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Comparing Disinfectants:

 Chlorination (Gas)

At normal pressures, elemental chlorine is a toxic, yellow-green gas, and is liquid at high pressures.


Chlorine is very effective for removing almost all microbial pathogens and is appropriate as both a primary and secondary disinfectant.


Chlorine is a dangerous gas that is lethal at concentrations as low as 0.1 percent air by volume. There is a resistance build up by some bacteria/virus.


Chlorine gas is released from a liquid chlorine cylinder by a pressure-reducing and flow-control valve operating at a pressure less than atmospheric. The gas is led to an injector in the water supply pipe where highly pressurized water is passed through a Venturi orifice creating a vacuum that draws the chlorine into the water stream. Adequate mixing and contact time must be provided after injection to ensure complete disinfection of pathogens. It will be necessary to control the pH of the water. 


A basic system consists of a chlorine cylinder, a cylinder-mounted chlorine gas vacuum regulator, a chlorine gas injector, and a contact tank or pipe. Prudence and/or state regulations would require that a second cylinder and gas regulator be provided with a change-over valve to ensure continuity of disinfection. Additional safety and control features may be required. A gas chlorinator should be installed in a room or chamber with direct emergency access to outside air and fitted with an exhaust fan ventilation system. Federal and state safety regulations must be observed. If not onsite, a self-contained breathing apparatus and a chlorine cylinder-repair kit should be available within a reasonable time frame and/or distance. 


Chlorine gas is supplied as liquid in high-pressure cylinders. 

Chlorination (Sodium-hypochlorite solution)

Sodium-hypochlorite is available as a solution in concentrations of 5 to 15 percent chlorine, but is more expensive than chlorine gas (available as chlorine).


Sodium hypochlorite is easier to handle than gaseous chlorine or calcium hypochlorite.


Sodium hypochlorite is very corrosive and should be stored with care and kept away from equipment that can be damaged by corrosion. Hypo-chlorite solutions decompose and should not be stored for more than one month. It must be stored in a cool, dark, dry area. There is a resistance build-up by some bacteria/virus


Sodium-hypochlorite solution is diluted with water in a mixing/holding tank. The diluted solution is injected by a chemical pump into the water-supply pipe at a controlled rate. Adequate mixing and contact time must be provided. 


A basic liquid chlorination system, or hypochlorinator, includes two metering pumps (one serving as a standby), a solution tank, a diffuser (to inject the solution into the water), and tubing. 


Sodium-hypochlorite solution is readily available. Sodium hypochlorite can also be generated onsite by electrolysis of sodium-chloride solution in specialized proprietary equipment. The only supplies required are common salt and electricity. Hydrogen is given off as a by-product and must be safely dispersed. 

Chlorination (Solid-calcium hypochlorite)

Calcium hypochlorite is a white solid that contains 65 percent available chlorine and dissolves easily in water. 


When packaged, calcium hypochlorite is very stable, allowing a year’s supply to be bought at one time. 


Calcium hypochlorite is a corrosive material with a strong odour that requires proper handling. It must be kept away from organic materials such as wood, cloth, and petroleum products. Reactions between calcium hypochlorite and organic material can generate enough heat to cause a fire or explosion. Calcium hypochlorite readily absorbs moisture, forming chlorine gas. Therefore, shipping containers must be emptied completely or carefully resealed.. There is a resistance build-up by some bacteria/virus 


Calcium hypochlorite may be dissolved in a mixing/holding tank and injected in the same manner as sodium hypochlorite. Alternatively, where the pressure can be lowered to atmospheric, such as at a storage tank, tablets of hypochlorite can be directly dissolved in the free-flowing water by a proprietary device that provides flow-proportional chlorination with gravity feed of the tablets. 


The equipment used to mix the solution and inject it into the water is the same as that for sodium hypochlorite. Solutions of 1 or 2 percent available chlorine can be delivered by a diaphragm-type, chemical feed/metering pump or by tablet chlorinator. 


Calcium hypochlorite can be purchased in granular, powdered, or tablet form. 


Chloramines are formed when water containing ammonia is chlorinated or when ammonia is added to water containing chlorine (hypochlorite or hypochlorous acid). 


An effective bactericide that produces fewer disinfection by-products, chloramine is generated onsite.

Usually, chloramine-forming reactions are 99 percent complete within a few minutes. 


Chloramine is a weak disinfectant. It is much-less effective against viruses or protozoa than free chlorine.

Chloramine is appropriate for use as a secondary disinfectant to prevent bacterial regrowth in a distribution system. Nitrogen trichloride appears to be the only detrimental reaction. It may be harmful to humans and imparts a disagreeable taste and odour to the water. The use of the proper amounts of each chemical reactant will avoid its production. There is a resistance build-up by some bacteria/virus 


Chlorine (gaseous solution or sodium hypochlorite) is injected into the supply main followed immediately by injection of ammonia (gaseous solution or as ammonium hydroxide). As before, adequate mixing and contact time must be provided. The mix of products produced when water, chlorine, and ammonia are combined depends on the ratio of chlorine to ammonia and the pH of the water. Chlorine-to-ammonia ratios of 5:1 should not be exceeded. If the pH drops below 5, some nitrogen trichloride may be formed. 


The generation of chloramines requires the same equipment as chlorination (gaseous or aqueous hypochlorination), plus equipment for adding ammonia (gaseous or aqueous). 


Chemicals used to generate chloramine from ammonia and chlorine gas depend on the ammonia-based chemical used. Anhydrous ammonia is the least expensive, while ammonium sulphate is the most expensive. 


Ozone, an allotrope of oxygen having three atoms to each molecule, it is the most powerful oxidizing and disinfecting agent practically available to man. It is formed by passing dry air through a system of high-voltage electrodes. There is no resistance build-up possible by any bacteria or virus as in the case of other mediums. 


Requiring the shortest contact time and lowest dosage of all, ozone is widely used, as a primary disinfectant in many parts of the world—but is relatively new to the U.S. Ozone leaves no residual toxins, flavours or chemicals and there is no resistance possible by any bacteria/virus. It also does not produce halogenated organic materials unless a bromide ion is present when used in combination, as a secondary disinfectant. 


Ozone gas is unstable and must be generated onsite. A secondary disinfectant, usually bromine or chlorine may be required when a long-term residual disinfectant in water needs to be maintained and where, for practical reasons, ozone cannot be reintroduced or recirculated. 


The five major elements of an ozonation system are: 

• Air preparation or oxygen feed;

• Electrical power supply;

• Ozone generation—usually using a corona discharge cell/s

• Ozone contact chamber; (if required)

• Ozone exhaust-gas destruction. (If Required) 


Ozonation equipment can include air-preparation equipment; an ozone generator, contactor, destruction unit; and instrumentation and controls. The capital costs of ozonation systems are relatively low. Operation and maintenance are relatively simple and the cheapest of all. Electricity represents 26 to 43 percent of total operating and maintenance costs for small systems and that consumption is very low being a high voltage, low amperage operation. 


For some applications, pure oxygen is a more attractive ozone feed gas than air because it: 

• has a higher production density;

• requires lower energy consumption;

• more than doubles the amount of ozone that can be generated per unit, and

• it requires smaller gas volumes for the same ozone output, thus lowering costs for ancillary equipment. 

Ultraviolet Light (UV)

Ultraviolet (UV) radiation is generated by a special lamp. When it penetrates the cell wall of an organism, the cell’s genetic material is disrupted and the cell is unable to reproduce. 


UV radiation effectively destroys bacteria and viruses. A secondary disinfectant must be used to prevent regrowth of micro-organisms. UV radiation can be attractive as a primary disinfectant for small systems because:

• it is readily available;

• it produces no known toxic residuals;

• it requires short contact times, and

• the equipment is easy to operate and maintain. 


UV radiation may not inactivate Giardia lamblia or Cryptosporidium cysts, and should be used only by ground water systems not directly influenced by surface water—where there is virtually no risk of protozoan cyst contamination. UV radiation is unsuitable for water with high levels of suspended solids, turbidity, colour, or soluble organic matter. These materials can react with or absorb the UV radiation, reducing the disinfection performance. 


The effectiveness of UV-radiation disinfection depends on the energy dose absorbed by the organism, measured as the product of the lamp’s intensity (the rate at which photons are delivered to the target) and the time of exposure. If the energy dosage is not high enough, the organism’s genetic material might only be damaged instead of destroyed. To provide a safety factor, the dosage should be higher than needed to meet disinfection requirements. 


UV lamps and a reactor. 


No chemical oxidant required; therefore, micro-organisms can be killed without generating by-products of chemical oxidation or halogenation. 

Source: National Drinking Water Clearinghouse.

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