Coagulants carry a charge and therefore attract charged clay particles. The particles begin to aggregate and form 'flocs'. Once the flocs reach a critical mass, they sink to the bottom of the settler. The outlet of the sedimenter is generally around the top of the structure, thus the clear water is removed by a surface channel. This system can be further refined with the use of modular or plate settlers which reduces the time require for settling by providing a wider surface area for aggregation of particles.
The most commonly used coagulants is aluminium
sulphate, although there are other coagulants available including ferric salts
(sulphates and chlorides) and polyelectrolytes. Coagulants are dosed in
solution at a rate determined by raw water quality near the inlet of a mixing
tank or flocculator. It is essential that the coagulant is rapidly and
thoroughly mixed on dosing, this is may be achieved through the use of a
hydraulic jump. The water then passes into the settler to allow aggregation of
the flocs. Increasing use is now being made of synthetic polymer compounds or
polyelectrolytes. As these are highly charged, there is a rapid increase in the
formation of flocs, particularly where clay makes up a large proportion of the
suspended solid load.
The advantages of the coagulation is that it reduces
the time required to settle out suspended solids and is very effective in
removing fine particles which are otherwise very difficult to remove from
water. Coagulation can also be effective in removing protozoa, bacteria and
viruses, particularly when polyelectrolyte is used, as the highly charged
coagulant attracts the charged microorganisms into the flocs. Coagulation can
also be effective in removing by precipitation certain contaminants such as
lead and barium.
The principle disadvantages of using coagulants are
the cost and the need for accurate dosing, jar testing and dose adjustment and
frequent monitoring. Coagulants can be expensive to buy (particularly
polyelectrolyte) and need accurate dosing equipment to function efficiently.
Staff need to be adequately trained to carry out jar tests to determine