Newsletter and Technical Publications
of Alternative Technologies for Freshwater Augumentation
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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.
Boiling is a very simple method of water disinfection. Heating water to
a high temperature, 100oC, 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 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
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
Figure 24: A Typical Hypochlorite Installation.
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.
Figure 25: A Typical Chlorine Cylinder Setup for Gas
Source: James M. Montgomery
Consulting Engineers, Water Treatment Principles and Design, Walnut Grove,
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
- Reading the product water flow meters and recording the amount of
- 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
- 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
- Reading the product water flow meters and recording the amount of
- 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.
The cost of boiling is related to the cost of the energy used in the
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.
||Tablets / Granules
||201 lb Cl2
||102 lb gas
|Total Cl residue
|Residue/ Cl2 ratio
||0.13 mg/lb Cl2
||0.46 mg/lb of Cl2
|% of available chlorine
Source: Margaret Dyer-Howe. General Manager,
Montserrat Water Authority, 1995.
Boiling is applicable everywhere, although it is now most often used in
emergencies or in rural areas where chlorinated public water supplies are
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.
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
- 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.
- 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
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
- 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
- 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.
Boiling is a widely accepted practice. Chlorination is a common practice
in water treatment plants in urban areas, but is rarely used in rural
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
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.
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
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.