Treatment processes, impurities removal, settlement

Treatment processes and the impurities they remove

 Raw -> Screen -> Aerator -> Sedimentation -> Coag/Floc
 water                                          |
 Supply <- Disinfection <- Filter <- Settling <-+

Factors affecting settlement in a sedimentation 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.

Hydraulic subsidence (settling) value: the overflow rate $v_0=Q/A$ is the minimum settling velocity that gives 100 % removal. A particle of velocity $v_s\ge v_0$ is fully removed; if $v_s<v_0$ only the fraction $v_s/v_0$ is removed.

Effect of temperature and diameter: from Stokes' law, $v_s = \dfrac{g(S_s-1)d^2}{18\nu}$. Settling velocity varies as the square of diameter (doubling the diameter gives four times the speed) and is inversely proportional to the kinematic viscosity, which falls as temperature rises (the viscosity at 10 °C is about 1.3 times that at 20 °C, so settling at 10 °C is about 23 % slower). Tanks are therefore designed for the lowest water temperature.