Slow sand filtration water treatment plants
Practical Action
7) Open the inlet valve to the first filter (IF1) and the inlet valve to the reservoir for
the normal filtration process.
• Normal filtration operation
During the filtering operation the inlet and outlet valves must be open, whereas the valves for
cleaning the bottom and the filtration bed should remain shut.
When necessary, algae and floating elements should be removed.
The incoming flow must be measured by unscrewing the universal union of the descending
pipe through which the water enters, whilst at the same time filling a bucket with a specific
capacity. If the flow entering the filtration units is different, the inlet valves must be
regulated so that the incoming flow is the same.
• Cleaning the filtration bed
If the water level in the filter rises to the brim, it means that the sand bed is obstructed and
needs to be cleaned (Canepa 1988; Marrón 1998a).
Two cleaning methods can be applied, depending on the duration of the preceding filtration
flow.
Dry filter-harrowing cleaning method.- Applied when the previous run has lasted more than a
month. It consists of de-compacting the sand and redistributing the filtered material to
regenerate the porosity of the sand bed. (Pardon et al, 1983). These steps should be
followed:
1) Close the water inlet and let the water filter at a declining rate throughout the
night.
2) The next day, remove floating matter and open the bottom cleaning valve until
the water reaches 15cm below the surface of the sand.
3) De-compact the 15cm deep sand bed with a pick, then rake the sand to loosen it
and make it more spongy.
4) Open the filter inlet valve.
Whilst the filter-harrowing method is being applied in a dry filter, care must be taken to
ensure that only the corresponding quantity of water enters the filter that is still in operation.
To this end, close the filter inlet valve until the correct operating flow is obtained. In this
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