Sedimentation with coagulation

Sedimentation with coagulation is the settling of fine suspended and colloidal impurities after chemicals (coagulants such as alum, ferric chloride or ferrous sulphate) have been added, so that the particles join into large, heavy flocs which settle quickly. It is also called chemical or aided sedimentation.

Why it is necessary

• Colloidal particles (0.001 to 1 µm) and very fine silt carry a negative charge and repel one another, so they stay suspended and plain sedimentation cannot remove them. By Stokes' law, their settling velocity is extremely small.
• It removes turbidity, colour, bacteria and some organic matter, so the filters are loaded less and run longer.
• It allows a much smaller and cheaper tank (higher overflow rate, 30 to 40 m³/m²/d, and detention of 2 to 4 h).

Processes involved

 Raw    +-------+   +-----------+   +---------+   +--------+
 water->| Rapid |-->|Flocculator|-->|Settling |-->| Filter |
        |  mix  |   |slow stir  |   |  tank   |   +--------+
        +-------+   +-----------+   +---------+
        coagulant     flocs form     flocs settle

1. Coagulation (rapid mixing): the coagulant is added and violently mixed for 30 s to 2 min. The positive metal ions neutralise the negative charge of the colloids (destabilisation), and metal hydroxide forms.

$$Al_2(SO_4)_3 + 3Ca(HCO_3)_2 \rightarrow 2Al(OH)_3\downarrow + 3CaSO_4 + 6CO_2$$

Alkalinity is needed (about 0.5 mg/l as CaCO$_3$ per 1 mg/l of alum); lime or soda ash is added if the water lacks it. 2. Flocculation (slow mixing): gentle stirring for 20 to 40 min brings the micro-flocs together to form large flocs by inter-particle collision (bridging and sweep floc). 3. Sedimentation: the large flocs settle in the settling tank under gravity; the clarified water is collected at the outlet weir and sludge is withdrawn from the hopper. 4. Filtration follows to remove the remaining fine flocs, then disinfection.