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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

Of the water quality improvement technologies used in SIDS, the most commonly used are filtration and disinfection. Standard water supply textbooks provide specific descriptions of these widely-used technologies. Notwithstanding, filtration may be a physical, chemical, or, in some instances, biological process for separating suspended impurities from water. The most common, physical means of filtration separates suspended matter from water by passage through porous media. Two general types of filters commonly used in community water treatment are the slow sand filter and the rapid sand filter.

Filtration is often required for water drawn from surface water sources. Pollution from animals and human activities cause surface waters to become contaminated with suspended matter (turbidity) and pathogens (disease-causing organisms). Pathogens are often associated with turbidity and, therefore, turbidity removal by settling and/or filtration is an essential step in making surface water safe for human consumption. Groundwater, by comparison with surface water, is usually low in pathogens and turbidity, and is often favoured over surface water sources because little or no treatment is normally required. Nevertheless, it should be borne in mind that groundwater sources can be contaminated with pathogens from poorly sited and/or malfunctioning wastewater disposal systems.

1.2.1 Rapid Sand Filtration

Technical Description

Rapid sand filtration (Figure 9) is a technique common in developed countries for treating large quantities of drinking water. It is a relatively sophisticated process usually requiring power-operated pumps for backwashing or cleaning the filter bed, and flow control of the filter outlet. A continuously operating filter will usually require backwashing about every two days when raw water of relatively low turbidity is used. Pretreatment of the raw water, using chemical flocculation agents in combination with setting tanks, is common where turbidity is high. Relatively large quantities of filter backwash water, as well as sludge from the settling process, may be generated and require some form of treatment before discharge to the environment.

Because of the higher filtration rates, the areal requirement of a rapid filtration plant is about 20% of that required for the slow sand filters described in the next section, although the latter usually do not require pretreatment of the raw water (Schulz, 1984).

Extent of Use

Rapid sand filters are used in some larger, urban, water supply systems in SIDS such as that on the main island of Mauritius, which is served by several systems utilizing rapid gravity filters as the primary treatment technology, and in the Seychelles. The total amount of water being treated on Mauritius is 57 000 m3/day, and plans have been prepared to increase the capacity of the rapid filters by another 120 000 m3/day. In the Seychelles, rapid sand filters were selected as the primary treatment technology due to insufficient land area to install slow sand filters.

Operation and Maintenance

Operation of a rapid sand filter consists of regular backwashing. The period between backwashes depends on the quality of the water being filtered. The purpose of backwashing is to remove the suspended material that has been deposited in the filter bed during the filtration cycle. Periodic repacking of the filter bed may be required at infrequent intervals to ensure efficient operation.

Figure 9
Figure 9. Simplified drawing of rapid sand filter (Schulz, 1984)

Level of Involvement

Operating a rapid sand filter requires trained personnel.

Costs

The construction cost of rapid sand filters is determined primarily by the cost of materials such as cement, building sand, gravel, reinforcing steel, filter media, pipes, and valves. The cost of labour is usually of lesser importance. However, the cost of land and transport of materials could add substantially to the total cost. The cost of energy required to operate a rapid sand filter may also add significant costs.

Maintenance costs will include repairs to the filters, and replacement of equipment. In general construction, operation and maintenance cost for rapid sand filters are higher than cost for slow sand filters.

Effectiveness of the Technology

The technology is proven and is very effective in removing suspended materials from the water. However, the technology often requires that the water be pretreated, usually by sedimentation of particulates in the raw water supply. The water is normally disinfected after filtration.

Suitability

This technology is most suited for larger urban water supply systems with a surface water source. It is also suitable in areas where there is a scarcity of land available for public works.

Advantages

The advantages of this technology are that it is a proven technology, effective in removing suspended solids, and that it requires a minimal land area for construction and operation compared to slow sand filters.

Disadvantages

Rapid sand filters have high capital and operation costs, which may be increased further if there is a need for pretreatment of the raw water. The technology uses energy for pumping, and requires a relatively high degree of training for the plant operator.

Cultural Acceptability

No cultural inhibitors have been identified.

Further Development of the Technology

The technology is well developed. Improvements may result from better backwash control devices.

Information Sources

American Water Works Association 1971. Water Quality Treatment. Third Edition. McGraw-Hill Book Company, New York.

James M. Montgomery Consulting Engineers, Inc. 1985. Water Treatment Principles and Design. John Wiley and Sons, New York.

Smethurst, G. 1992. Basic Water Treatment for Application World-Wide, Second Edition. Thomas Telford, London.

Schulz, C.R. and D.A. Okum 1984. Surface Water Treatment for Communities in Developing Countries. John Wiley and Sons, New York.

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