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PART B. TECHNOLOGY PROFILES

2.4 Disinfection by Boiling and Chlorination

Boiling and chlorination are the most common water and wastewater disinfection processes in use throughout the world. Boiling is primarily used in rural areas in developing countries to eliminate living organisms, especially bacteria, present in the water. It is also used in emergencies when other, more sophisticated methods of disinfection are not available. Prior to the development of chlorination, boiling was the principal method used to kill pathogenic organisms.

Technical Description

Boiling

Boiling is a very simple method of water disinfection. Heating water to a high temperature, 100^oC, kills most of the pathogenic organisms, particularly viruses and bacteria causing waterborne diseases. In order for boiling to be most effective, the water must boil for at least 20 minutes. Since boiling requires a source of heat, rudimentary or non-conventional methods of heat generation may be needed in areas where electricity or fossil fuels are not available.

Chlorination

Chlorination has become the most common type of wastewater and water disinfection. It should be noted that it is designed to kill harmful organisms, and generally does not result in sterile water (free of all microorganisms). Two types of processes are generally used: hypochlorination, employing a chemical feed pump to inject a calcium or sodium hypochlorite solution, and gas chlorination, using compressed chlorine gas.

Hypochlorination. Calcium hypochlorite is available commercially in either a dry or wet form. High-test calcium hypochlorite (HTH), the form most frequently used, contains about 60% available chlorine. Because calcium hypochlorite granules or pellets are readily soluble in water and are relatively stable under proper storage conditions, they are often favored over other forms. Figure 24 shows a typical hypochlorite installation.

Sodium hypochlorite is available in strengths from 1.5% to 15%, with 3% available chlorine as the typical strength used in water treatment applications. The higher the strength of the chlorine solution, the more rapidly it decomposes -and the more readily it is degraded by exposure to light and heat. It must therefore be stored in a cool location and in a corrosion-resistant tank. Typically, 30 minutes of chlorine contact time is required for optimal disinfection with good mixing. Water supply treatment dosages are established on the basis of maintaining a residual concentration of chlorine in the treated water.

Water-based solutions of either the liquid or the dry form of hypochlorite are prepared in predetermined stock solution strengths. Solutions are injected into the water supply using special chemical metering pumps called hypochlorinators. Positive displacement types are the most accurate and reliable and are commonly preferred to hypochlorinators employing other feed principles (usually based on suction). Positive-displacement-type hypochlorinators are readily available at relatively modest costs. These small chemical-feed pumps are designed to pump (inject under pressure) an aqueous solution of chlorine into the water system. They are designed to operate against pressures as high as 100 psi, but may also be used to inject chlorine solutions under ambient (atmospheric) or negative head conditions. Hypochlorinators come in various capacities ranging from 3.8 to 227 l/day . Usually, the pumping rate is manually adjusted by varying the stroke of the pump's piston or diaphragm. Once the stroke is set, the hypochlorinator accurately feeds chlorine into the system at that rate, maintaining a constant dose. This works well if the water supply rate and the output of the pump are fairly constant.

(larger image)

Figure 24: A Typical Hypochlorite Installation.
Source: Liguori P. Small, Water Systems Serving the Public, Washington, D.C., U.S. Environmental Protection Agency, 1978.

Montserrat has been using floating chlorinators, but in response to concern expressed by the Director of Health Services that they leave chlorine residues in the water supply and that "the chlorine values are generally too low to guarantee safety," the Montserrat Water Authority looked into various other methods and decided on gaseous chlorine. It is now proceeding cautiously to replace floating chlorinators with gas chlorination as treatment plant operators are trained in the new system.

Gas Chlorination. In gas chlorination systems, chlorine is supplied as a liquefied gas under high pressure from containers varying in size from 100 lb to 1 ton or from tank cars for larger sizes. Cylinders in use should be set on platform scales flush with the floor; the loss of weight is used as measure of the dosage. The following precautions have to be taken when handling chlorine gas:

• Chlorine gas is both very poisonous and very corrosive; adequate exhaust ventilation at floor level must be provided since chlorine gas is heavier than air.

• Chlorine-containing liquids and gases can be handled in wrought-iron piping; however, chlorine solutions are highly corrosive and should be handled in rubber-lined or corrosion-resistant plastic piping with hard rubber fittings where necessary.

• Pressurized chlorine gas should never be piped in silver, glass, Teflon, or other piping material that cannot handle the pressure; exposure to concentrated chlorine gas can be fatal.

(larger image)

Figure 25: A Typical Chlorine Cylinder Setup for Gas Chlorination Treatment.
Source: James M. Montgomery Consulting Engineers, Water Treatment Principles and Design, Walnut Grove, Cal., 1985.

A gas chlorinator meters the gas flow and mixes the gas with water. The resulting chlorine solution is then injected into the product water. Small water supplies can be effectively served by a 100 or 150 lb container; larger containers are not recommended for small systems, as they require special hoists and cradles. (Chlorine gas is a highly toxic lung irritant compound and special facilities are required for storing and housing gas chlorinators.) The advantage of this method, however, is the convenience afforded by the relatively large quantity of chlorine gas available for continuous periods of operation lasting several days or weeks, without the need to mix chemicals. Figure 25 shows a typical chlorine gas cylinder system for gas chlorination treatment.

Extent of Use

Boiling is a primary technology used to control the spread of waterborne diseases. It is a traditional technology that was used prior to the advent of existing technologies. It is still used in areas where the energy supplies and modern facilities needed for other technologies are lacking, and in areas where the quality of the water supply is questionable.

The most common system of disinfection in Latin America and the Caribbean is chlorination. Chlorine tablets, liquid, powder, and gas are widely used. Chlorination of water supplies on an emergency basis was practiced in the region as early as about 1850. At present, chlorination of both water supplies and wastewater is widespread. Chlorination for disinfection is used to prevent the spread of waterborne diseases and to control algal growth and odors. Economics, ease of operation, and convenience are the main factors used to evaluate disinfection processes.

For safety, and to ensure a constant supply of chlorine, on-site generation is recommended. Most commercially available chlorine generation equipment will operate on waters ranging in salinity from freshwater to seawater, and also on brine solutions prepared for the purpose. Hypochlorite solutions prepared from seawater are usually limited to about 1 800 mg/l of available chlorine, and those produced from brine to about 8 000 mg/l. Heavy metal ions present in seawater interfere with the stability of hypochlorite solutions prepared using water from this source.

Operation and Maintenance

Gas Chlorinators. Gas chlorinators have an advantage in situations where water flow rates are variable, because the chlorine feed rates may be synchronized to inject variable quantities of chlorine into the product water. Capital costs of gas chlorination, however, are somewhat greater, but chemical costs may be less. Normal operation of a gas chlorinator requires routine observation and preventive maintenance. Daily duties of an operator should include the following tasks:

• Reading the chlorinator rotameter daily and recording the information.

• Reading the product water flow meters and recording the amount of water pumped.

• Checking the chlorine residual levels in the distribution system and, as necessary, adjusting the rotameter to increase the feed rate if they are too low and decrease it if they are too high.

• Calculating the chlorine usage, and ordering further chlorine stocks if necessary.

• Cleaning the equipment and the building weekly, cleaning the "Y" strainer three times a week, and replacing the gaskets periodically.

• Performing preventive maintenance on the equipment.

Hypochlorinators. Because of its oxidizing potential, calcium hypochlorite should be stored in a cool, dry location, away from other chemicals, in corrosion-resistant containers. Operators should perform the following maintenance tasks:

• Reading and recording the level of the solution tank at the same time every day.

• Reading the product water flow meters and recording the amount of water pumped.

• Checking the chlorine residual levels in the system and adjusting the chlorine feed rate as necessary, in order to maintain a chlorine residual level of 0.2 mg/l at the most remote point in the distribution system (the suggested free chlorine residual for treated water or well water is 0.5 mg/l at the point of chlorine application, provided that the 0.2 mg/l concentration is maintained throughout the distribution system). The chlorine feed rate of a floating chlorinator must be adjusted daily to increase or decrease the dosage in conformity with the water output of the treatment plant.

• Checking and adjusting the chemical feed pump operation; most hypochlorinators have a dial indicating the chlorine feed rate, with a range from 0 to 10, the pointer of which should initially be set to approximately 6 or 7, when using a 2 % hypochlorite solution. The pump should be operated in the upper ranges of the dial to ensure that the strokes or pulses from the pump are frequent enough so that the chlorine will be fed continuously into the water being treated.

• Replacing the chemicals and washing the chemical storage tank as necessary so that a 15-to 30-day supply of chlorine is on hand to meet future needs; hypochlorite solutions, however, should be prepared only in quantities needed for two to three days of operation, in order to preserve their potency.

• Checking the operation of the check valve.

• Inspecting and cleaning the feeder valves. Commercial sodium hypochlorite solutions (such as Clorox) contain an excess of caustic soda (sodium hydroxide, NaOH); when diluted with highly alkaline water, they produce a solution that is supersaturated with calcium carbonate, which tends to form a coating on the valves in the solution feeder. Similarly, in systems using calcium hypochlorite (HTH), when sodium fluoride is injected at the same point as the hypochlorite solution the calcium and fluoride ions combine and form a coating. The coated valves will not seat properly and the feeder will fail to chlorinate the product water properly. (Small hypochlorinators are sealed so that they cannot be repaired without replacing the entire unit. Otherwise, they require very little maintenance, mostly consisting of a periodic oil change and lubrication.)

Frequent visits are required to the chlorination points in the distribution system to make adjustments, to clear PVC tubing of sludge formation that stops tablets from dissolving, and to recharge tablets.

Level of Involvement

Boiling is exclusively the responsibility of individual users.

Chlorination is normally conducted by the private sector in small-scale hypochlorite treatment systems. Regional or large-scale systems require the involvement of a public utility or regional water supply authority, particularly if gas chlorination is used. For large systems, government involvement and financing are required.

Costs

The cost of boiling is related to the cost of the energy used in the process.

The cost of chlorination systems varies considerably depending on the geographic location and the type of chlorination system used. Table 11 shows a comparison of capital costs of two different chlorination systems.

TABLE 11. Comparison of Capital Costs of Chlorination Systems ($).

Source: Margaret Dyer-Howe, General Manager, Montserrat Water Authority, 1995.

Effectiveness of the Technology

Boiling is a very effective disinfection technology, but it is recommended only as a backup to other technologies because of its volume limitations and energy requirements.

Chlorination is a very effective and well-known technology. Its effectiveness is a function of the quality of the water that is being chlorinated and the method of chlorination used. Normally gas chlorination is a more efficient method of disinfection, although a system based on the use of hypochlorite tablets is easier to operate and maintain and is preferred by individual users. Table 12 shows a comparison of the two methods as used on the Caribbean island of Montserrat.

TABLE 12. Technological Efficiency of Chlorination Methods.

Source: Margaret Dyer-Howe. General Manager, Montserrat Water Authority, 1995.

Suitability

Boiling is applicable everywhere, although it is now most often used in emergencies or in rural areas where chlorinated public water supplies are not available.

Chlorination can be used in most areas depending on the availability of chemicals. Gas chlorination, however, is best used in controlled situations such as provided by a public water utility.

Advantages

As was noted above, boiling, while an effective technology, is generally considered to be a secondary or emergency means of disinfecting water supplies. For this reason, the following advantages refer to chlorination systems:

• The systems are extremely reliable; the hypochlorite system is somewhat easier to operate than the gas system because the operators need not be as skilled or as cautious.

• Chlorination is less costly than other disinfection systems and is generally easier to implement; chlorine (Cl2) can be made in the region and safety considerations for its production, transportation, and use are well known.

Hypochlorinator system:

• Hypochlorite compounds are non-flammable.

• Hypochlorite does not present the same hazards as gaseous chlorine and therefore is safer to handle; spills may be cleaned up with large volumes of water.

• Floating chlorinators can be adapted to small community systems or individual rainwater collector systems. They easy to construct and to transport. However, they cannot easily guarantee uniform residual chlorine concentrations.

Gas feeder system:

• Gas feeder systems are fitted with valves to automatically close the vacuum regulator in case of leaks or accidental breaks in the vacuum line, stopping gas flow at source.

• The systems have an automatic shut-off in case of interruption of feedwater supplies.

• The use of chlorine gas is cheaper and cleaner. Chlorine supplies last approximately three months.

• Dosage rates and the resulting chlorine residual can be accurately controlled.

Disadvantages

• Boiling requires a reliable source of energy and is limited in terms of the volume able to be treated.

• The use of chlorine in gaseous form or in solution can cause safety hazards; all operating personnel should be made aware of these hazards and trained in their mitigation.

• Chlorine is reactive and interacts with certain chemicals present in the product water, depending on pH and water temperature; this results in the depletion of the chlorine concentration, leaving only residual amounts of chlorine for disinfection (over-chlorination may result in the formation of chlorinated hydrocarbons, such as trihalomethanes, which are known to be carcinogenic).

• Chlorine will also oxidize ammonia, hydrogen sulfide, and metals present in the product water to their reduced states.

• Chlorine gas is heavier than air, and is extremely toxic and corrosive in moist atmospheres. Dry chlorine can be safely handled in steel containers and piping, but where moisture is present (as it is in most treatment plants), corrosion-resistant materials such as silver, glass, Teflon, and certain other plastics must be used-though not, as was said above, for pressurized gas.

• Hypochlorite may cause damage to eyes and skin upon contact, and, because it is a powerful oxidant, may cause fires if it comes into contact with organic or other easily oxidizable substances.

Cultural Acceptability

Boiling is a widely accepted practice. Chlorination is a common practice in water treatment plants in urban areas, but is rarely used in rural areas.

Further Development of the Technology

Boiling and chlorination are very well known technologies used by most of the world's population for the routine and/or emergency disinfection of water supplies and wastewaters. Nevertheless, chlorination systems could be improved primarily in the area of safety both in the production of chlorine gas and the methods of handling and distributing the gas within the treatment plants. Development of corrosion-resistant materials that are not affected by chlorine could increase the frequency of utilization of gas chlorination, which is a more efficient method of disinfection than hypochlorite. Hypochlorite production methods, using seawater and brackish water as source waters for the production of chlorine solutions, could also be improved, to reduce the cost and to make use of the by-products of this process.

Sources of Information

Contacts

Margaret Dyer-Howe, General Manager, Montserrat Water Authority, Post Office Box 324, Church Road, Plymouth, Montserrat, BWI. Tel. (809)491-8440. Fax (809)491-4904.

José Payero, Profesor, Investigador, Departamento de Recursos Naturales, Instituto Superior de Agricultura (ISA), Apartado 166, La Herradura, Santiago, República Dominicana. Tel. (809)247-0082. Fax (809)247-2626.

Bibliography

Bello, J.D., and M. Acosta. 1993. Análisis de la Aceptación de las Empresas Purificadoras de Agua en la Ciudad de Santiago. Santiago, República Dominicana, Pontificia Universidad Católica Madre y Maestra. (Tesis)

Man, H.T., and D. Williamson. 1986. Water Treatment and Sanitation: Simple Methods for Rural Areas. London, Intermedia Technology Publications.

Martin, Edward J. 1988. Handbook for Appropriate Water and Wastewater Technology for Latin America and the Caribbean. Washington, D.C., PAHO and IDB.

-, and E.T. Martin. 1983. Examination of the Water Supply and Sewerage Rehabilitation Needs for Selected Cities in Ecuador. Washington, D.C., PAHO and IDB.

-, and -. 1985. Water and Wastewater Cost Analysis Handbook for Latin America and the Caribbean. Washington, D.C., PAHO and IDB.

Montgomery, James M., Consulting Engineers. 1985. Water Treatment Principles and Design. Walnut Grove, California.

PAHO. 1978. "Evaluation of the Utilization of New Technology in Water Treatment in Latin America." Paper presented for the Seventeenth Meeting of the PAHO Advisory Committee on Medical Research, Lima, Peru. Washington, D.C.

Small, Liguori P. 1978. Water Systems Serving the Public. Washington, D.C., USEPA.

USEPA. 1980. Innovative and Alternative Technology Assessment Manual. Washington, D.C. (Report No. EPA-430/9-78-009)

White, G. 1986. The Handbook of Chlorination, 2nd ed. New York, Van Nostrand Reinhold.

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