Water Treatment Notes

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

FormulaDiscrete settling

Stokes' law (laminar, $R e < 1$ , small particles):

v_{s} = \frac{g ( G _{s} - 1 ) d ^{2}}{18 ν}

Ideal settling basin (Camp–Hazen):

Surface overflow rate (SOR) $v_{0} = \frac{Q}{A _{s}}$ (m³/m²/day) — equal to the settling velocity of the smallest particle removed completely.
Particles with $v_{s} \geq v_{0}$ are removed 100%; particles with $v_{s} < v_{0}$ are removed in the fraction $\frac{v _{s}}{v _{0}}$ .
Removal depends on surface area, not on depth (in theory).

Detention time $t = \frac{V}{Q}$ ; horizontal velocity $v_{h} = \frac{Q}{B H}$ (kept below the scour velocity); weir loading = $Q$ ÷ 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 $A l_{2} (S O_{4})_{3} \cdot 18 H_{2} O$	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:

A l_{2} (S O_{4})_{3} + 3 C a (H C O_{3})_{2} \to 2 A l (O H)_{3} ↓ + 3 C a S O_{4} + 6 C O_{2}

1 mg/L of commercial alum ( $A l_{2} (S O_{4})_{3} \cdot 18 H_{2} O$ , 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

FormulaVelocity gradient (Camp and Stein)

G = \frac{P}{μ V}

$P$ = power input (W); $μ$ = dynamic viscosity; $V$ = volume of the tank.

Paddle flocculator: $P = \frac{1}{2} C_{D} A ρ v_{r}^{3}$ , where $v_{r}$ = relative velocity of paddles with respect to water (≈ 0.75 × paddle tip velocity).

The dimensionless product $Gt$ measures the opportunity for particle collisions.

Rapid (flash) mixing — intense agitation for a few seconds to about a minute (high $G$ ) 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 $G$ (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.

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Water Treatment