Access to safe drinking water is a fundamental human right. Conventional water treatment removes suspended solids, colloidal matter, pathogens, colour, odour, and taste from raw surface water or groundwater to meet IS 10500:2012 (Drinking Water Specifications). This guide covers every unit operation in a conventional WTP, design criteria, chemical dosages, and quality standards — essential knowledge for environmental engineering, public health engineering, and GATE/ESE preparation.
Sources of Water and Typical Contamination
| Source | Typical Turbidity | Typical Coliform | Main Treatment Challenge |
|---|---|---|---|
| River (plains) | 50–5000 NTU (monsoon) | High | High turbidity, organics, bacteria |
| Reservoir/lake | 5–50 NTU | Moderate | Algae, colour, taste/odour |
| Groundwater | 1–5 NTU | Low–High | Hardness, iron, fluoride, arsenic |
| Rainwater | < 5 NTU | Low | pH, dissolved gases |
Conventional WTP — Flow Sequence
- Intake works (raw water collection)
- Screening (coarse and fine screens)
- Aeration (if needed for iron/CO₂ removal)
- Coagulation (chemical addition + rapid mixing)
- Flocculation (slow mixing for floc formation)
- Sedimentation / Clarification (settling of floc)
- Filtration (removal of remaining turbidity)
- Disinfection (chlorination / UV)
- pH correction / Fluoridation (if needed)
- Storage and distribution
Unit 1: Screening
Removes gross solids — fish, leaves, debris — protecting downstream equipment.
- Coarse screens: Bar screens, 25–75 mm spacing
- Fine screens: Travelling band screens, 6–10 mm spacing
- Velocity through screen: 0.6–0.9 m/s (to prevent organic growth)
Unit 2: Coagulation
Destabilises colloidal particles (typically 0.001–1 μm, negatively charged) by neutralising their charge. Without coagulation, particles take days to settle by gravity.
Common Coagulants:
| Coagulant | Dose Range | Optimal pH | Notes |
|---|---|---|---|
| Alum [Al₂(SO₄)₃·18H₂O] | 5–50 mg/L | 6.5–7.5 | Most widely used in India, cheap |
| Ferric sulphate [Fe₂(SO₄)₃] | 10–100 mg/L | 5–9 | Works in wider pH range, coloured floc |
| Ferric chloride [FeCl₃] | 5–50 mg/L | 5–9 | Corrosive, heavy floc, settles faster |
| Poly-aluminium chloride (PAC) | 2–15 mg/L | 6.5–8.0 | Pre-polymerised, better performance |
Coagulant aid: Polyelectrolytes (anionic/cationic, 0.1–1 mg/L) improve floc formation and settling.
Rapid Mixing Design:
- Mixing time: 30–120 seconds
- Camp Number (velocity gradient G): 300–1000 s⁻¹
- G × t (Camp number): 30,000–150,000
Unit 3: Flocculation
Gentle agitation brings micro-flocs together to form macro-flocs large enough to settle.
- Detention time: 20–40 minutes
- G value: 10–100 s⁻¹ (much lower than coagulation)
- G × t: 10,000–100,000
- Types: mechanical (paddle flocculators), hydraulic (baffled chambers)
- Avoid over-mixing — breaks flocs apart
Unit 4: Sedimentation (Clarification)
Flocculated water flows into settling tank where floc settles under gravity.
Design Criteria:
| Parameter | Conventional Tank | Tube/Plate Settler |
|---|---|---|
| Surface overflow rate | 15–30 m³/m²/d | 60–120 m³/m²/d |
| Detention time | 2–4 hours | 20–60 min |
| Horizontal velocity | < 9 m/hour | < 15 m/hour |
| Depth | 3–5 m (+ 0.5 m sludge zone) | 2–3 m |
| Length:Width ratio | 3:1 to 5:1 | — |
Unit 5: Filtration
Removes residual turbidity, floc, and most pathogens. Produces water with turbidity < 1 NTU before disinfection.
Types of Filters:
| Type | Filtration Rate | Media | Cleaning |
|---|---|---|---|
| Slow Sand Filter (SSF) | 0.1–0.4 m³/m²/hr | Fine sand D₁₀=0.15–0.3 mm | Scraped every 1–3 months |
| Rapid Sand Filter (RSF) | 5–15 m³/m²/hr | Coarse sand D₁₀=0.5–1.0 mm | Backwashed every 24–72 hr |
| Dual media filter | 12–20 m³/m²/hr | Anthracite + sand | Backwashed |
| Pressure filter | Up to 25 m³/m²/hr | Sand ± anthracite | Backwash under pressure |
Note: Slow sand filters have biological action (schmutzdecke layer) that removes pathogens — efficiency 99.9% for Giardia and Cryptosporidium. Rapid filters are physical straining only — disinfection is mandatory downstream.
Unit 6: Disinfection
Ensures pathogen-free water to the consumer. Chlorination is universal in India.
Chlorination — IS 10500 Requirements:
- Free residual chlorine at consumer tap: ≥ 0.2 mg/L (IS 10500:2012)
- Recommended dose: 0.5–1.5 mg/L free residual chlorine after 30-min contact time at distribution inlet
- CT value (concentration × time): ≥ 0.5 mg·min/L for Giardia inactivation (WHO)
Breakpoint Chlorination:
Breakpoint occurs when chlorine demand from organics + NH₃ is satisfied
Chlorine dose beyond breakpoint = Free Residual Chlorine
Typical dose = 5–10× ammonia nitrogen content to reach breakpointIS 10500:2012 Key Drinking Water Standards
| Parameter | Acceptable Limit | Permissible Limit (no alt) |
|---|---|---|
| Turbidity | 1 NTU | 5 NTU |
| pH | 6.5–8.5 | No relaxation |
| Total Dissolved Solids | 500 mg/L | 2000 mg/L |
| Total Hardness (as CaCO₃) | 200 mg/L | 600 mg/L |
| Fluoride | 1.0 mg/L | 1.5 mg/L |
| Nitrate | 45 mg/L | No relaxation |
| Arsenic | 0.01 mg/L | No relaxation |
| Iron | 0.3 mg/L | No relaxation |
| Total Coliform | Absent/100 mL | No relaxation |
| E. coli/Thermotolerant coliform | Absent/100 mL | No relaxation |
Frequently Asked Questions
What is the per capita water supply standard in India?
CPHEEO Manual (2013) recommends 135 litres per capita per day (lpcd) for cities with house service connections and sewerage, 70 lpcd for towns with public stand posts. For Delhi NCR and metro cities, actual supply is 150–200 lpcd. WHO minimum for survival is 50 lpcd.
How is alum dose determined for a specific water source?
Jar test (IS 3025 Part 44) is used to determine optimum coagulant dose. Jars containing raw water samples are dosed with varying alum concentrations (5, 10, 20, 30, 50 mg/L), rapidly mixed for 1 min, slowly mixed for 15 min, then allowed to settle for 30 min. The dose giving minimum residual turbidity is the optimum dose. Full-scale WTPs typically use on-line turbidity monitoring and automatic coagulant dosing systems.
What is the difference between MPN and membrane filtration for coliform testing?
MPN (Most Probable Number — IS 1622) uses multiple tubes with lactose broth and counts statistical probability of coliforms. Membrane Filtration (IS 5401 Part 1) filters 100 mL sample through 0.45 μm membrane, incubates on selective media, and counts colonies directly. Membrane filtration is more accurate for potable water (<200 coliforms/100 mL). Both methods are acceptable under IS 10500:2012.