Slow sand filtration water treatment plants
Practical Action
The selection of water treatment processes as a possible solution for supplying drinking water
to rural communities depends on the quality of the raw water.
Alternatives
C
FL+C
PC+C
PC+FL+C
S+PC+FL+C
S+PS+FL+C
P+S+PS+FL+C
Water-
related
endemic
diseases
NO
-
NO
-
-
-
-
Raw water quality limits
Faecal coliforms
(uc)
≤ 10
≤ 10,000
≤ 100
≤ 10,000
≤ 10,000
≤ 10,000
≤ 10,000
90% of
the time
≤ 25
≤ 25
≤ 50
≤ 100
≤ 150
≤ 300
≤ 500
Turbidity limits (ut)
80% of the Sporadically
time
≤ 10
≤ 50
≤ 10
≤ 50
≤ 20
≤ 100
≤ 50
≤ 150
≤ 80
≤ 200
≤ 200
≤ 500
≤ 200
≤ 1000
P=
S=
PC =
PS =
FL =
C=
Pre-sedimentation
Sedimentation
Gravel Pre-filter in layers with an ascending flow
Gravel Pre-filter in series with vertical or horizontal flows
Slow sand filter
Sieve
If the water contains faecal coliforms and turbidity levels that exceed the values
shown in the table, it is advisable to look for other water sources.
At the entrance to the plant, a triangular weir structure should be considered to
measure the flow, in which the desired water should be marked with paint. The
operator can open the inlet valve to reproduce the flow required to operate the
system.
AERATION
Aeration consists of blending the water with the air in order to:
Increase the oxygen content in the water
Increase the pH in the water by reducing its carbon dioxide content
Remove the iron, magnesium, hydrogen sulphate, methane and various volatile
organic compounds responsible for the flavour and smell of the water (Hofkes et al,
1987).
The simplest aeration methods are described below:
1) Multiple tray aerator
This is an economical solution that occupies little
space. It consists of four to eight trays with mesh
bottoms, placed one on top of another with a 30-
50cm space between them. The water is evenly
distributed in the first tray through porous pipes, or
by stopping the gush with a bar (as shown in the
figure), so that the water trickles into the tray at a
rate of 0.02 m3/s per square metre of the tray’s
surface. Small droplets are then sprayed into the
6
Wooden trays break up
the water stream into
droplets.