The settlement of fine colloidal and suspended solids can be assisted by the use of flocculation as described in last post but with dilute suspensions, such as lowland river waters, the opportunities for collisions and agglomeration are limited. In such circumstances flocculation does not significantly improve the settling characteristics of the suspension. The addition of a chemical coagulant, which precipitates flocculent solids in the water, followed by flocculation and sedimentation, can provide a high degree of clarification. The process of chemical coagulation is thus carried out in a sequence of operations, the first of which involves rapid mixing of the coagulant with the flow of water to permit enmeshment of the colloidal solids in the rapidly precipitating floe. This can be achieved by means of hydraulic turbulence or by a high-speed rotating-blade mixer with a retention time of 30-60 s. Following the initial precipitation stage, the suspension is passed to flocculation and sedimentation units, which may be separate or combined.
The most popular coagulant for potable water treatment is aluminium sulphate, often referred to as alum. When it is added to water in small doses of around 20-50 mg/l, a reaction takes place with the natural alkalinity present, and insoluble aluminium hydroxide is formed. This is a highly flocculent precipitate which responds well to controlled flocculation. The chemical reactions which occur are complex but may be simplified as
Al2(SO4)3 + 3Ca(HCO3)2 → 2Al(OH)3 + 3CaSO4 + 6CO2
When using commercial alum which has 16-18 molecules of water of crystallization, and expressing alkalinity in terms of calcium carbonate, each mg/1 of alum reacts with 0.5 mg/1 of alkalinity. If insufficient alkalinity is present, the precipitation of aluminium hydroxide will not be complete, coagulation will be poor and residual aluminium will appear in the treated water.
The solubility of aluminium hydroxide is lowest in the pH range of 5-7.5, and outside this range coagulation with alum will not be satisfactory. An acid or alkali, as appropriate, can be added to bring the pH into the correct range. Alternatively, another coagulant such as ferric sulphate, ferrous chloride or ferric sulphate may be employed. The size and strength of floc particles can often be enhanced by the use of a small dose (<< 1 mg/1) of a coagulant aid such as a polyelectrolyte. These compounds are very large organic molecules which serve to bind floc particles together and thus improve their settling characteristics and resistance to shear.
There is no theoretical basis for the calculation of coagulant doses and they must be determined experimentally using a jar-test apparatus which simulates the stages of rapid mixing, flocculation and sedimentation. By using a range of coagulant doses and pH values, it is possible to determine the optimum conditions to achieve the required water quality.