Sedimentation and coagulation

Raw surface water is turbid with silt, clay, algae and organic matter. Sedimentation tanks let heavier particles settle under gravity. Particle settling velocity follows Stokes' law for small, slow particles, rising with the square of particle size and with density difference. An ideal settling tank removes all particles whose settling velocity exceeds the surface overflow rate, the flow divided by plan area. Depth affects detention time but not, in theory, removal efficiency, which is why shallow tube settlers and plate settlers improve performance.

Fine clay and colloids settle far too slowly, because they are tiny and carry negative charges that repel each other. Coagulation neutralises those charges by adding a coagulant, usually aluminium sulphate (alum), poly-aluminium chloride or ferric salts, mixed rapidly so it disperses within seconds. Flocculation follows: gentle mixing for 20 to 40 minutes encourages destabilised particles to collide and stick into large, heavy flocs that settle readily. The velocity gradient G and its product with time, Gt, guide flocculator design.

The right coagulant dose depends on raw water quality and is found by jar tests, which mimic rapid mix, flocculation and settling in beakers. pH and alkalinity matter, because alum consumes alkalinity. Settled water then passes to filters. Clariflocculators combine flocculation and settling in one unit; sludge from the tank bottom is removed and treated.

Real tanks fall short of the ideal because of short-circuiting, density currents, wind and inlet turbulence, so designers use baffled inlets, uniform outlet weirs with limited weir loading and safety margins on overflow rate.