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
1.2.2 Slow Sand Filtration
A distinguishing feature of slow sand filters is the presence of a thin
layer, called the schmutzdecke, which forms on the surface of the sand bed
and includes a large variety of biologically active microorganisms. A slow
sand filter comprises approximately 1.2 m depth of fine sand supported on
two or three gravel layers. It is a very simple and effective technique
for purifying surface water. It will remove practically all of the
turbidity from the water as well as most of the pathogens without the
addition of chemicals. Slow sand filters can frequently be constructed
largely from locally-available materials. The effective size of the sand
used in slow sand filters is about 0.2 mm, but may range between 0.15 mm
and 0.35 mm, and with a coefficient of uniformity of between 1.5 and 3.0.
In contrast, the range of effective size for the rapid sand filters
described in the preceding section is 0.35 mm to 1.0 mm, with a
coefficient of uniformity of 1.2 to 1.7.
The walls of the filter can be constructed of concrete or stone. Sloping
walls dug into the earth, supported or protected by chicken wire
reinforcement, and lined with sand or a sand-bitumen mixture could be a
cost-effective alternative to concrete. Inlets and outlets should be
provided with controllers to keep the raw water level and the filtration
rate constant. Bottom drains consist of a system of manifolds and lateral
pipes (Figure 10). The filtration rate usually employed in developing
countries is between 2.5 and 6.0 m3/m2/day. Higher
rates may be used, but should be tested to ensure that the higher rates
yield a good quality product water. The system should be designed for
flexibility. Highly turbid water may need some form of pre-treatment such
as settling or rough filtration.
Home filters using the principles of slow sand filtration can be
constructed using a 200 l drum. Such systems are a common way of treating
water for household drinking. The drum has a layer of 3 mm sized gravel on
the bottom surrounding the outlet pipe, 600 mm of fine sand on top of the
gravel, and a space for the inlet water. An overflow and a lid to prevent
mosquito breeding and algal growth should be provided. Often large stones
are placed on top of the sand to prevent the flow from the inlet pipe from
scouring the sand. This type of filter is often used in rainwater systems
(see above) to remove sediment before the rainwater is conveyed to a
Figure 10. Simple drawing of slow sand filter (Schultz,
Extent of Use
Slow sand filters are often used in both urban and rural water supply
systems on larger islands. On La Digue, Seychelles, the installation of
slow sand filters has proven to be the most cost-effective method for the
treatment of raw water. Water supplied to Apia and part of the west coast
of Upolu, Western Samoa, is treated using slow sand filters which work
well under normal flow conditions. In Rarotonga, Cook Islands, water from
many of the stream sources is treated with small versions of a slow sand
filter, while, in French Polynesia, slow sand filters have been operated
successfully in small communities supplied by stream catchments on the
high islands. A small-scale slow sand filter unit has been designed and
operated in a 500 mm steel pipe.
Operation and Maintenance
Filter cleaning, by scraping off a 25 mm surface layer of sand, is
required at intervals of between 30 and 100 days depending on the
turbidity of the water being filtered. Trials in India found that, at a
turbidity of 10 NTU, the filter could be run for up to 90 days, whereas,
at a turbidity of 30 NTU, the filter run time would be reduced to 30 days.
Filter surface fabrics have been developed for slow sand filters to
improve and ease the cleaning process. However, because aliquots of the
sand are removed in the cleaning process, the sand layer will need to be
topped up from time to time. The depth of sand should not be allowed to
become less than 600 mm. Topping up can be done using washed sand, removed
during previous filter cleanings, or fresh sand. Pathogen removal is
achieved within the biological slime layer (schmutzdecke) which forms on
the surface of the sand. This will take a few days to re-form after each
cleaning and there will be a slight decline in the performance of the
filtration system during this time; usually, this decline is not
Level of Involvement
The most attractive aspect of slow sand filtration is its simplicity of
operation and control. After a short training period, the operation of the
system can be mastered by a local caretaker.
Slow sand filter costs can be low if appropriate local materials are
available. For large slow sand treatment plants, the land area required
for constructing the system may add significantly to the cost. A slow sand
filter may take up to five times the equivalent land area of a rapid sand
filtration plant. Hence, the technology is best adopted by small
communities where land cost may not be a problem.
Because slow sand filters usually require no chemicals or energy inputs,
the capital and operational costs can be very low in comparison to rapid
sand filters. Maintenance costs will include minor repairs to the filters,
and replacement of the sand washed out or removed during the scraping of
the silt from the filter surface. Other maintenance costs relate to the
replacement of the few moving parts present in the filter system. The
costs will be higher in pumped schemes. Examples of specific costs for
small islands are not available.
Effectiveness of the Technology
This is a proven technology that is very effective in removing both
suspended materials and bacteria. However, the improvement in water
quality brought about by slow sand filtration will differ from place to
place due to raw water quality, sand grain size, rate of filtration,
temperature, and the oxygen content of the water. An indication of the
purification effect of a slow sand filter is summarized in Table 2 and
shown in Figure 11.
TABLE 2. Performance of Slow Sand Filters.
|Parameter of water quality
||Purification effect of slow sand
||30% to 100% reduction
||Turbidity is generally reduced to less than 1 NTU
||95% to 100%, and often 99% to 100%, reduction in the
level of faecal coliforms
||Virtual removal of cercariae of schistosomes, cysts and
||Virtually complete remova
||60% to 75% reduction in COD
|Iron and manganese
||30% to 95% reduction
Slow sand filtration is normally used for the treatment of surface water
supplies only, and, thus, would not be applicable to many of the small
low-lying islands where surface waters do not exist. Slow sand filtration
is most suited for use with gravity operated, surface water supply
systems, or systems with large, clear water storage facilities, as the
filters require a continuous flow of water.
Figure 11. Effect of sand size on removal of total
coliform bacteria in slow sand filtration (Visscher, et al., 1987).
The cost of construction is low, and its simplicity of design and
operation means that slow sand filters can be built and used with limited
technical supervision. Little special pipework, equipment, or
instrumentation is needed, and the labour required for maintenance can be
unskilled as the major labour requirement is in cleaning the beds, which
can be done by hand. Imported materials and equipment is usually
negligible and no chemicals are required. Likewise, power is not required
if a gravity head is available, and there are no moving parts or
requirements for compressed air or high-pressure water. Variations in raw
water quality and temperature can be accommodated, provided turbidity does
not become excessive, and overloading for short periods does no harm.
Compared to rapid sand filtration, there is a net savings of water as
large quantities of backwash water are not required.
Slow sand filtration units require large land areas for plants treating
large flows (about five times that of rapid sand filtration plants).
Clogging may occur if the source water is excessively turbid or if certain
(filamentous) types of algae are present in the raw water. Pre-treatment
with roughing filters or settling tanks may be necessary if such clogging
occurs frequently. Also, toxic chemical contamination of the raw water may
affect the biological surface layer (this could be a good indication of
water source problems!)
Land ownership issues could become a problem, but there are no known
cultural concerns associated with the use of this technology.
Further Development of the Technology
Although this technology is well known, there have been many new
developments in slow sand filtration. In association with roughing
filters, slow sand filters have been used very effectively for cleaning
very turbid pond water for small village, community water supplies in West
Bengal, India. Small-scale applications of this technology could benefit
small islands which need low technology surface water treatment
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James M. Montgomery Consulting Engineers, Inc. 1985. Water Treatment
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Filtration: A Low Cost Treatment for Water Supplies in Developing
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Communities in Developing Countries. John Wiley and Sons, New York.
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Traditional Surface Water Sources for Domestic Consumption. All India
Institute of Hygiene and Public Health, Calcutta.
Visscher, J.T., R. Paramasivam, A. Raman, and H.A. Heijnen 1987. Slow
Sand Filtration for Community Water Supply. International Water and
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