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Chapter 2 of 6

Water Treatment & Distribution

In the GATE Civil syllabus under Environmental Engineering · 2 parts

Water Treatment · Conveyance & Distribution of Water

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📑 Contents (22 sections)

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

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.

Part 2 of 2

Conveyance & Distribution of Water

Last reviewed 16 Sept 2026 · 9 min read

Requirements of a good distribution system

Supply water to all consumers at adequate pressure (and fire flow when needed).
Deliver the required quantity at all times, including peak hours.
Maintain water quality — watertight, no contamination, adequate residual chlorine.
Be reliable — a break should affect as few consumers as possible (loops, valves).
Allow easy maintenance, repairs and extensions; economical.
Minimise leakage and unauthorised use.

Methods of distribution

Method	Description	Suitability
Gravity system	Source (reservoir) at sufficient elevation; water flows by gravity	Hill sources above the town — cheapest and most reliable
Pumping without storage	Pumps deliver directly into mains	Not preferred — pumps must follow demand, power failure stops supply
Combined (pumping with storage)	Water pumped at a uniform rate to elevated/service reservoirs; excess stored during low demand and supplied during peaks	Most common — pumps run at uniform rate, storage covers peaks, fire and breakdowns

Systems of supply

Continuous (24 × 7) supply — water available at all times; best for quality (positive pressure keeps contamination out) and convenience; needs good leak control and metering.
Intermittent supply — water supplied for a few hours; pipes empty and contaminated water can be sucked in through leaks; consumers store water wastefully; used where sources are inadequate.

Layouts of distribution systems

Layout	Description	Merits	Demerits
Dead-end (tree) system	Main → submains → branches ending in dead ends	Simple, cheap, easy design, fewer valves	Stagnant water at dead ends; a break cuts off all downstream consumers; pressure falls at remote ends
Grid-iron (reticulation) system	Interconnected mains and branches forming loops	No dead ends; water reaches points from several directions; small area affected by repairs; good for fire flow	More pipes and valves; complex design (Hardy Cross); costlier
Ring (circular) system	A main ring around a district with branches inside	Every point supplied from two directions; good for well-planned towns	More pipe length
Radial system	Area divided into zones, each with a central reservoir supplying radially outward	High pressure and quick service; efficient	Needs several reservoirs

Planned cities with rectangular street patterns suit the grid-iron system; old irregular towns often have dead-end systems.

Service reservoirs

Service (distribution) reservoirs store treated water within the distribution area.

Functions: balance the difference between uniform supply (pumping) and fluctuating demand; maintain pressure; store water for fire fighting and emergencies/breakdowns; allow pumps to run at uniform rate and during off-peak power periods; reduce pipe sizes.

Types:

Surface (ground-level) reservoirs — at high ground; masonry/RCC; large capacity.
Elevated reservoirs (overhead tanks) — on staging (columns or shafts) where no high ground exists; provide the pressure head.
Standpipes — tall cylindrical tanks resting on the ground.

Fittings: inlet and outlet pipes, overflow pipe, washout (scour) pipe, ventilators, water level indicator, manholes, ladder, lightning conductor.

Capacity

Total capacity = balancing (equalising) storage + breakdown storage + fire storage.

Balancing storage is found by the mass curve method (or the analytical/hydrograph method):

Tabulate the hourly (or 2-hourly) supply and demand.
Compute the cumulative (supply − demand).
Balancing storage = maximum cumulative surplus + maximum cumulative deficit (i.e. the range of the cumulative difference).

Pipes

Pipe materials

Material	Features
Cast iron (CI)	Durable, corrosion-resistant; heavy and brittle; largely replaced by DI
Ductile iron (DI)	Strong, tough, less brittle than CI; cement-mortar lined; widely used for mains
Mild steel (MS)	High strength, large diameters, lighter than CI; needs corrosion protection (lining and coating); used for trunk mains
Prestressed concrete (PSC)	Large diameters, high pressures, long life; heavy
Reinforced cement concrete (RCC)	Low-pressure mains, gravity conduits
Asbestos cement (AC)	Formerly common; light, corrosion-resistant; brittle; health concerns over asbestos have restricted its use
uPVC / PVC	Light, cheap, smooth, corrosion-free; for small diameters and moderate pressures; affected by sunlight and heat
HDPE	Flexible, light, joints by fusion welding; corrosion-free; good for house connections and difficult terrain
GRP / FRP	Light, corrosion-resistant, large diameters

Joints

Spigot and socket (lead or rubber ring), flanged joints (pumping stations, valves), mechanical joints, flexible joints (settlement, river crossings), expansion joints (steel pipes exposed to temperature), push-on (rubber gasket) joints, welded joints (steel), solvent cement joints (PVC), butt or electrofusion joints (HDPE).

Hydraulic design

FormulaHead loss in pipes

Hazen–Williams (SI):

V = 0.849 C_{H} R^{0.63} S^{0.54} h_{f} = \frac{10.67 L Q ^{1.852}}{C _{H}^{1.852} D ^{4.87}}

$C_{H}$ ≈ 140–150 for PVC/HDPE; about 130–140 for new DI/CI and steel with smooth lining; lower for old or tuberculated pipes.

Darcy–Weisbach: $h_{f} = \frac{f L V ^{2}}{2 g D}$

Manning: $V = \frac{1}{n} R^{2/3} S^{1/2}$ (gravity conduits, sewers)

Residual pressure at the ferrule point commonly recommended (CPHEEO): 7 m for single-storey, 12 m for two-storey and 17 m for three-storey buildings. Higher buildings use their own sumps and pumps.
Velocities are kept moderate (roughly 0.6–2 m/s in mains) to avoid deposits at low velocities and excessive head loss and water hammer at high velocities.
Equivalent pipe method — replaces a series/parallel combination with a single pipe of equal head loss.
Hardy Cross method — iterative balancing of flows in loops: $Δ Q = - \frac{\sum r Q ^{n}}{\sum n r ∣ Q ∣ ^{n - 1}}$ ( $n$ = 1.852 for Hazen–Williams, 2 for Darcy).
Other methods: circle method, Newton–Raphson; software such as EPANET, LOOP and similar hydraulic models.

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