Traditionally, three methods have been available to disinfect water.
- Boiling (including distillation),
- Adding oxidizing agents like chlorine, iodine, or ozone,
- Exposing water to ultraviolet light.
Each method has some advantages and some disadvantages. None of these methods are ideal for a point-of-use (POU) water treatment system. Boiling water uses a lot of fuel.
- Boiling (including distillation) takes a reliable source of fuel. In times of emergency the fuel required may not be available leaving you without a water supply at the time you need it most. Boiling water also creates mineral deposits on containers that can take a lot of effort and caustic chemicals to clean.
- Oxidizing agents like chlorine, chloramine, and iodine require the addition of chemicals and can leave disinfection by-products that can be very hazardous to health.
- Ozone requires not only power, but also is such a strong oxidizer that it tends to destroy the plumbing used to dispense it.
- Ultraviolet light requires water that is already very clean as well as a source of power.
Now there is a new method of filtration that is revolutionizing water disinfection technology. Up until a few years ago this was not a reliable or practical method, however new developments in capillary membrane technology
make filtration an ideal method of disinfecting water supplies. The process is failsafe, no power is required, there is no water waste, and the technology can be made to work on almost any water supply.
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Boiling water is extremely effective as a disinfectant. Vigorous boiling for one to three minutes (depending upon altitude) kills bacteria, and disease-causing organisms like cryptosporidium and giardia cysts.
Any heat source, such as electric or gas ranges, camp stoves or wood fires can be used to boil water. Even microwave ovens can heat water to boiling. Distillers use this method to create potable drinking water. This makes it the most widely available form of disinfection.
Mineral deposits may build up in vessels used for boiling water. Soaking these vessels in a weak acid solution such as vinegar or lemon juice can help dissolve the mineral scale.
Boiled water can taste stale. It consumes a lot of fuel and is a batch treatment system that requires separate water storage.
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Various Oxidizing Agents
Chlorine kills bacteria, including disease-causing organisms as well as the nuisance organism known as iron bacteria. However, low levels of chlorine, normally used to disinfect water, are not an effective treatment for giardia and/or cryptosporidium cysts. A chlorine level of over 10 mg/l must be maintained for at least 30 minutes to kill cysts.
Chlorine has been used since 1908 to disinfect water supplies in the United States to protect public health. The effectiveness of chlorination depends on the demand of the water, the concentration of the chlorine solution added, and the time that chlorine is in contact with the organism and water.
- As the concentration of the chlorine increases, the required contact time to disinfect decreases.
- Chlorination is more effective as water temperature increases.
- Chlorination is less effective as the water's pH increases (becomes more alkaline).
- Chlorination is less effective in cloudy (turbid) water.
When chlorine is added to the water supply, part of it combines with other chemicals in water (like iron, manganese, hydrogen sulfide, and ammonia) and is not available for disinfection. The amount of chlorine that reacts with the other chemicals plus the amount required to achieve disinfection is the chlorine demand of the water.
The safest way to be sure that the amount of chlorine added is sufficient is to add a little more than is required. This will result in a free chlorine residual that can be measured easily. This chlorine residual must be maintained for several minutes depending on chlorine level and water quality.
Kits are available for measuring the chlorine residual by looking for a color change after the test chemical is added. The test is simple and easy for a homeowner to perform. If chlorination is required for the water supply, the chlorine residual should be tested regularly to make sure the system is working properly.
The kit should specify that it measures the free chlorine residual and not the total chlorine. Once chlorine has combined with other chemicals it is not effective as a disinfectant. If a test kit does not distinguish between free chlorine and chlorine combined with other chemicals, the test may result in an overestimation of the chlorine residual.
Chlorine will kill bacteria in water, but it takes some time. The time needed depends on the concentration of chlorine. Two methods of chlorination are used to disinfect water: simple chlorination and superchlorination.
To ensure the proper contact time of at least 30 minutes, a holding tank can be installed. Pressure tanks, while often thought to be sufficient, are usually too small to always provide 30 minutes of contact time.
When the water cannot be held for at least 30 minutes before it is used, superchlorination is an alternative. For superchlorination, a chlorine solution is added to the water to produce a chlorine residual of between 3.0 and 5.0 mg/l, which is about ten times stronger than for simple chlorination. The necessary contact time for this concentration is reduced to less than five minutes. The water will have a very strong chlorine smell. If this is not desirable, the chlorine can be removed just before it is used with a carbon filter.
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Types of Chlorinators
Chlorine can be purchased in two formulations: calcium hypochlorite, which is a dry powder or tablet, and sodium hypochlorite, which is a liquid (commonly called chlorine bleach). Calcium hypochlorite dissolved in water or sodium hypochlorite are added to the water system through an injection pump. These pumps can be adjusted to add the prescribed amount of chlorine and are activated by the well pump. Other liquid chemicals (such as soda ash solutions) in addition to chlorine can be injected using the same pump.
Other types of chlorinators add chlorine tablets to the water supply, often at the well. They are called erosion and pellet chlorinators.
Pellet chlorinators consist of a canister to hold a supply of chlorine tablets and a chamber to allow water to flow over and dissolve the tablets. These units have the advantage of using chlorine tablets that are easy to handle and store. However, the chlorine dose they deliver tends to fluctuate greatly and is difficult to control. Tablet bridging occurs when the tablets get damp in the storage canister and stick together. Tapping the storage canister occasionally can help break down the bridging that occurs.
Pellet chlorinators can also be fitted to the top of a well and drop chlorine tablets directly into the well. A preset number of tablets are dropped in response to water being pumped. The well must be clear of obstructions to ensure that the tablets do not become lodged before reaching water level.
Some water supplies (mostly ponds and streams) contain some natural organic chemicals from the breakdown of plants and leaves. These organic chemicals (called precursors or dissolved organic contaminants - DOC's) can combine with chlorine to form chemicals called THMs or trihalomethanes. Trihalomethanes are suspected cancer-causing agents. Activated carbon filters can be used to remove THMs.
As with chlorine bleach, both solid and liquid formulations of chlorine are irritating to the skin and are poisonous in their concentrated form. They must be carefully handled and stored. Chlorine tablets must be stored in a dry location and both liquids and solids should be stored in their original labeled container away from children and animals. All chlorine solutions should be stored in a dark place, because light can cause a photochemical reaction which reduces their potency.
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One of the major disadvantages of chlorination is the production of disinfection by-products (DBPs) that are formed when chlorine interacts with contaminants in water. Some DBPs in chlorine-treated water have been found to raise the risks of various cancers, as well as birth and developmental defects.
To reduce this risk the EPA has encouraged water treatment systems to replace the use of chlorine with an alternative like ozone or chloramine, a
combination of chlorine and ammonia.
Chloramine has been used for more than 100 years to disinfect water. As with the other alternatives including chlorine-dioxide and ozone
these chemical treatments react to compounds present in a drinking water source, and the result is a variety of chemical disinfectant byproducts.
Some 600 DBPs have been identified since 1974. Scientists believe they've identified maybe 50 percent of all DBPs that occur in chlorine-treated water, but only 17 percent of those occurring in chloramines-treated water, 28 percent in water treated with chlorine-dioxide, and just 8 percent in ozone-treated water. Of the structurally identified DBPs, he said, the quantitative toxicity is known for maybe 30 percent.
When many municipal systems began to use chloramine instead of chlorine, the dangers of DBPs that result from the process were unknown. Now two classes of DBPs are known to be produced by the process, and they may be the most toxic ever discovered. For a more complete discussion of chloramines in drinking water, click here.
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Iodine kills bacteria and disease-causing organisms. Iodine is, however, ineffective as an algicide.
Iodine has been used to disinfect water since the early 1900s. In its natural state, iodine is a solid black crystal. Iodine crystals will dissolve in water, dependent on the water temperature. The higher the temperature, the more will dissolve. The simplest method of disinfecting water with iodine is by dissolving iodine in water to form a saturated solution and then injecting the iodine solution into a water system.
Iodine does not kill bacteria on contact; a holding time of at least 20 minutes is needed depending on the iodine concentration. An iodine residual of 0.5 to 1.0 mg/l should be maintained and iodine at this level gives the water little or no iodide taste or odor. Iodine can be removed from water with a carbon filter just before drinking.
Iodine dosage is very temperature dependent because iodine crystals are more soluble at higher temperatures. Iodine remains effective over a wide range of pH and does not lose effectiveness until the pH of water reaches 10. Iodine residuals in water can easily be measured using a test kit that indicates a color change.
Iodine tablets were developed during World War II to disinfect small amounts of water for emergency or temporary use. A few drops of tincture of iodine or iodine tablets are popular with campers and the military for disinfecting water.
The advantages of iodine are that:
- it does not require power
- it requires little maintenance
- provides residual treatment
- the residual is easy to measure
The disadvantages of iodine are that:
- health effects are undetermined
- effectiveness is affected by water temperature
- gives water a slight color at high levels
- gives water an iodine taste
- not effective as an algicide.
Ozone – or O3
– is often called Mother Nature’s purifier and disinfectant. The 3 stands for the three oxygen atoms that compose Ozone. The normal Oxygen we breathe is called O2
, and is made up of only two chemically linked Oxygen atoms.
You may have noticed that a sudden summer storm leaves behind a very distinct smell, sort of a “fresh scent” which lasts for about an hour. In this case, you smell Ozone, which has been creating from lighting bolts during the electrical storm. Ozone is also created by the Sun’s ultra violet rays.
While atmospheric Oxygen or O2
is a relatively stable molecule, Ozone or O3
is quite unstable or reactive by comparison. The extra oxygen atom wants to break away and combine with other substances, including organic substances like bacteria, and viruses. When this single Oxygen atom binds with the other substance, it causes it to oxidize or turn into something else. (Rust is an example of Iron oxidizing into Iron Oxide). When Ozone reacts with bacteria or viruses it destroys their cell membranes and DNA thereby killing them.
Ozonation water purification systems create Ozone with something called an Ozone Generator, which creates O3
in much the same way as the sun does. Inside ozone generator’s chamber is a high intensity Ultraviolet (UV) light. Compressed air is forced into the generator’s chamber, which then converts some of the oxygen in the air into Ozone. This process is part of the reason why the layer of Ozone in the Earth’s upper atmosphere protects us from most of the harmful UV rays emanating from our Sun.
The Ozone that has now been created inside the Ozone generator is then sent through a line into a diffuser, which creates ozone-saturated bubbles. Water is drawn in to mix with the bubbles, and then fed into the water purification tank. The weak Oxygen molecule in the Ozone attaches to other organic molecules in the water and oxidizes them. The result is fresh, purified drinking water.
The advantages of Ozone are that:
- it is powerful (over 3,000 times more effective than chlorine at disinfection and over 50 times more powerful as chlorine at oxidizing contaminants,)
- it is effective on all forms of microorganisms,
- it does not require long retention times,
- it does not add "off-tastes" or odors to water,
- it does not produce the toxic disinfection by-products of chlorine.
The disadvantages of Ozone are that:
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- it requires power to generate it at the point of use because it is a very unstable molecule that cannot be stored,
- it is highly corrosive so it requires special parts that are highly resistant to corrosion and a great deal of maintenance,
- while it may not produce the toxic disinfection by-products of chlorine, recent research indicates it does create aldehydes and other by-products that may have health consequences that are currently unknown,
- it requires water that is already fairly clear and clean.
Ultraviolet (UV) light has disinfection properties that kill bacteria, viruses, and some cysts. However, it will not kill giardia cysts.
While not yet allowed under the current rules of some state Department's of Health for private water supplies, the concept of using light to treat water supplies has been around for over 75 years. It has not been until recent times that home ultraviolet treatment systems have been available.
Water is passed through a disinfecting chamber containing a quartz mercury lamp that emits ultraviolet light rays. The ultraviolet irradiation kills or inactivates microorganisms almost instantly.
Ultraviolet light is a very effective disinfectant. However, disinfection only occurs inside the unit. No residual disinfectant is retained in the water to continue to kill bacteria that may be introduced into the water after it is disinfected.
The major differences in UV units are the capacity and optional features. Some units are equipped with UV detectors to warn the user when the unit is dirty or the light source is failing. These detectors must be properly calibrated and should not take the place of annual light source replacement and regular cleaning.
Since the ultraviolet light must reach the bacteria in order to kill them, the light source must be kept clean and the bulbs must be replaced on a regular schedule.
Bacteria could be shielded in cloudy water or water that is contaminated by large numbers of bacteria. An upper limit of the use of UV for disinfection is 1000 total coliforms/100 ml or 100 fecal coliforms/100 ml.
Substantial pretreatment is normally required for UV units. Prefilters are needed to remove discoloration, turbidity and organic particles. The water must be clear in order for the light to penetrate to kill the microorganisms. Water containing high mineral levels will cause a coating on the lamp sleeve, reducing the effectiveness of the treatment. Water softeners or phosphate injectors may be needed to prevent coating of the lamp.
The advantages of UV light are that:
- it does change the taste or odor of water,
- it kills bacteria almost immediately,
- it is compact and easy to use.
The disadvantages of UV light are that:
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- it requires power,
- there are no disinfection residuals,
- it requires pretreatment of cloudy or colored water,
- it requires cleaning and a new lamp at least annually.
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