Water Treatment: Settling, Filtration, Disinfection

A conventional water treatment plant removes particles, then germs: screening → aeration → coagulation and flocculation → sedimentation → filtration → disinfection.

Sedimentation

Discrete particles settle at the Stokes' velocity:

v_s = g (G_s − 1) d²/(18 ν)

(valid for small particles, Re < 1).

An ideal settling tank removes all particles whose v_s is at least the surface overflow rate:

v₀ = Q/A (plan area A) — removal does not depend on depth.

• Particles with v_s < v₀ are removed in the fraction v_s/v₀.
• Detention time t = V/Q.
• Weir loading = Q/(weir length).

Coagulation and flocculation

• Coagulants (alum, ferric salts) neutralise the charge on colloids so they stick together. The dose is found by the jar test.
• Alum consumes alkalinity: Al₂(SO₄)₃·14H₂O + 3Ca(HCO₃)₂ → 2Al(OH)₃ + 3CaSO₄ + 6CO₂ + 14H₂O.
• Velocity gradient G = √(P/(μV)) (P = power input). Flash mixing: high G for seconds; flocculation: G ≈ 20–80 s⁻¹ for 20–40 minutes. Gt (dimensionless) is about 10⁴–10⁵.

Filtration

Effective size D₁₀ and uniformity coefficient D₆₀/D₁₀ describe the filter sand. Backwash expands the bed by about 20–50%.

Softening and other processes

• Lime–soda process: lime removes carbonate hardness; soda ash removes non-carbonate hardness.
• Ion exchange (zeolite): removes all hardness, gives zero-hardness water.
• Defluoridation (Nalgonda technique), iron and manganese removal by aeration.

Disinfection

• Chlorine forms HOCl (strong disinfectant), which dissociates to OCl⁻ at high pH. Disinfection works best below pH about 7.5.
• Chlorine demand = applied − residual. Breakpoint chlorination: add chlorine until free residual appears after destroying ammonia compounds.
• Residual of 0.2 mg/L free chlorine at the consumer end is the usual target.
• Alternatives: ozone, UV.

Worked example

A tank treats 6 MLD with plan area 250 m² and depth 3 m.

• Overflow rate = 6000/250 = 24 m³/m²/day; detention time = 750/6000 × 24 = 3 hours.

Practice