is the removal of suspended solids through the settling of particles moving
through a tank at a slow rate. There are a number of forms of sedimentation. In
water treatment plants treating source water a high proportion of suspended
solids of coarser grades (e.g. sand and coarse silt) a grit chamber may be used
to remove the largest particles through simple sedimentation. In this process,
water is passed through a tank at a slow rate and suspended solids fall out of
suspension. In small supplies, simple sedimentors may also be used, which functioning
in a similar fashion to grit chambers, although with a slower rate of water
throughflow. Simple sedimentation will not remove fine grained particles
because the flow rates remain too high and the retention time is insufficient.
A further common fault with simple
sedimenters is that design flow rates are rarely
achieved in practice and a certain element of 'short-circuiting' can occur unless
construction, operation and maintenance is very careful.
As a result of the drawbacks in simple
sedimentation, it is common to find that the sedimentation process is enhanced
through the addition of chemicals - or coagulation.