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Newsletter and Technical Publications

<Sourcebook of Alternative Technologies for Freshwater Augumentation
in Small Island Developing States>


PART B - ALTERNATIVE TECHNOLOGIES

1. TECHNOLOGIES GENERALLY APPLICABLE TO ISLAND STATES

1.2 Water Quality Improvement Technologies

1.2.3 Disinfection

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.

Technical Description

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 employed.

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.

Costs

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.

Suitability

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.

Advantages

Chlorination can result in the destruction of disease-producing organisms, and can be relatively easily applied to a range of treatment situations.

Disadvantages

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.

Cultural Acceptability

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.

Information Sources

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, 2:86-90.

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.

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