Household water treatment systems
Practical Action
Two types of sedimentation tank are shown in Figures 3 and 4: Horizontal flow settling tank and
an upward flow clarifier.
In a horizontal tank water travels from one end of the tank to the other with a rentention time of
4-6 hours. An upward flow tank makes use of a “sludge blanket”, where a layer of floc
(coagulated sediment) is formed and filters the water passing upwards. This is more efficient
than the horizontal flow tanks since it has a retention time of 1-3 hours. It does however cost
more and is less easy to operate than the horizontal flow tank. Water that has a silt content of
more than 1000mg/l should be treated using a horizontal flow tank, since this can cause an
upward flow tank to be less efficient and difficult to operate.
Coagulation
Coagulation can be achieved by using an additive to the water which helps to bring solid matter
together. This makes the clusters of particles heavier and achieves a faster settlement time.
Adding iron or aluminium sulphate 10-70mg per litre results in a positive charged that will
combine with the negative charged bacteria, viruses and fine clay which then fall out of the
water as sediment. Water must be turbulent to mix the coagulant with the water and this can
usually be done by passing water over a weir or through constrictions.
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