Slow sand filtration water treatment plants
Practical Action
way the surplus water in the pre-filters will overflow away. This operation must take place
every time one of the two filters is out of service.
Damp filter-harrowing method with removal of filtered material
This is applied when the previous run has lasted less than a month. It consists of making the
water flow against the current (ascending) whilst the sand bed is raked until the cleaning
water is less cloudy. The cleaning water will drag the dirt and other matter raked up from the
sand. Then the same method applied for the dry filter-harrowing method should be applied.
The steps to be followed for cleaning the first filter are:
1) Close the water inlet and let the water filter at a declining rate throughout the night.
2) The next day, remove the floating matter and open valve to clean the filtration bed.
Shut the inlet valves to both filters, the outlet valve (SF2) of the filter that will not be
cleaned and the inlet valve to the reservoir (R). Open the filter by-pass valves (BF) so
that the effluent from the pre-filters enters through the bottom of the filter to be
cleaned – ie. through the filtered water outlet pipe (see sketch).
3) Rake the entire depth of the sand bed (30 to 40cm) whilst the water flows against
the current until it is clearer, taking care not to rake the layer supporting the gravel
below the sand. Rake 15 - 20cm of the whole filter surface by rows; then remove the
raked sand from the surface of half the filter to the other half, as shown in the
drawing, and rake the remaining 15 to 20cms. Replace the sand and then repeat
the operation on the other half of the sand bed. Try and prevent the cleaning water
from overflowing and losing sand. This operation can take about two hours and two
people are needed to take turns raking the sand.
4) Close the water outlet (SF 1) and the cleaning valve of the filtration bed in the filter
undergoing cleaning (filter 1). Open the inlet valve to the reservoir (IR) and the inlet
(IF2 and outlet (SF2) valves of filter 2, putting it into normal operating mode.
5) Next, apply the dry filter-harrowing method to filter 1.
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