LESSON 6.1 — Water Supply Systems
§A — Water Demand & Per Capita Norms
§A.1 Design Demand: The Infrastructure Dependency
The quantity of water to be supplied is a function of service level, not just population. The primary determinant in Indian practice is whether a sewerage system exists, since water-seal toilets add a substantial flushing component (roughly 30–40 lpcd) to domestic demand.
URDPFI 2014 / CPHEEO Norms — Per Capita Supply:
| Settlement Classification | Norm (lpcd) | Key Rationale |
|---|---|---|
| Towns: piped supply, no sewerage | 70 | Dry/pour-flush latrines; no flushing demand |
| Cities: piped supply with sewerage (existing or proposed) | 135 | Includes water-seal toilet flushing; domestic uses |
| Metropolitan / Mega cities: piped supply + sewerage | 150 | Higher commercial/institutional load; larger NRW allowance |
Exam anchor — the 150 vs 135 trap: 150 lpcd applies to Metro/Mega cities only. A “city with sewerage” not meeting metro population thresholds is 135 lpcd. The difference is approximately 15 lpcd attributed to scale-related demand growth and system losses.
CPHEEO Additional Demand Components (within the above aggregate):
| Component | Demand (lpcd) |
|---|---|
| Drinking & cooking | 5 |
| Bathing | 55 |
| Ablution / toilet flushing | 30–45 |
| Washing clothes & utensils | 20 |
| Public and misc. standpost | 10 |
| Institutional & commercial (included) | ~15 |
| Fire fighting (design reserve, not per capita) | As per NBC |
Real-world gap: Actual supply in many Indian cities is 50–80 lpcd due to intermittent supply and non-revenue water (NRW) exceeding 40%. Design norms represent planned maximum, not operational reality.
§A.2 Derived Demand Figures for Design
From the per capita rate, the following design flows are calculated:
| Flow Type | Factor Applied | Use |
|---|---|---|
| Average Day Demand (ADD) | Population × lpcd | Treatment plant capacity |
| Maximum Day Demand (MDD) | 1.5 × ADD | Storage reservoir sizing |
| Maximum Hour Demand (MHD) | 2.7 × ADD (or 1.8 × MDD) | Distribution pipe sizing |
| Minimum Night Flow | ~0.25 × ADD | Leak detection baseline |
WTP Land Requirement (URDPFI 2014):
| Plant Capacity | Land (ha) |
|---|---|
| 5 MLD | 0.10 |
| 50 MLD | 0.93 |
| 100 MLD | 1.87 |
| 200 MLD | 3.73 |
| 500 MLD | 9.34 |
Land excludes raw water pumping station and clear water reservoir.
§B — Water Distribution Systems
Four pipe network configurations are used in municipal water supply. Selection depends on city morphology, population density, reliability requirements, and maintenance capacity.
§B.1 Network Layout Types
| Layout | Description | Geometry |
|---|---|---|
| Dead-End (Tree/Branch) | Main supply pipes branch progressively; pipes terminate at ends | Tree-like; no loops |
| Grid Iron (Interlaced) | Mains laid in two sets of parallel pipes intersecting at right angles, forming a rectangular grid | Rectangular mesh |
| Ring Main (Loop) | Main pipeline forms a closed loop around the supply area; sub-mains branch inward | Peripheral loop |
| Radial | Water feeds from a central reservoir into radial mains extending outward to the periphery; cross-connections may link radial arms | Star / spoke pattern |
§B.2 Comparative Analysis
| System | Advantages | Disadvantages | Ideal Condition |
|---|---|---|---|
| Dead-End | Low cost; simple design & maintenance | Stagnant water at ends → bacterial growth; long shutdown for repairs; pressure drops at terminal ends | Very small towns; low-density rural supply; temporary schemes |
| Grid Iron | Good pressure uniformity; any section isolatable; multiple flow paths | Higher pipe length → higher capital cost; more valves required | Medium-to-large cities; planned grid-layout urban areas (e.g., Chandigarh, Gandhinagar) |
| Ring Main | High reliability; isolation of any segment without full shutdown; pressure equalisation around the ring | More expensive than dead-end; design hydraulics more complex | High-density urban cores; industrial estates; areas requiring continuous supply (hospitals, high-rise zones) |
| Radial | Simple to design; good pressure at periphery when combined with ring connections | Flow convergence at centre can overload central mains; failure at the hub affects whole zone | Circular or fan-shaped service areas; satellite townships fed from a central overhead tank |
GATE MCQ pattern: “Which distribution system is most suitable for a densely developed rectangular urban block requiring continuous supply with minimal maintenance shutdown?” → Grid Iron or Ring Main (both acceptable; Grid Iron for planned rectangular layout, Ring Main for maximum isolation reliability).
§B.3 Pressure Zones — Gravity vs. Pumped Distribution
Why pressure zoning matters: A city’s topographic variation creates pressure differentials. Without zoning, high-elevation areas receive inadequate pressure while low-elevation areas experience excess pressure causing pipe bursts and high leakage.
| Method | Working Principle | Suitable Condition |
|---|---|---|
| Gravity distribution | Overhead reservoir / elevated tank at higher elevation; water flows entirely by gravity to service area | Favourable topography; source or storage above service area (e.g., hill stations — Shimla, Dehradun) |
| Pumped distribution | Pumps lift water from treatment plant / sump to distribution network | Flat terrain; source at or below service level (e.g., Delhi, Mumbai, Chennai) |
| Combined (gravity + pumped) | Pumps fill elevated service reservoir (ESR) during off-peak hours; ESR supplies by gravity during peak demand | Most common in Indian practice; ESR provides surge buffer and gravity pressure |
Pressure Zone Design Principles:
– Minimum residual pressure at consumer tap: 7 m head (low-pressure zone floor)
– Normal service pressure: 10–20 m head at consumer
– Maximum working pressure: 50–70 m head (to avoid pipe stress and excess leakage)
– Pressure zones are separated by pressure-reducing valves (PRVs) or break-pressure tanks
– High-rise zones (above 15–18 storeys) require a dedicated boosted zone or on-site pressure boosting
Individual Connection Sequence (CPHEEO standard):
Municipal Main → Ferrule → Goose neck → Service pipe → Stop cock → Water meter → Building plumbing
§C — Water Treatment Sequence
Raw surface water (river, lake, reservoir) carries suspended solids, colloids, organic matter, microorganisms, and dissolved minerals. Treatment removes these in a defined sequence — each stage conditions the water for the next.
§C.1 Treatment Train (Conventional)
Raw Water Intake
↓
[1] COAGULATION & FLOCCULATION
↓
[2] SEDIMENTATION (Clarification)
↓
[3] FILTRATION
↓
[4] DISINFECTION
↓
[Optional: SOFTENING if water is hard]
↓
Clear Water Reservoir → Distribution
§C.2 Stage-by-Stage Detail
Stage 1 — Coagulation & Flocculation
- Purpose: Destabilise fine colloidal particles that carry negative charges and remain suspended indefinitely.
- Chemical: Alum (potassium aluminium sulfate, KAl(SO₄)₂·12H₂O) — the most common coagulant in Indian practice.
- Mechanism: Alum releases Al³⁺ ions → neutralises negative surface charges on colloids → particles clump into flocs.
- Flocculation follows coagulation: slow, gentle mixing promotes floc growth to settleable size.
- Rapid mix (flash mixing): 1–2 minutes; slow mix (flocculation): 20–40 minutes.
Stage 2 — Sedimentation (Clarification)
- Purpose: Allow flocs and other settleable solids to settle under gravity.
- Type: Rectangular or circular clarifier / settling tank.
- Removes: 50–70% of suspended solids; 25–40% BOD.
- Settled sludge is periodically removed; clarified water flows to filters.
- Detention time: 2–6 hours in conventional tanks.
Stage 3 — Filtration
Three principal filter types (CPHEEO classification):
| Filter Type | Flow Rate | Pre-treatment Needed | Mechanism | Remarks |
|---|---|---|---|---|
| Rapid Gravity Sand Filter | 5–15 m³/m²/hr | Yes (coagulation required) | Physical straining | Most common in large municipal plants; backwash cleaning |
| Upflow Sand Filter | 5–10 m³/m²/hr | Yes | Physical straining | Less common; coarse-to-fine filtration path |
| Slow Sand Filter | 0.1–0.2 m³/m²/hr | No chemical pre-treatment | Schmutzdecke (biological layer) provides primary purification | Best quality output; large land area; suitable for small/medium towns |
Exam anchor: Slow sand filter = no chemicals + schmutzdecke = biological purification. Rapid gravity = requires alum pre-treatment. Rate difference: ≈50–100× faster for rapid vs. slow.
Stage 4 — Disinfection
- Purpose: Inactivate residual pathogenic microorganisms (bacteria, viruses, protozoa).
- Agent: Chlorine is the universal disinfectant. In Indian practice: bleaching powder [Ca(ClO)₂] for small plants; liquid chlorine or sodium hypochlorite for large plants.
- Residual chlorine in distribution: 0.1–0.3 ppm free residual (maintained throughout the network).
- Too little → recontamination risk in distribution system.
- Too much → taste/odour complaints; formation of trihalomethanes (THMs).
- Chlorination point: After filtration; sometimes also at intake (pre-chlorination) for heavily contaminated sources.
Stage 5 — Softening (Conditional)
- Applied where hardness (Ca²⁺, Mg²⁺ salts) is high.
- Base exchange (ion exchange) method: Raw water passed through sodium zeolite bed → zeolite exchanges Na⁺ for Ca²⁺/Mg²⁺ → softened water exits.
- Zeolite regeneration: flush with brine (NaCl solution).
§D — Water Quality Parameters & Acceptable Limits
§D.1 Key Parameters Table (IS 10500:2012 / WHO 4th Ed. 2017)
| Parameter | Acceptable Limit (IS 10500) | WHO Guideline | Significance |
|---|---|---|---|
| pH | 6.5–8.5 | 6.5–9.5 | Controls corrosivity (low pH) and scale formation (high pH); affects chlorine efficacy |
| Turbidity | 5 NTU (desirable: 1 NTU) | < 1 NTU (treated) | Indicates suspended particles; high turbidity shields microbes from disinfection |
| Total Coliform | Absent in 100 mL | Absent in 100 mL | Indicator of fecal contamination; presence implies potential pathogen risk |
| E. coli / Fecal Coliform | Absent in 100 mL | Absent in 100 mL | Direct fecal contamination indicator; more specific than total coliform |
| Total Hardness (as CaCO₃) | 300 mg/L (max 600 mg/L) | No health limit; aesthetic ≤ 500 | Causes scaling in pipes/boilers; reduces soap lather |
| Total Dissolved Solids (TDS) | 500 mg/L (max 2000 mg/L) | 600 mg/L (taste threshold) | Higher TDS = salty/brackish taste; mineral accumulation |
| Fluoride | 1.0 mg/L (max 1.5 mg/L) | 1.5 mg/L | Excess → fluorosis (dental/skeletal); deficiency → dental caries |
| Arsenic | 0.01 mg/L | 0.01 mg/L | High in Bengal delta groundwater; carcinogenic at elevated concentrations |
| Iron | 0.3 mg/L | 0.3 mg/L | Staining, taste, pipe clogging; promotes bacterial growth |
| Residual Chlorine | Min 0.2 mg/L at consumer end | 0.2–0.5 mg/L | Disinfection protection through distribution; must not fall to zero |
| BOD (raw source water) | — | < 3 mg/L (drinking source) | Organic load indicator; treated water BOD should approach zero |
Exam traps:
– Turbidity limit: 5 NTU (not 5 mg/L — NTU is the unit for turbidity).
– Coliform: limit is absence (zero), not a concentration threshold.
– Hardness limit: 300 mg/L desirable; exam may offer 200/300/500 — choose 300.
– pH: remember the lower bound (6.5) is as important as the upper — too acidic water corrodes distribution pipes.
§E — Rainwater Harvesting (RWH)
§E.1 Definition & Legal Mandate
RWH is the collection, storage, or groundwater recharge of rainwater from a defined catchment (rooftop, paved surface, open ground) before it enters the storm drain system.
Legal basis: Section 15 of the Environment (Protection) Act, 1986 empowers the Central Ground Water Authority (CGWA) to mandate RWH. Tamil Nadu was the first state to make RWH compulsory statewide (amendment to Chennai City Municipal Corporation Act). Gujarat, Rajasthan, and Delhi followed with mandates for new buildings above specified plot sizes.
§E.2 Two Principal Collection Methods
| Method | How It Works | When to Use |
|---|---|---|
| Surface storage | Collected rainwater → first-flush diverter → settlement tank → storage cistern/tank for direct reuse | Impermeable subsoil; saline/deep groundwater; water needed for direct use (toilet flushing, gardening, irrigation) |
| Groundwater recharge | Collected rainwater → settlement/filter pit → recharge structure (pit, trench, well, shaft) → aquifer replenishment | Permeable subsoil; depleted groundwater table; long dry seasons requiring sustained baseflow |
6 Recharge Structure Types (CGWB classification):
| Type | Description |
|---|---|
| Abandoned/dug well | Existing disused well used as recharge point |
| Hand pump shaft | Connects RWH pipe to hand pump borehole |
| Recharge pit | 1–2 m dia., 2–3 m deep; gravel-filled; for shallow aquifers |
| Recharge trench | Long, shallow, filter-filled trench; suitable for gentle slopes |
| Vertical shaft | Bored through impermeable overburden to reach deep aquifer |
| Trench + bore | Hybrid: surface trench feeds vertical bore; maximum depth reach |
§E.3 Runoff Calculation Formula
The rational formula for RWH yield:
$$boxed{Q = C times i times A}$$
Where:
– Q = runoff / harvestable quantity (m³/hr)
– C = runoff coefficient (dimensionless; fraction of rainfall becoming runoff)
– i = rainfall intensity (mm/hr)
– A = catchment area (hectares)
Unit consistency check: If A is in m², convert: 1 hectare = 10,000 m². If i is in mm/hr and A in m², Q in litres/hr = C × i (mm/hr) × A (m²) ÷ 1000.
Standard Runoff Coefficients (IS 15797:2008):
| Surface Type | C Value |
|---|---|
| Metal / concrete / tile roof | 0.75–0.95 |
| Paved surface (asphalt, concrete) | 0.70–0.90 |
| Compacted earth / unpaved | 0.20–0.40 |
| Lawn / grass | 0.05–0.25 |
| Mixed urban catchment | 0.40–0.70 |
Composite Runoff Coefficient (for mixed catchments):
$$C_{composite} = frac{A_1 C_1 + A_2 C_2 + cdots + A_n C_n}{A_{total}}$$
§E.4 Storage Tank Sizing Approaches
| Approach | Basis | Method |
|---|---|---|
| Supply-side (catchment-based) | Volume available from roof/catchment | Annual harvestable volume = C × annual rainfall (m) × A (m²); size tank for 30–50% of monsoon surplus |
| Demand-side (requirement-based) | Water needs of users | Daily demand (lpcd × users) × dry days without rain; size tank to bridge the longest dry spell |
| Cost-optimal | Balance construction cost vs. water security | Marginal benefit per additional tank volume; select where benefit-cost ratio is maximised |
First-Flush Diversion: The first 1–2 mm of rainfall from any event carries the highest concentration of dust, bird droppings, and atmospheric contaminants. All RWH systems must incorporate a first-flush diverter that discards this initial runoff before directing flow to storage or recharge.
§D (Required Section) — Per Capita Demand Table + RWH Worked Numericals
D.1 Consolidated Per Capita Demand Reference
| Settlement Type | Supply Norm (lpcd) | MDD Factor | MHD Factor | Planning Use |
|---|---|---|---|---|
| Town — no sewerage | 70 | 1.5 × ADD | 2.7 × ADD | Small towns, rural towns |
| City — with sewerage | 135 | 1.5 × ADD | 2.7 × ADD | Class I/II cities |
| Metro / Mega city | 150 | 1.5 × ADD | 2.7 × ADD | Mumbai, Delhi, Kolkata etc. |
(ADD = Average Day Demand = Population × lpcd norm)
D.2 Worked Numerical — NAT Example 1: Mixed Catchment RWH Runoff
Problem: A housing society in Nagpur has:
– Rooftop area: 1,200 m² (concrete tiles, C = 0.85)
– Paved driveway: 600 m² (asphalt, C = 0.80)
– Open lawn: 1,000 m² (grass, C = 0.10)
A storm event produces a rainfall intensity of 60 mm/hr. Calculate:
(a) Composite runoff coefficient
(b) Total runoff generated per hour (m³/hr)
(c) Volume of runoff in 45 minutes of this storm (litres)
Solution:
(a) Composite C:
$$C = frac{(1200 times 0.85) + (600 times 0.80) + (1000 times 0.10)}{1200 + 600 + 1000}$$
$$C = frac{1020 + 480 + 100}{2800} = frac{1600}{2800} = mathbf{0.571}$$
(b) Total area: A = 2800 m² = 0.28 hectares
Apply Q = C × i × A:
$$Q = 0.571 times 60 text{ mm/hr} times 0.28 text{ ha}$$
$$Q = 0.571 times 60 times 0.28 = mathbf{9.59 text{ m}^3/text{hr}}$$
(Check via m²: Q = 0.571 × 60 × 2800 / 1000 = 95,928 L/hr = 95.93 m³/hr — note: when A in m², i in mm/hr, divide by 1000 for litres/hr, or by 1,000,000 for m³/hr)
Corrected working (units reconciled):
Q (litres/hr) = C × i (mm/hr) × A (m²) = 0.571 × 60 × 2800 = 95,928 L/hr
Q = 95.93 m³/hr
(c) Volume in 45 minutes:
$$V = 95,928 times frac{45}{60} = 95,928 times 0.75 = mathbf{71,946 text{ litres}} approx mathbf{71.9 text{ m}^3}$$
Answer: C = 0.571; Q = 95.93 m³/hr; V (45 min) = 71.9 m³ ≈ 71,946 litres
D.3 Worked Numerical — NAT Example 2: Storage Tank Sizing (Demand-Side)
Problem: A school in Jaipur has 500 students and 50 staff. Non-potable RWH water is used for toilet flushing at 30 lpcd. The school operates 8 months/year; the remaining 4 months (dry season) have no usable rainfall. The rooftop catchment is 800 m² with C = 0.85. Annual rainfall = 650 mm.
Calculate:
(a) Annual non-potable water demand
(b) Annual harvestable RWH volume from rooftop
(c) Minimum storage tank capacity to cover the 4-month dry period
Solution:
(a) Annual non-potable demand:
Total users = 500 + 50 = 550
Daily demand = 550 × 30 lpcd = 16,500 L/day
School operating days (8 months) ≈ 8 × 25 = 200 working days/year
Annual demand = 16,500 × 200 = 33,00,000 L = 3,300 m³/year
(b) Annual harvestable RWH volume:
Q_annual = C × Annual rainfall (m) × A (m²)
Q_annual = 0.85 × 0.65 m × 800 m² = 442 m³/year = 4,42,000 litres/year
Note: This is less than annual demand (3,300 m³) — confirms RWH is a supplementary source, not full supply.
(c) Storage tank for 4-month dry season:
Dry period = 4 months × 25 working days = 100 days
Storage required = 16,500 L/day × 100 days = 16,50,000 litres = 1,650 m³
Practical recommendation: A minimum tank of 1,650 m³ (or a system of multiple cisterns totalling this volume) is needed to bridge the dry period entirely from stored RWH water. If RWH is insufficient to fill this tank from the 8-month operational period (442 m³ << 1,650 m³), the storage must be supplemented from mains supply.
Answer: Annual demand ≈ 3,300 m³; Annual harvestable = 442 m³; Required dry-season tank = 1,650 m³
§F — Mini-Check: Practice Questions
NAT 1 — RWH Runoff Calculation
A terrace in Hyderabad has:
– RCC roof: 500 m², C = 0.90
– Paved parking: 300 m², C = 0.75
– Unpaved earth: 200 m², C = 0.30
Rainfall intensity = 80 mm/hr for 30 minutes.
(a) Calculate the composite runoff coefficient.
(b) Calculate the total runoff volume generated in 30 minutes (in litres).
Answer key:
– A_total = 500 + 300 + 200 = 1,000 m²
– C = (500×0.90 + 300×0.75 + 200×0.30) / 1000 = (450 + 225 + 60) / 1000 = 735/1000 = 0.735
– Q (L/hr) = 0.735 × 80 × 1000 = 58,800 L/hr
– Volume in 30 min = 58,800 × 0.5 = 29,400 litres = 29.4 m³
NAT 2 — Per Capita Demand & Daily Requirement
A Class I city (population 8 lakh) has a functioning sewerage system. Calculate:
(a) Average Day Demand (ADD) in MLD
(b) Maximum Day Demand (MDD) in MLD
(c) Maximum Hour Demand (MHD) in MLD
Answer key:
– lpcd norm = 135 (city with sewerage)
– ADD = 8,00,000 × 135 L = 1,08,00,00,000 L/day = 108 MLD
– MDD = 1.5 × 108 = 162 MLD
– MHD = 2.7 × 108 = 291.6 MLD
MCQ 1 — Distribution System Selection
A new township of 80,000 population is being planned with a rectangular grid street layout. The supply system must ensure continuous supply (hospitals and institutions are major users), and any section must be isolatable for repair without interrupting the rest of the network. Which distribution system is most appropriate?
(A) Dead-end system
(B) Ring main with sub-mains only
(C) Grid iron system with looped interconnections
(D) Radial system from a central overhead tank
Answer: (C)
Rationale: Grid iron with looped interconnections combines the regularity of a grid (matching the rectangular town layout) with the isolation benefit of loops — any section can be valved off without full shutdown. Ring main alone covers only the periphery. Dead-end cannot be isolated without service interruption. Radial creates flow convergence at the hub, unsuitable for high-density uniform demand.
MCQ 2 — Water Quality Parameter
Which parameter’s acceptable limit in drinking water (IS 10500) is stated as “Absent in 100 mL”?
(A) Turbidity
(B) Total Hardness
(C) pH
(D) Total Coliform
Answer: (D)
Rationale: Total Coliform (and E. coli) limits are expressed as absence in a 100 mL sample — there is no acceptable concentration; any detection indicates contamination. Turbidity is in NTU; hardness in mg/L CaCO₃; pH is dimensionless.
MCQ 3 — Treatment Sequence
Which is the correct conventional sequence for surface-water treatment before distribution?
(A) Filtration → Coagulation → Disinfection → Sedimentation
(B) Coagulation → Sedimentation → Filtration → Disinfection
(C) Sedimentation → Coagulation → Disinfection → Filtration
(D) Disinfection → Filtration → Coagulation → Sedimentation
Answer: (B)
Rationale: Standard train: coagulation/flocculation → sedimentation (settle flocs) → filtration (remove remaining particles) → disinfection (kill pathogens). Reversing steps breaks process logic.
§G — Exam Traps & Anchors
| Trap | Correct Answer |
|---|---|
| “150 lpcd applies to all cities with sewerage” | No — 150 lpcd is Metro/Mega only; other cities with sewerage = 135 lpcd |
| “Slow sand filter requires chemical pre-treatment” | No — slow sand filter uses biological schmutzdecke; no alum required |
| “Turbidity limit = 5 mg/L” | Wrong unit — turbidity is measured in NTU, limit is 5 NTU |
| “Ring main = gravity distribution” | No — ring main is a pipe layout pattern; gravity vs. pumped is a separate classification |
| “First flush is beneficial for storage” | No — first flush is the most contaminated runoff; it must be diverted away |
| “Dead-end system is best for large cities” | No — dead-end is suitable for small/rural settlements only; stagnation risk at terminals |
| “RWH can replace municipal supply” | No — RWH is an augmentation measure; yield is always far below full demand |
| “Q = C × I × A; A must be in hectares” | Units must be consistent; if i in mm/hr and A in m², divide by 1000 for litres; hectares apply only when using the tabulated rational formula version |
| “Coliform limit is 1/100 mL” | No — limit is Absent (zero); any coliform = non-compliance |
| “Hardness limit in IS 10500 is 200 mg/L” | No — desirable limit is 300 mg/L; max permissible is 600 mg/L |