Slow sand filtration water treatment plants
Practical Action
FILTRATION
Slow modified filter with filter-harrowing cleaning
The slow sand filter proposed below differs from the conventional variable head slow filter in
the following aspects:
Its water inlet and outlet structures are simpler than those of conventional filters,
without altering their function.
Considering that the main biological activity in the sand bed occurs in the first layers
and that the high temperature in the Amazon region is favourable for this activity, it
is suggested that the height of the bed should be 35cm so that 95% of the coliforms
can be removed.
The ascending flow filter-harrowing cleaning method is applied, as shown on page
22.
With the above-mentioned modifications, the direct cost of building the filters is reduced by
approximately 40% (the structures are simplified and the height of the filter box is reduced)),
in addition to making its operation easier.
The selection of sand for the filtration bed is a critical point, as the need to transport the
selected sand from distant places raises the costs significantly; this could represent 30% of
the direct construction costs of the filters. Technical literature is very demanding regarding
the characteristics of the sand. With a simple method, however, locally-available sand can be
sifted and cleaned to obtain an adequate filtration bed without affecting the effectiveness of
the filter (see the Operation and Maintenance chapter).
Description
During a normal filtration operation, the water enters through the top part of the filter to the
supernatant layer. The inlet pipe has two holes to release the possible accumulation of air at
the top of the inlet pipe. The water will remain in the supernatant layer for several hours,
during which time the suspended particles will settle.
The obstruction of the filter will cause the supernatant layer to rise to the brim, at which time
it will be necessary to clean the sand bed. There must be 20cm of free space above the
brim.
The greatest biological activity takes place on the surface layer of sand, where the majority of
pathogenic organisms in the water are removed. The bed must be between 30 and 40cm
high, depending on the quality of the gross volume of water. The inner walls of the filter box
in the section containing the filtration bed must have a rough finish to prevent the formation
of short-circuits.
The sand must go through a sifting process to eliminate grains that are too thick or too fine.
Both the gravel and the sand must then be washed before being placed in the filter, in order
to eliminate organic matter and clay (see Operation and Maintenance chapter).
The water goes through the filtration sand bed to the drainage system underneath, which is
comprised of porous pipes leading to the next unit. Layers of gravel are placed on top of the
porous pipes at the bottom to support the sand.
Outside the filter, the outlet pipe has a 10-20cm tee pipe above the sand bed to prevent any
accidental discharge from the filter which could affect the layer of micro organisms and avoid
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