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Continuous Chlorination

Information on related products can be found on the following pages:
Chlorine Injection Systems
Contact Tanks
Dechlorination Filters

Continuous chlorination can be an effective method for disinfection of drinking water. It also can be one step in the process of removing iron, manganese or hydrogen sulfide. Continuous chlorination should not be a substitute for a sanitary water supply. Protecting the water supply from contamination should be the primary goal for assuring good water quality.

What Contaminants Does Chlorination Remove

Purification of drinking water containing pathogenic (disease-causing) organisms requires disinfection treatment. Disinfection of drinking water destroys pathogenic bacteria, nuisance bacteria, viruses and other microorganisms, to produce a drinking water considered by public health officials to be essentially pathogen free. Chlorination is the most commonly used method for drinking water disinfection. In addition, chlorine will oxidize iron and manganese so they can be filtered out and also oxidize hydrogen sulfide to reduce nuisance odors.

Water used for drinking and cooking should be free of pathogenic organisms that can lead to illnesses such as typhoid fever, dysentery, infectious hepatitis and gastroenteritis. Pathogenic bacteria and viruses can be transmitted to humans by several routes, including contaminated drinking water supplies. Concentrated human and/or livestock populations near wells may result in contaminated water supplies. Contamination can also occur during or after well construction or repair, or flooding. Water can also be contaminated with naturally-occurring nuisance organisms such as iron bacteria, slime bacteria and sulfate-reducing bacteria. Disinfection by a process such as continuous chlorination is necessary for surface water supplies. It also is used to treat ongoing bacterial contamination and the process is similar to that used by public water utilities. Shock chlorination is used for new wells or after well repairs and is recommended for treating non-recurring bacterial contamination.

What Contaminants Does Chlorination Not Remove

Chlorine will not remove nitrate from water. Chlorination will not remove heavy metals, calcium and magnesium (hard water minerals), fluoride, and many other compounds. The concentration of chlorine typically applied for disinfection will not adequately destroy protozoan cysts such as Giardia and Cryptosporidium. These contaminants are not normally found in groundwater but may be present in contaminated surface water.

Water Testing

Regardless of the water treatment system being considered, the water first should be tested to determine what substances are present. Public water systems routinely are tested for contaminants. In contrast, monitoring of private water systems is the responsibility of the homeowner. Therefore, contamination is more likely to go undetected in a private water supply. Knowledge of what contaminants may be present in the water should guide the testing, since it is not economically feasible to test for all possible contaminants. It is essential to know what contaminants are present, their quantities and reasons for their removal (i.e., to reduce contaminants posing health risks, to remove tastes or odors, etc.) before selecting treatment methods or equipment. Private well owners should have their drinking water tested for bacteriological contamination at least once each year and after any repair or improvements in the well.

Coliform bacteria testing is used to indicate the presence of disease-causing bacteria in drinking water. Coliform bacteria are organisms found in soil and in the intestinal tract of warm-blooded animals. Coliform bacteria are not necessarily pathogenic, but are indicators of possible contamination (For example, if the water is positive for coliform bacteria, it indicates the possibility of contamination by soil or waste from humans or animals, which may contain pathogenic organisms).

Coliform presence in a drinking water sample does not necessarily mean the water is unsafe to drink. Since the test is a screening tool, a positive result should initiate additional testing, perhaps at different locations (well, pressure tank, etc.) in order to identify the source of the contamination. If the results reported show high coliform numbers, there is most likely considerable contamination and the water should not be consumed until the source of contamination is identified and eliminated, and/or the water is purified. In such situations, further water sample analysis for fecal coliform or E. coli should be initiated.

Chlorination Principles

The disinfecting effectiveness of chlorine depends on the concentration in the water, the amount of time the available chlorine is in contact with the water prior to use (contact time), the water temperature, water pH and the characteristics of the contaminants and water supply. When chlorine is added to water it reacts with microorganisms, certain chemicals, plant material, and compounds that can cause taste, odor or color in the water. These components "tie up" some of the chlorine and this is called the chlorine demand. The chlorine that does not react with these contaminants is free, or residual, chlorine. The breakpoint is that concentration of chlorine that just meets the chlorine demand so that a higher concentration would allow for some residual chlorine. It is important to have enough chlorine in the water to meet the chlorine demand and allow for residual disinfection. Test kits are available for testing chlorine in private systems; be certain the kit you purchase tests free chlorine, not total chlorine.

The concentration of free chlorine necessary for adequate disinfection is system-dependent; the chlorine concentration is depended on the amount and type of contamination present, water pH and temperature, etc. Household chlorination systems may provide a higher free chlorine concentration than the typical 0.3 - 0.5 ppm (parts per million) concentration used for chlorination of public water supplies. The distribution system of a public water system provides a much longer contact time than a household plumbing system so a lower concentration may be used for disinfection. Piping in home water systems generally provides very limited contact time for chlorination since the time between the pump and the nearest faucet is usually one minute or less. For this reason, it is usually recommended to install a separate storage tank to increase the contact time.

The reaction of chlorine with trace concentration of naturally occurring organic matter can produce compounds such as trihalomethanes (THMs) as by-products. These disinfection by-products may increase the risk of certain cancers. The EPA mandates that public water systems have less than 80 parts per billion (ppb) of THMs in their treated water. Activated carbon filtration can be effective in removing chlorine and some disinfection by-products from drinking water.

Chlorine concentrations used for disinfection in water are not toxic to humans or animals. The concentration can be high enough, however, to create a taste or odor that some people find objectionable. Activated carbon filtration following chlorination may be used to remove the taste and odor.

Equipment

Chlorine is available in dry form as either a powder or pellets (calcium hypochlorite) or in liquid form (sodium hypochlorite). Both forms of chlorine must be stored in accordance with the manufacturers' recommendations for safety purposes and to maintain the chemical integrity of the product.

The common types of chlorinators for continuous chlorination of a home drinking water supply include chemical feed pumps, and tablet chlorinators. Because the effectiveness of the disinfection is a function of contact time, in each type of chlorinator the chlorine should be introduced into the water as close to the source as possible. This will allow the chlorine a longer contact time with the water.

Figure 1 below shows a chemical feed pump chlorinator. A fixed amount of chlorine solution is delivered with each pump discharge stroke. The amount delivered can be adjusted by changing the length of the discharge stroke, the speed of the pump, or the running time of the pump. The feed pump should be wired to the water pump pressure switch so that the chemical pump operates only when the water pump is operating. A contact tank for additional contact time, and a carbon filter, for dechlorination and removal of precipitated contaminants, are also shown.

Figure 1: Chlorine Injection System
chlorination schematic


Figure 2 illustrates a dry pellet chlorination system. Chlorine pellets are dropped directly into the well. The diagram also shows a dechlorinating carbon filter and a contact tank, although the tank is often not required due to the additional contact time inside the well.

Figure 2: Pellet Chlorination System
pellet chlorination schematic

Summary

Drinking water treatment using continuous chlorination disinfects a water supply. It destroys pathogenic bacteria, nuisance bacteria, viruses, some parasites and other microorganisms. It also oxidizes iron and manganese so they can be filtered out, and oxidizes hydrogen sulfide in order to reduce nuisance odors.

 
 

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