Part 1 of 2
Water Treatment
Last reviewed 16 Sept 2026 · 13 min read
Treatment flow sheet
A conventional surface water treatment plant:
Intake and screens → (plain sedimentation for very turbid water) → aeration (if needed) → rapid mixing with coagulant → flocculation → sedimentation (clarification) → rapid sand filtration → disinfection → clear water reservoir → distribution
Groundwater often needs only aeration, iron/manganese removal and disinfection; hard or fluoride-rich water needs special treatment.
| Unit | Main impurity removed |
|---|---|
| Screening | Large floating matter |
| Aeration | Dissolved gases (CO₂, H₂S), tastes and odours; oxidises iron and manganese |
| Plain sedimentation | Settleable suspended solids |
| Coagulation–flocculation–sedimentation | Colloidal turbidity and colour |
| Filtration | Remaining fine flocs, turbidity, some bacteria |
| Disinfection | Pathogens |
| Softening | Hardness |
Screening
- Coarse screens (bar racks) at the intake to exclude floating debris, and fine screens or micro-strainers for smaller matter and algae.
- Cleaned manually or mechanically; velocity through screens kept low.
Aeration
Brings water into intimate contact with air:
- Removes CO₂ (reduces corrosiveness), H₂S and volatile tastes and odours.
- Adds oxygen — oxidises dissolved iron and manganese to insoluble forms that can be settled and filtered.
Types: spray aerators (nozzles), cascade aerators (water flows down steps), multiple tray aerators (perforated trays with coke/gravel), diffused air aerators (compressed air bubbled through water), mechanical aerators.
Sedimentation
Types of settling
| Type | Description | Example |
|---|---|---|
| Type I — discrete settling | Particles settle individually without interaction at constant velocity | Plain sedimentation of sand/silt, grit chambers |
| Type II — flocculent settling | Particles coalesce, grow and settle faster | Coagulated water, primary sewage settling |
| Type III — zone (hindered) settling | High concentration; particles settle as a mass with a distinct interface | Secondary clarifiers (activated sludge) |
| Type IV — compression | Particles form a structure compressed by the weight above | Sludge thickeners, bottom of clarifiers |
Stokes' law (laminar, , small particles):
Ideal settling basin (Camp–Hazen):
- Surface overflow rate (SOR) (m³/m²/day) — equal to the settling velocity of the smallest particle removed completely.
- Particles with are removed 100%; particles with are removed in the fraction .
- Removal depends on surface area, not on depth (in theory).
Detention time ; horizontal velocity (kept below the scour velocity); weir loading = ÷ weir length.
Typical design values (clarifiers after coagulation, CPHEEO guidance): detention time about 2–2.5 h; surface loading about 30–40 m³/m²/day; weir loading not exceeding about 300 m³/m/day. Plain sedimentation tanks without coagulant need longer detention.
Types of tanks
- Horizontal-flow rectangular tanks — long, narrow; inlet and outlet baffles; sludge scraped to a hopper.
- Circular radial-flow tanks — central feed, peripheral weir; rotating scraper.
- Upflow (hopper-bottom) tanks and sludge blanket clarifiers.
- Tube (lamella) settlers — inclined tubes or plates reduce the settling distance, greatly increasing capacity in a small area.
Coagulation and flocculation
Colloidal particles (clay, colour, bacteria) are negatively charged and repel each other, so they do not settle. Coagulation destabilises them; flocculation gently brings them together into large, settleable flocs.
Mechanisms
- Charge neutralisation — positively charged hydrolysis products of coagulants neutralise particle charges.
- Double-layer compression.
- Sweep (enmeshment) coagulation — particles trapped in precipitating metal hydroxide flocs (dominant at usual alum doses).
- Inter-particle bridging — by long-chain polymers.
Coagulants
| Coagulant | Features |
|---|---|
| Alum — aluminium sulphate | Most common; cheap; best pH about 6.5–8.5; consumes alkalinity |
| Ferrous sulphate (copperas) + lime | For high pH waters; used where lime is added anyway |
| Chlorinated copperas (ferric sulphate + ferric chloride) | Effective over a wide pH range; removes colour |
| Ferric chloride / ferric sulphate | Wide pH range, dense flocs; corrosive |
| Sodium aluminate | For waters low in alkalinity; also used in softening |
| Poly-aluminium chloride (PAC) | Pre-hydrolysed; effective at low doses and low temperatures; less alkalinity consumed |
Coagulant aids: activated silica, bentonite clay, polyelectrolytes (cationic, anionic, non-ionic polymers), lime or soda ash (to supply alkalinity or adjust pH).
Alum reaction with natural alkalinity:
1 mg/L of commercial alum (, molecular weight ≈ 666) consumes about 0.45 mg/L alkalinity as CaCO₃.
Jar test — samples dosed with different coagulant amounts are rapidly mixed, flocculated and settled in a gang stirrer; the dose giving the best clarity at least cost is the optimum dose.
Mixing and flocculation
= power input (W); = dynamic viscosity; = volume of the tank.
Paddle flocculator: , where = relative velocity of paddles with respect to water (≈ 0.75 × paddle tip velocity).
The dimensionless product measures the opportunity for particle collisions.
- Rapid (flash) mixing — intense agitation for a few seconds to about a minute (high ) to disperse coagulant uniformly: mechanical flash mixers, hydraulic jumps, baffles, in-line mixers.
- Flocculation (slow mixing) — gentle agitation for about 10–40 minutes with low (commonly about 10–75 s⁻¹) so flocs grow without breaking: paddle flocculators, baffled channels.
- Clariflocculator — a circular unit combining a central flocculation zone with an outer clarifier; widely used in India.