Treatment process; sedimentation factors; coagulants

Treatment processes and impurities removed

Factors affecting sedimentation in a tank

1. Size of particle: $v_s \propto d^2$; bigger particles settle quicker.
2. Specific gravity of particle: heavier particles settle faster ($v_s \propto S_s-1$).
3. Temperature (viscosity): warmer water has lower viscosity, so settling is faster; $v_s \propto 1/\nu$.
4. Surface overflow rate / tank area: larger surface area for the flow reduces the overflow rate and improves removal.
5. Detention time: the longer, the better the removal.
6. Velocity of flow: should be low (below about 0.3 m/min) so that the settled sludge is not scoured.
7. Tank shape and depth, inlet and outlet arrangement (short-circuiting, density currents, wind).
8. Flocculation: coagulated particles are bigger and settle faster.
9. Hindered settling at high concentration and tank turbulence.

Why coagulants are used

• Fine silt and colloids (0.001 to 1 µm) have a negative charge and do not settle by themselves in a reasonable time.
• A coagulant such as alum supplies positive ions (Al$^{3+}$) that neutralise the charge, and forms a jelly-like Al(OH)$_3$ floc which enmeshes the particles:

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

• The large flocs settle quickly, which removes turbidity, colour, bacteria and organic matter, reduces tank size and detention time, and increases the length of filter runs.