Slow sand filtration water treatment plants
Practical Action
PRE-FILTER IN ASCENDING FLOW LAYERS
Pre-filters serve to eliminate impurities that cloud the water. If impurities enter the filter
directly they will obstruct it: frequent cleansing is therefore necessary.
Dirty water enters through the lower part and rises through the gravel filtration bed, which
itself retains algae, a good percentage of bacteria, and all suspended matter larger than
10mm in diameter. Then the upper pipes collect the water to convey it to the next treatment
unit.
Operation of the pre-filter in ascending flow layers
Start-up
To fill the pre-filter, shut off the drainage valve and slightly open the inlet valve so
that a moderate flow does not drag the solids accumulated on the bed. This flow can
be increased gradually as filling progresses.
Normal pre-filtering operation
During the pre-filtration operation the inlet and outlet valves should be open. The
drainage valve should remain shut.
When necessary, remove algae and floating elements.
Hydraulic cleaning
The discharge descending from the pre-filter will drag the particles that settled in the
gravel during normal operation. For thorough cleaning it is important to completely
empty the pre-filter. The operation should be repeated two or three times until the
water is clear.
The pre-filter should not remain empty for too long,
because adhered material will be compacted, making the
subsequent hydraulic cleaning process less effective.
In order to prevent accumulated mud from compacting
(and to reduce the regularity of manually cleaning the
gravel), hydraulic cleaning should take place
approximately once a week during the rainy season (high
turbidity) or every two months during a dry season (low
turbidity).
The following steps should be followed:
1) Shut the inlet valve and open the drainage valve
2) Wait until the unit is completely empty
3) Fill the unit again, as indicated in the start-up chapter, ie. with a moderate
flow.
4) If the cleaning water is cloudy, repeat the operation.
Manual cleaning
There comes a time when, after successive hydraulic cleaning processes, not all the
material accumulated in the filtration bed is eliminated because it is stuck to the
19