WATER TREATMENT: FLOCCULATORS
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 coagulant dosage.
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