Historically, chlorine compounds were first used as disinfectants around 1800 by De Moveua in France and by Cruikshank in England. Most early applications were for odor control in waste treatment. In 1854, chlorinated lime was being used in a London sewage facility for deodorizing sanitary waste.20 Chlorine for drinking water treatment is commonly available in three forms: gas, liquid solution and solid tablet or powder. Public water systems frequently use chlorine gas (Cl2), but this form is expensive and involves safety risks in handling for small-scale water treatment. Chlorine is also available in dry form, such as calcium hypochlorite, which is commercially available as a soluble powder or as tablets sold under such trade names as HTH or Perchloron. These compounds are classed as high-test hypochlorites (HTH), ranging from 65 to 75 percent available chlorine by weight.21
Liquid sodium hypochlorite solution is commonly used in residential and business chlorination systems. This is sold in supermarkets as household bleach and consists of a 4.0 to 5.25 percent solution of available chlorine. Swimming pool chlorine solutions run from 10 to 15 percent available chlorine solutions by weight.22
The use of chlorine feed has been mentioned in earlier chapters of this text, and its versatile use as a strong oxidizing agent has been discussed for other applications. The liquid chlorine solutions will be covered in this section in conjunction with the chemical feed pump method.
The most common feeder is the positive-displacement hypo chlorinator, which uses a piston or diaphragm pump to inject the solution into the water stream. This type of equipment, which is adjustable during operation, is reliable and accurate for measured feed levels. The action of the electrically powered hypochlorinator can be synchronized with the operation of the water pumping system. These feed pumps are adjustable so that it is possible to increase the feed rate to the proper level (Figure 10-8).
Hypochlorination involves several factors: dosage levels in ppm-mg/L, chlorine demand, residual and necessary contact time. Dosage is the amount of chlorine fed into the water system, expressed as ppm or as mg/L. A set amount of chlorine fed into water will oxidize or combine with other water constituents, such as ferrous iron, manganese, hydrogen sulfide, or organic matter, withdrawing them from availability during the disinfection task and action. The amount of chlorine consumed in reactions with water constituents is known as chlorine demand.
The chlorine remaining in the water after the demand is filled is known as residual. If ammonia nitrogen is present in the water supply, some chlorine will combine with it to form chloramines, which have only mild germicidal ability. When there is no ammonia in the water supply, the remaining chlorine is called free-chlorine residual. This has 25 to 100 times the disinfecting capability of chloramines.23
Contact time is the period that a chlorine residual is maintained in the water. Generally, the suitable contact time for disinfection is 20 to 30 minutes, and the chlorine dosage level should be great enough to provide a free-chlorine residual of 0.2 to 0.5 ppm-mg/L at the end of the contact time. When the pressure tank is not large enough to provide adequate time, a second retention tank can be installed in series.24
Chlorination is popular as a disinfection process because it is an effective biocide and is able to maintain a residual level that provides ongoing antibacterial protection as water flows through water pipes and plumbing to the point of use. Chlorine is effective on many types of pathogens, has adequate pH and temperature ranges, is reasonably priced and readily available, and can be easily monitored with simple test kits.25
Chlorinated organics in the water do create trihalomethane (THM) by-products. At higher levels, some of these chlorinated compounds are considered suspect carcinogens.