Newsletter and Technical Publications
<Sourcebook of Alternative Technologies for
in Small Island Developing States>
PART B - ALTERNATIVE TECHNOLOGIES
1. TECHNOLOGIES GENERALLY APPLICABLE TO ISLAND STATES
1.2 Water Quality Improvement Technologies
Boiling water may be effective as a method of disinfection, but it is
not practical for large quantities. Sunlight can also act as a natural
method of disinfection, but it is difficult to control and manage. Thus,
chemical disinfection, principally using chlorine products, is practised
extensively in SIDS. Disinfection also may be achieved with ultraviolet
(UV) light, and is best suited to individual household applications,
although larger-scale units are being used in some countries. However, UV
disinfection requires a power supply is required and does not provide the
residual level of disinfection in the water as chlorine does.
Chlorine compounds, at their optimum dosage, will inactivate
disease-causing organisms within 30 minutes. They are widely used and are
relatively inexpensive. If carefully applied, chlorine has the advantage
that a measurable residual of chlorine in solution can be maintained in
the water supply. This residual provides further protection from possible
re-contamination and is also an important indicator of successful
application. There is a very wide range of chlorination methods in use on
small islands, ranging from the very simplest of methods to the most
complicated. In general, the most commonly used methods are bleaching,
injection of hypochlorite solution, and injection of chlorine gas.
Bleaching, using powdered or liquid chlorinated lime and sodium
hypochlorite of different commercial brands, is widely used for
sterilizing small water supplies such as household storage tanks.
Injection of hypochlorite solution, either by gravity feed or by special
chlorine pumps, add measured amounts of a sodium hypochlorite solution to
the product water to achieve disinfection. This is the method most widely
used on small islands as it is the most suitable method for use in small,
unsophisticated water work. In large water supply systems, chlorine gas is
often injected into the product water. The chlorinator is a fairly
complicated apparatus which reduces the pressure of the gas leaving the
cylinders, controls the rate of flow, mixes the gas with water, and
delivers it to the pump or injector which forces it into the filtered
water prior to discharge to storage.
Extent of Use
Chlorination, as a disinfection method, is widely used throughout SIDS,
and is, in many cases, the only means of water quality improvement
Operation and Maintenance
Chlorination requires trained operators as it is important that the
dosing is correct in order to achieve the optimum effect of the
chlorination. Over chlorination can lead to the production of
trihalomethanes which have been implicated as potential carcinogens.
Operation of a liquid chlorination unit mainly consists of preparing the
correct solution of sodium hypochlorite and testing for residual chlorine
on a regular basis to ensure that the dosing is correct. The maintenance
of the unit consists of keeping all parts operable and clean as chlorine
can be very corrosive. Gaseous as well as granular chlorine is very
corrosive and requires careful handling and storage.
Level of Involvement
Trained personnel are required to operate and maintain the various types
of chlorinators to ensure proper chlorine dosage and adequate safety
procedures. With some training, individuals may be able to disinfect their
water supply by adding the correct dosages of dilute liquid chlorine to
their storage tanks.
The costs involved are usually restricted to the purchase of sodium
hypochlorite, laboratory test equipment to measure chlorine residuals, and
power when pumps are used.
Effectiveness of the Technology
The effectiveness of disinfection is influenced by the turbidity of the
water being treated, the concentration of the disinfectant, the contact
time provided, and the chemical character, pH and temperature of the water
being treated. To be effective, chlorine must be applied to water with a
low turbidity. If there is high turbidity and/or chlorine resistant
organisms such as Giardia cysts likely to be present in the source water,
sand filtration prior to chlorination may be required. The resistance of
different species of organisms to a disinfectant varies considerably. For
instance, cysts and viruses can be quite resistant to chlorination,
requiring storage dosage and/or longer contact times than would be
required to eliminate bacterial organisms. Normal conditions of
chlorination (i.e., resulting in a free residual chlorine of > 0.5 mg/l
after at least 30 minutes contact at a pH of less than 8.0, and a
turbidity of less than 1 NTU) can bring about more than a 99% reduction in
E. coli numbers as well as numbers of certain viruses, but not of the
cysts or oocysts of parasitic protozoa (WHO, 1993). Nevertheless, in
general, chlorination is a very efficient method of disinfecting water
supplies if the above-mentioned factors are taken into consideration.
Chlorination is, in most cases, a very suitable means of disinfecting
potable water. However, in the case of rainwater catchment tanks and dug
wells, the effectiveness of the method varies depending on the method and
dosage of chlorine. Furthermore, the suitability of chlorination as a
disinfection technology may be determined by the availability of a ready
and dependable supply of the disinfectant at reasonable cost.
Chlorination can result in the destruction of disease-producing
organisms, and can be relatively easily applied to a range of treatment
Testing is required to monitor the level of residual chlorine. Further,
a ready and reliable supply of sodium hypochlorite is not always
available, and special care is required in the handling and storage of
chlorine and hypochlorite, especially gaseous chlorine.
The taste and odour of chlorine can become offensive to consumers,
especially if the chlorine dosage is too high.
Further Development of the Technology
For rural water supplies, there is a need to develop more reliable and
simple methods of chlorination, as well as simplified methods for
monitoring and adjusting the chlorine residual in the product water.
American Water Works Association 1971. Water Quality Treatment.
Third Edition. McGraw-Hill Book Company, New York.
APHA [American Public Health Association] 1985. Standard Methods for
the Examination of Water and Wastewater. 16th Edition. American Public
Health Association, American Public Works Association, and Water Pollution
Control Federation, Washington.
Bridgen, J. 1989. High Volume Potable Water Disinfection with Medium
Pressure U.V. Systems. In: Proceedings of the Seminar on Water
Management in Small Island States. Cyprus Joint Technical Council and
Commonwealth Engineer's Council, 103-105.
James M. Montgomery Consulting Engineers, Inc. 1985. Water Treatment
Principles and Design. John Wiley and Sons, New York.
Myhrsytad, J.A. and O. Haldorsen 1984. Drinking Water in Developing
Countries - The Minimum Treatment Philosophy: A Case Study. Aqua,
Parr, J., et al. 1995. Chlorination of Community Water Supplies.
Technical Brief No. 46, Waterlines, 14(2).
Schultz, C.R. and D.A. Okun 1984. Surface Water Treatment for
Communities in Developing Countries. John Wiley and Sons, New York.
Smethurst, G. 1992. Basic Water Treatment for Application
World-Wide, Second Edition. Thomas Telford, London.
WHO [World Health Organization] 1993. Guidelines for Drinking-water
Quality. Vol 1: Recommendations, Vol. 2: Health Criteria and Other
Supporting Information, Vol. 3: Drinking-water Quality Control in Small
Community Supplies. World Health Organisation, Geneva.
Winter, S.J. 1988. Operation and maintenance of the Moen water treatment
plant chlorination equipment. The development of a VIP toilet for use in
the rural areas of Micronesia. ATE.
WRC [Water Research Commission] 1989. Disinfection of Rural and
Small-community Water Supplies. A Manual for Design and Operation.
World Health Organization, Geneva.