Ultraviolet irradiation units are automatic, require little maintenance, and do not add undesirable materials to the water. However, these units offer no germicidal residual, so determining the system’s effectiveness is difficult.
AerationThis process treats water through intimate contact with air. Aeration may be accomplished through several methods, including spraying, cascading, aspirating or bubbling the water supply to bring it in direct contact with air. Either pressure (closed system) or gravity (open system) aerators may by used. Pressure systems primarily are used for oxidation, while gravity systems commonly are used for degassing (e.g., removing dissolved radon, carbon dioxide, hydrogen sulfide or methane).
pH NeutralizationIn order to increase the overall efficiency of a water conditioning system, acidic water may be pre-treated by passing it through a tank containing a bed of granular lime, calcium carbonate or marble before entering the remainder of the treatment process. Similarly, alkaline water may be treated with an acid drip or injection process to neutralize the water.
This article is provided through the courtesy of the American Ground Water Trust; its mission is to protect ground water, promote public awareness of the environmental and economic importance of ground water, and provide accurate information to assist public participation in water resources decisions. Visit www.agwt.org for a wealth of ground water information.