Water Treatment

Water treatment turns raw water into safe drinking water. Learn water demand, quality parameters, the treatment train (screening, coagulation–flocculation, sedimentation, filtration, disinfection), design parameters (overflow rate, detention time, filtration rate) and distribution.

Safe drinking water has saved more lives than almost any medical advance. Rivers, lakes and groundwater contain particles, microorganisms and chemicals that must be removed before water reaches taps.

A conventional treatment plant is a sequence of physical and chemical processes, each sized with simple loading rates and detention times.

An analogy

Treatment is like making a clear soup: let the big bits settle, add something to clump the small bits, settle again, strain, then boil (disinfect) for safety.

Quality and standards

Physical (turbidity, colour, taste), chemical (pH, hardness, fluoride, nitrate, arsenic, iron) and biological (coliforms) parameters are compared with standards (WHO, IS 10500, EPA). Turbidity should be < 1 NTU and E. coli absent.

The treatment train

Intake and screening → aeration (iron, taste) → coagulation (alum/PACl neutralises particle charges) → flocculation (gentle mixing forms flocs) → sedimentation (flocs settle) → rapid sand filtration (5–15 m/h) → disinfection (chlorine, UV, ozone) → storage and distribution.

Sedimentation theory

In an ideal settling tank, a particle is removed if its settling velocity exceeds the overflow rate Q/A — depth doesn't matter for removal efficiency. Typical SOR 20–40 m³/m²·d and detention 2–4 h.

Advanced treatment and distribution

Membranes (MF/UF, RO for desalination), activated carbon for organics, ion exchange for hardness, advanced oxidation for micropollutants. Distribution networks maintain pressure (≥ 7–20 m residual head) and chlorine residual (0.2 mg/L); hydraulic models (EPANET) design and manage them.

Formulas

• Demand: Q = population × per-capita demand (× peak factor)
• Stokes' law: v_s = g(ρ_s − ρ_w) d² / (18 μ)
• Settling tank area: A = Q / SOR (particles with v_s ≥ SOR are removed)
• Detention time: t_d = V / Q
• Chemical dose: mass/day = Q (MLD) × dose (mg/L) [kg/d]

Worked example: Plant capacity and settling tank

Given: Population 100 000, 135 lpcd; SOR = 30 m³/m²·d, depth 3 m

Find: Average demand, tank area, detention time.

• Q = 100 000 × 135 = 13.5 MLD = 13 500 m³/d
• A = 13 500/30 = 450 m²
• t_d = (450 × 3)/(13 500/24) = 2.4 h

Answer: 13.5 MLD, 450 m², 2.4 h

Worked example: Settling velocity of silt

Given: d = 0.05 mm, SG = 2.65; μ = 1.0 × 10⁻³ Pa·s

Find: v_s and whether removed at SOR 30 m/d.

• v_s = 9.81 × 1650 × (5 × 10⁻⁵)²/(18 × 10⁻³) = 2.25 × 10⁻³ m/s = 194 m/d
• 194 > 30 → removed

Answer: Removed easily (colloids need coagulation)

Common mistakes

• Wrong: Designing for average flow only. — Right: Size units for maximum daily flow and check peak hourly for distribution.
• Wrong: Skipping chlorine residual in the network. — Right: Maintain residual to prevent recontamination.

Applications

• Municipal supply: City treatment plants and distribution networks.
• Rural & emergency: Slow sand filters, chlorination units and household treatment.
• Industry & desalination: Process water, RO desalination for coastal cities.

Key takeaways

• Demand = population × lpcd × peak factors.
• Coagulate, flocculate, settle, filter, disinfect.
• A = Q/SOR; t_d = V/Q.
• Keep residual chlorine and pressure in the network.

Going further

• CT concept: Disinfection effectiveness = concentration × contact time for target log-removal.
• Membrane fouling: Pretreatment and cleaning regimes govern membrane plant economics.
• Water safety plans: Risk-based management from catchment to consumer (WHO).