Depending on the water source, disinfection may be a simple or complex matter. Ground water from deep, drilled wells may not need disinfection if regular bacteriological tests show that it is safe. Shallower wells or dug wells almost certainly will require disinfection, and surface waters probably would benefit from fine filtration as well.
There are a number of ways of purifying water. In evaluating the methods of treatment available, the following points regarding water disinfectants should be considered:
Because no single type of disinfectant meets all these criteria, a certain amount of compromise is necessary in making a selection. The type of equipment used to dispense the disinfectant during the water-treatment process is as important as the disinfectant. Desirable characteristics include the following:
Because no system can meet all these criteria, the system selected, like the disinfecting agent itself, will strike a compromise among the various criteria.
There are two kinds of filtration: particulate removal, which is for the most part a physical sieving or straining process, and adsorption, which is mainly chemical in nature. Adsorption is the attraction and bonding of certain dissolved substances, and somtimes of extremely tiny particles, onto the surfaces of large particles, using a force which acts something like magnetism. Adsorption filtration is commonly used to remove taste and odor causing organic substances, chlorine, pesticides, other toxic organics, and even asbestos fibers and viruses. Depending on the design, construction, and type of medium used, a filter may be able to remove any one, or possibly many types of contaminants from water, to varying degrees.
In the treatment of surface waters which may contain large, resistant parasites such as protozoan cysts and worm eggs, fine filtration (fine particulate removal) is a neccesary companion to chemical disinfection. Although filters capable of removing 99.9 percent of bacteria from water may be purchased, they are not intended for this purpose. No filter is perfect; some bacterial and viral pathogens can always be expected to pass through, so one should never depend on filtration to do the entire job of disinfection. The types and ranges of fine-particulate filtration methods are the following:
NOTE: Filters designated as "granular filters," "bed filters," or "depth filters," that do not have a specific (absolute) micrometer rating of 3uM or less are not acceptable for the removal of protozoan cysts and larger organisms.
Chlorine is a disinfecting agent used extensively to treat water for municipal and individual supplies. Its popularity is owed to the fact that both the disinfectant and the treatment system meet nearly all the desirable criteria described in the preceeding section.
Chlorine is commonly available in three forms: solid, liquid, and gas. Municipalities and community water systems frequently use chlorine gas (CL2) for water purification. In its gaseous state, however, chlorine is not safe to handle, and the equipment required to deal with it is too expensive for use in treating individual water-supply systems. Liquid sodium hypochlorite is commonly used in domestic chlorination systems. Sold in grocery stores as household bleach, this product consists of a 5.25% solution of sodium hypochlorite, which is equivalent to 5% available chlorine. Other available sodium hypochlorite solutions range in strength from 3-15% available chlorine by weight. These solutions can be diluted with potable water to produce the desired solution strength. Chlorine solutions should be stored in a cool, dark place if they are to maintain their designated strength because light produces a photochemical reaction that reduces their potency.
The most common kind of positive displacement hypochlorinator uses a piston or diaphragm pump to inject the solution. This type of equipment, which is adjustable during operation, can be designed to operate at reliable and accurate feed rates. The starting and stopping of electrically powered hypochlorinators can be synchronized with those of the pumping unit. Although hypochlorinators can be used with any water system, they are especially desirable in systems where water pressure is low or fluctuating.
Determination of Proper Chlorine Dosage
Proper hypochlorination involves four basic factors: dosage, demands, residual, and contact time. Dosage is the amount of chlorine fed into the water system, expressed as milligrams of chemical per liter (mg/L). A set amount of chlorine, fed into the water, will oxidize or combine with chemicals such as ferrous iron, manganese, hydrogen sulfide, or nitrate, withdrawing them from availability during disinfection action. The amount of chlorine required is known as the chlorine demand.
The chlorine remaining in the water after the demand is filled, is known as the residual. If ammonia nitrogen is present in the water, some chlorine will combine with it to form chloramines, which have only mild germicidal capability. When there is no ammonia in the water, the remaining chlorine is called the free-chlorine residual. This has a 25-to-100-times greater disinfecting ability that do the chloramines.
Contact time is a period that elapses between the addition of chlorine and the water's use. Suitable contact time is required for the disinfecting action to occur. Contact times of 20 to 30 minutes are recommended, and the chlorine dosage should be great enough to provide a free-chlorine residual of 0.2 to 0.5 mg/L.
If a pressure tank does not provide adequate contact time for chlorination, the time of detention can be increased by adding a gravity tank or reservoir, by placing multiple tanks in series, or by adding lengths of tubing. In some cases contact time can be increased by feeding the chlorine into the well below the pump. To avoid corrosion, the feed tube in a well should extend at least a foot below the pump assembly.
The primary factors that determine the biocidal efficiency of chlorine include the following:
Determination of Chlorine Residual
The practice of free-residual chlorination became widespread around 1939. This practice consists of adding enough chlorine to produce a residual made up almost entirely of free available chlorine. Because of its many advantages, including ease of control, the practice of free residual chlorination is recommended for individual water supply systems. If ammonia is present in the water, a free-chlorine residual can be obtained by adding sufficient chlorine to combine with all the ammonia nitrogen and form a compound known as nitrogen trichloride. Once this is done, the addition of any further chlorine will produce a free-chlorine residual.
Levels of free-chlorine residual can be easily and accurately determined by the DPD Colorimetric Method. The reagents required for a free-residual chlorine determination are contained in one tablet. When this tablet is added to a sample containing chlorine residual, a red color is produced that can be matched to a standard color comparator. Test kits for measuring free-residual chlorine levels of 0 to 10 mg/L are commercially available. We also offer free water testing in our store.