Slow sand filtration water treatment plants
Practical Action
gravel walls. In this case the pre-filter should be cleaned manually. As a reference,
this should be done at least once a year after the rainy season.
First of all, remove the water from the inlet pipe. Extract the top layer and wash it in
a water container. The best procedure for washing gravel is to stir it mechanically in
a container so that the friction loosens the solids that are stuck to the gravel surface.
A cylinder could be used, as shown in the figure. The gravel is then scraped with the
help of a stick whilst the clean water is entering the cylinder through a hose. The
water with the impurities from the gravel will go out through another hose placed
further up in the cylinder.
The same procedure is followed for
the next layer, trying not to mix the
gravel of the different layers. If for
any reason they do mix, they should
be sifted again to separate them.
The pre-filter must contain water in
order to prevent the filtered material
from compacting the unwashed gravel onto the wall in the pre-filter.
Once the pre-filter is empty, the walls must be brushed down and the layers replaced
on the bottom.
The gravel should be replenished carefully in layers of corresponding size. This
should be done immediately after this cleaning process, in order to avoid possible
contamination.
Maintenance of pre-filters
• Cut down weeds, shrubs and trees around the pre-filters.
• Protect the soil against erosion.
• Repair cracks in the structure walls.
• Check all the valves and grease them if necessary.
• Extract floating elements from the water.
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