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
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
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. Simplified drawing of rapid sand filter (Schulz, 1984)
Level of Involvement
Operating a rapid sand filter requires trained personnel.
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.
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.
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.
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.
No cultural inhibitors have been identified.
Further Development of the Technology
The technology is well developed. Improvements may result from better
backwash control devices.
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.