Slow sand filtration water treatment plants
Practical Action
next tray. The trays can be made of various materials: asbestos-cement sheets, plastic pipes
with small diameters or parallel wooden slats. To obtain a finer spray, the aerator trays can
be filled with thick gravel approximately 10cm deep.
2) Cascade aerator
This consists of either four or six steps, each 30cm high and having a capacity of
approximately 0.01 m3/sec per metre of width. An obstacle is often placed at the edge of
each step to produce turbulence and thus improve the aeration efficiency.
Although more space is required than for multiple tray aerators, less head is lost and no
maintenance is required.
PRE-FILTRATION
Many designs of water treatment plants fail to take into account the differences in the quality
of water, particularly during the rainy season when turbidity increases, obstructing filters and
reducing their effectiveness. Hence the need to previously clear the water through different
processes, including pre-filtration, which not only removes turbidity but also algae and
bacteria, lengthening the life of the filters and reducing the cleaning frequency (Collins et al,
1991).
Although the efficiency of ascending and descending flow pre-filters is similar, the former are
easier to maintain as the hydraulic retro-washing operation is simpler.
The alternative of an ascending flow in layers is recommended for water with less than 50 UT
turbidity with short peaks. On the other hand, the series pre-filter is better for poor quality
water as it has a better capacity for storing solids (AARAUV 1985).
There are two alternatives for achieving a uniform flow in the pre-filtration units:
Structure with rectangular tanks equipped with manual sluice valves to isolate the
units. Although a small investment is required, this is the most comfortable solution
for the operator and the most reliable in the long term.
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