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Brief Overview of Residential Water Treatment Methods

Water Disinfection

Water Purification

Water Disinfection: a purification process that kills or removes biological contaminants (cysts, bacteria, viruses, protozoans, etc) from a water source. Water that has been disinfected (by UV treatment, boiling, chlorination, micro-filtration, etc.) may still be polluted with other contaminants that are not affected by the disinfection treatment. In some cases, additional contaminants may actually be added to the water by the disinfection process. For instance, the process of chlorination nearly always adds some disinfection byproducts (e.g. trihalomethanes).

Ultra Violet Light: UV filtration is recognized by the EPA as one of four approved methods of disinfecting water, and is preferable over the other three methods - chlorine, iodine, and distillation - due to its instantaneous effect and the fact that it does not leave any residual chemicals in the water. It simply removes the risk of illness caused by microbial contamination, making water safe to drink or use commercially.

Water enters an ultraviolet sterilization chamber where any viruses, bacteria, mold, or other living microorganisms are bombarded with special, intense light of a specific wavelength (254íùm). The UV light penetrates the cell walls of the organisms and attacks their DNA structure leaving the organism dead or unable to reproduce - effectively neutralized.
The effectiveness of a UV system in eliminating microbiological contamination is directly dependent on the physical qualities and/or clarity of the water supply.

Suspended solids or particulate matter can cause a shielding problem in which a microbe may pass through the UV filter without actually having any direct UV penetration. Iron/Manganese will cause staining on the quartz sleeve that houses the UV bulb, at levels as low as 0.3 PPM of iron and 0.05 PPM of manganese. UV filters are best used after adequate sediment, turbidity, iron, and manganese filtration.

Boiling: Parasites that might be lurking in your water will be killed if the water is boiled long enough. Boiling will also drive out some of the Volatile Organic Compounds (VOCs) that may be in the water. This method works well to make water that is contaminated with living organisms safe to drink, but it is not routinely used to purify drinking water - except in emergencies. This is due to the inconvenience, and because it does not remove many other water contaminants. In fact boiling is liable to concentrate some contaminants like lead, asbestos, mercury, etc. and the many toxic organic chemicals that do not vaporize as the relatively pure water vapor boils off.

Distillation:

Distillation is almost the reverse of boiling. To remove impurities from water by distillation, the water is boiled causing the mostly pure steam to vaporize leaving the non-volatile contaminants behind. The steam is then cooled until it condenses, and the resulting distillate drips into a container. Salts, sediment, heavy metals - anything that won't boil or evaporate - remain in the distiller and must be removed. VOCs are are carried along with the steam and are a good example of a contaminant that distillation won't remove. A carbon filter or other device must be used with a distiller to ensure the complete removal of all contaminants.

A good distillation unit produces very pure water. Distillation also kills parasites in the water. However, distillation takes a comparatively long time to purify the water, uses electricity all the time the unit is operating, and requires cleaning. It can take about five hours to make a gallon of distilled water. The cost of ownership is high because of the initial high cost of the distillation unit, and the ongoing electrical energy consumption for each gallon of water produced.

Ozonation: The formation of oxygen into ozone occurs with the use of an electric discharge field as in the CD-type ozone generators (corona discharge simulation of the lightning), or by ultraviolet radiation as in UV-type ozone generators (simulation of the ultra-violet rays from the sun). Ozone is a naturally occurring component of fresh air. It can be produced by the ultra violet rays of the sun reacting with the Earth's upper atmosphere, which creates the protective ozone layer, by lightning, or it can be created artificially with an ozone generator. The ozone molecule (O3) contains three oxygen atoms whereas the oxygen molecule (O2) contains only two. Ozone is a very reactive and unstable gas with a short half-life before it reverts back to oxygen. Ozone is the most powerful and rapid acting oxidizer man can produce, and will destroy (oxidize) all bacteria, mold and yeast spores, organic material and viruses with which it comes in contact. Ozone also oxidizes and precipitates iron, sulfur, and manganese so they can be filtered out of solution. Ozone will oxidize and break down many organic chemicals as well, but ozone treatment creates its own set of undesirable byproducts - formaldehyde and bromate - that can be harmful if left untreated. Ozonation is typically used as a point of entry water treatment method for this reason.

Water Purification: to remove specific contaminants from a water source.

Water Filters: The basic concept behind nearly all filters, is fairly simple the contaminants are physically prevented from moving through the filter either by screening them out with very small pores and/or, in the case of carbon filters, by trapping them within the filter matrix by attracting them to the surface of carbon particles (the process of adsorption)

There are two main types of filters (sediment and activated carbon), and sometimes they are combined into a single unit. When it comes to how good the filter is at removing particles from the water - smaller is better. A 1 micron filter will remove more particles than a 10 micron filter, but it will also clog faster. One micron is about 1/100 the diameter of a human hair. (For an illustration of micron purification levels click HERE). A filter that removes particles down to 5 microns will produce fairly clean-looking water, but most of the water parasites, bacteria, cryptosporidium, giardia, etc will pass through the pores if present. A filter must trap particles one micron or smaller to be effective at removing cryptosporidium or giardia cysts. (Reverse osmosis filters are 0.001 micron filters). One benefit of home filtration systems is that they are passive. That is, they require no electricity to filter the water - normal home water pressure is used to push the water though the filter. The only routine maintenance required is periodic replacement of the filtration element. As long as the cost of the replacement filter elements is reasonable, owning a even a high-end water filter can be very inexpensive if you look at the long term costs and compare it with other solutions.

Sediment Filters: Solid particles are strained out of the water. Fiber sediment filters contain cellulose, rayon or some other material spun into a mesh with small pores. Suspended sediment (or turbidity) is removed as water pressure forces water through tightly wrapped fibers. These filters come in a variety of sizes and meshes from fine to coarse, with the lower micron rating being the finer. Depth type sediment filters are constructed typically from melt blown polypropylene, and trap sediment in an ever-constricting matrix as the water flows through the filter structure. The finer the filter, the more particles are trapped and the more often the filter must be changed. Sediment filters will not remove contaminants that are dissolved in the water, like lead, mercury, trihalomethanes or other organic compounds.

Ceramic sediment filters are much like fiber filters using the process where water is forced through the pores of a ceramic filtration media. This type of filter can reduce some asbestos fibers, cysts (if the pores are one micron or smaller), some bacteria (with pore sizes in the - 1 micron range) and other particulate matter. Ceramic filters will not remove contaminants that are dissolved in the water, like lead, mercury, trihalomethanes or other organic compounds. These filters may be used as a back-end to an activated carbon filter to provide a more thorough removal of contaminants.

Activated carbon filters: particles of carbon that have been treated to increase their surface area and increase their ability to adsorb a wide range of contaminants. Coconut shell carbon and bituminous (coal) carbon are the most common sources of carbon in activated carbon filters. While bituminous carbon is the more cost effective type of carbon, coconut shell carbon is preferred in home applications due to the superior taste it imparts on the product water. The raw carbon source is slowly heated in the absence of air to produce a high carbon material. Passing oxidizing gases through the material at extremely high temperatures activates the carbon. The activation process produces the pores that result in such high adsorptive properties.

Activated carbon is particularly good at adsorbing organic compounds. You will find two basic kinds of carbon filters - Granular Activated (GAC) and Solid Block Activated. It is important to note - particularly when using counter-top carbon filtration systems - that hot water should NEVER be run through a carbon filter, because hot water will tend to release trapped contaminants into the water flow potentially making the water leaving the filter more contaminated than the water going in. The lone exception to this rule is carbon mixed with KDF 55.

Granular Activated (GAC): In this type of filter, water flows through a bed of activated carbon granules which trap some particulate matter and remove chlorine, organic contaminants, chemicals, and undesirable tastes and odors. The main problems associated with GAC filters are: channeling, and dumping. Water flowing through the filter can "channel" around the carbon granules and avoid filtration. Pockets of contaminated water can form in a loose bed of carbon granules. With changes in water pressure and flow rates, these pockets can collapse, "dumping" the contaminated water through the filter. The GAC filters used in the Home Master series, Value Line Chemical Fighter series, and the Home Master Jr F2 water filters use spring loaded pressure disks to compress the granular activated carbon and prevent channeling and dumping. High quality GAC filters will expose each drop of water to more carbon surface area than in solid carbon block filters.

KDF Media: KDF filter additive employs a matrix (generally small granules) of a zinc/copper alloy, which eliminates contaminants from water by utilizing electrochemical oxidation-reduction. Chemical properties of KDF include the ability to remove chlorine, kill algae and fungi, and control bacterial growth in the filter. KDF controls and inhibits microorganisms by setting up an electrolytic field and also by forming peroxide and hydroxyl radical by redox reactions. Types of KDF will also remove hydrogen sulfide, iron, lead, cadmium, aluminum, mercury, arsenic, and other inorganic compounds. Zinc and copper are the preferred metals used in the KDF alloy since both are relatively good reducing agents with respect to common inorganic contaminants (such as chlorine), and both can be tolerated in solution in moderate concentrations without adverse side effects. Zinc chloride is soluble, while cupric sulfide and ferric oxide are insoluble therefore, both copper sulfide and ferric oxide must be backwashed off the KDF granules so the insoluble contaminants will not inhibit the working efficiency of the KDF granules.

KDF is the only filter medium that removes contaminants from running hot water (unlike carbon filters where hot water can release trapped contaminants into the water stream). This makes them ideal for use in the shower. The filters change the chlorine some people are allergic to into a form (zinc chloride) that is much more easily tolerated. KDF filter media must be used in conjunction with other filtration technologies, such as GAC and/or reverse osmosis, in order to remove organic chemicals (pesticides, disinfection byproducts, MTBE, etc), or parasitic cysts (giardia and cryptosporidium).

Carbon Block: Activated carbon is the primary raw material in solid carbon block filters; but instead of carbon granules comprising the filtration medium, the carbon has been specially treated, compressed, and bonded to form a uniform matrix. The effective pore size can be very small, down to 1 micron. Carbon block filters provide a small pore size to physically trap particulates. In some cases bacteria that become trapped in the pores do not have enough room to multiply, eliminating the problem described above for GAC filters. By combining other specialized materials with carbon block, greater capacity ratings for certain contaminants like lead, mercury, etc can be achieved.

Reverse Osmosis (RO): Water pressure is used to force water molecules through a very fine membrane leaving the contaminants behind. Purified water is collected from the "clean" or permeate side of the membrane, and water containing the concentrated contaminants if flushed down the drain from the "contaminated" or concentrate side. The average RO system is a unit consisting of a sediment/chlorine pre filter, the reverse-osmosis membrane, a storage tank, and an activated-carbon post filter.

Reverse osmosis removes salt and most other inorganic material present in the water. For that reason, RO lends itself to use in places where the drinking water is brackish (salty), or contains nitrates that are difficult to remove by other methods. With a quality carbon filter used to remove any organic materials that get through the filter, the purity of the treated water approaches that produced by distillation. Microscopic parasites (including viruses) are usually removed by RO units, but any defect in the membrane will allow these organisms to pass into the permeate water. Though slower than a water filter, RO systems can typically purify more water per day than distillers. They also do not use electricity, but RO systems do waste water. Three or more gallons of concentrate wastewater are flushed down the drain for each gallon of filtered water produced. Zero waste RO units are available, however problematic, as they re-inject the concentrate wastewater back into the water feeding the RO thus forcing the RO to work harder and shortening its service life.

Two common types of household RO membranes are the Thin Film Composite (TFC or TFM) membrane and the Cellulose Triacetate (CTA) membrane. The main differences between the two types are filtration ability and chlorine tolerance. The CTA membrane is chlorine tolerant, but it more susceptible to fouling from bacteria, and it only rejects 93% of standard contaminants. The TFC/TFM membranes reject 98% of standard contaminants, are less susceptible to organic fouling, but it can only treat chlorine free water. Carbon pre-treatment must be used with a TFC/TFM membrane when purifying chlorinated municipal water supplies.

Neutralizing filters: Neutralizing filters are used to treat corrosive and acidic water. The most common home treatment is to run water through granular calcite (including materials such as marble, lime, or calcium carbonate). Some systems use soda ash, sodium carbonate, or caustic soda (sodium hydroxide) if the water is very acidic. Disadvantages to using neutralizers are increased water hardness because of added calcium, or added sodium.

Water Softening: Water softeners reduce the hardness of water by replacing the hardness minerals - calcium and magnesium ions (other cations will be removed as well) - with another ion, frequently sodium. Hard water creates unsightly scale on water fixtures; coats water heating elements causing power loss and increased electricity consumption; wastes soap by reducing its lathering ability, and it leaves a residue on your skin and hair. Very hard water (>10gpg) can also cause premature clogging in reverse osmosis membranes. Water is considered slightly hard if it has between 1-3.5 grains per gallon (gpg) of hardness minerals, moderately hard 3.5-7gpg, hard 7-10.5gpg, and very hard 10.5+ gpg.

Anion exchange: Anion exchange resins are materials that attract negatively charged particles (anions), and remove them from the water. This method effectively removes nitrate from drinking water.

Cation exchange: Cation exchange is a process similar to anion exchange, but the particles removed are positively charged (cations). Water softeners, which remove calcium, magnesium, and iron from water, but add sodium, are cation exchange units. Filters become saturated with the removed contaminants, and must be flushed with sodium periodically. Water softeners have some filtering ability, but if the water contains a high level of particulate matter or heavy turbidity, then it should be filtered before entering the softening system.

Aeration: Aeration is a process that allows volatile contaminants to be broken down when water is separated into small droplets (spray) in the air. This type of purification is most commonly seen on sink faucet aeration tips. Water that looks bubbly as it leaves the tap is being aerated by the faucet tip. However, contaminants that are removed from water by aeration are transferred to the air, where they may be capable of posing a health threat.

In larger aeration equipment, fresh air is then drawn through the spray, and collected in a storage tank, repressurized, and is then passed through a filter. The contaminants are carried to a storage tank. Be sure that any aeration device you purchase vents the air to the outdoors or collects the contaminant in a filter.

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