Slow sand filtration water treatment plants
Practical Action
Water measuring devices at the entrance of each unit and the subsequent regulation
of the intake valve. No additional investment required.
1) Gravel pre-filter in layers, with an ascending flow
Description
The intake pipe goes through the centre of the pre-filter to the bottom of the
unit. From this pipe, the water is distributed evenly throughout the unit by
means of secondary porous pipes. The filtration zone consists of four layers of
gravel of different sizes, with the thickest gravel forming the bottom layer and the
finest gravel the top layer.
The water goes through the filtration zone in an ascending manner and is
collected by the outlet pipe which has 12.7mm diameter holes at 50mm from
the centre and 0.40m from the gravel filtration bed. This pipe can be larger than
the intake pipe to reduce the head loss and ease the outgoing flow.
The drainage zone is at the bottom of the pre-filter. The floor of the unit has a
12.5% incline to ease the discharge of sediment to the cleaning water discharge
channel. Slabs or bricks are placed on the discharge channel to support the
gravel, at intervals of two or three centimetres (CRHEA 1991; Marrón 1998).
Design criteria
PARAMETER
Number of pre-filters
Filtration rate /flow/area) (m/h)
Layers
thickness (m)
Layer 1 (gravel)
Layer 2 (gravel)
Layer 3 (gravel)
Layer 4 (gravel)
Total bed thickness (m)
Height of water above the bed surface (m)
Height of free border (m)
Bottom slope (%)
Distance between slabs/bricks (cm)
Width of slabs/bricks (cm)
Cleaning water discharge velocity (m/min)
Velocity in cleaning channel (m/s)
VALUES
size (mm)
8