CHAPTER 9

Disinfection — Chlorination and Alternatives

Q: What is typical chlorine dose?

1.0-2.5 mg/L for treated water. Dose = chlorine demand (0.5-1.5 mg/L) + required residual (0.5-1.0 mg/L at plant outlet, 0.2 mg/L at consumer). Varies with raw water quality.

Q: What residual chlorine at consumer tap?

Minimum 0.2 mg/L at farthest consumer point (per IS 10500:2012). Higher residual 0.5 mg/L near WTP, declining through distribution. Below 0.2 mg/L indicates contamination risk — investigate and increase dose.

Q: Can I use bleaching powder instead of chlorine gas?

Yes, for small schemes (< 5 MLD). Bleaching powder 30-35% available chlorine. Dose: for 1 MLD plant with 2 mg/L dose, need 2 kg chlorine/day = 6-7 kg bleaching powder/day. Prepare daily solution; unstable in storage.

Q: Is UV disinfection sufficient?

UV inactivates bacteria, viruses, Cryptosporidium but provides NO residual — cannot protect distribution system. Use UV as primary at treatment plant outlet + small chlorine dose (0.2 mg/L) for distribution residual.

Q: What is breakpoint chlorination?

Process of adding chlorine in excess of chlorine demand + ammonia demand, such that all ammonia is destroyed and free chlorine appears. Beyond the breakpoint, residual chlorine accumulates as free chlorine (more effective than chloramines).

Q: What are THMs (trihalomethanes)?

Disinfection byproducts from chlorine reacting with organic matter (humic acids). IS 10500 limit 100 μg/L. Risk: carcinogenic at high exposure. Reduce via (a) remove organics before chlorination via coagulation, (b) switch to UV/ozone primary disinfection.

Disinfection

Specifies disinfection methods for drinking water — chlorination (gas, liquid sodium hypochlorite, bleaching powder, on-site hypochlorite generation), UV irradiation, ozonation. Covers dose calculation, contact time, residual monitoring, and safety.

⚗ Water TreatmentManual on Water Supply and Treatment3rd Edition (1999) with 2024 revision updates

Key formulas

Chlorine demand + Residual = Chlorine dose. Example: 1.0 mg/L demand + 0.5 mg/L residual = 1.5 mg/L dose.
CT value (disinfection efficacy): CT = C × T, where C = residual concentration (mg/L), T = contact time (minutes). Example: 0.5 mg/L × 30 min = 15 mg.min/L.
Chlorine dose (kg/day) = Flow (MLD) × Dose (mg/L). Example: 100 MLD × 2 mg/L = 200 kg/day chlorine.
Chlorine from bleaching powder (calcium hypochlorite): 1 kg of 30% strength bleaching powder = 300 g available chlorine.
Sodium hypochlorite dose: 5% NaOCl solution provides 50 g/L available chlorine; 10% provides 100 g/L.

Key values & thresholds

chlorine dose typical mg per L: 1.0 - 2.5
chlorine residual WTP outlet mg per L: 0.5 - 1.0
chlorine residual consumer end mg per L: 0.2 - 0.5
contact time minutes: 30
CT value mgmin per L: 15 (E.coli), 30 (virus), 100+ (Cryptosporidium)
chlorine demand typical mg per L: 0.5 - 1.5
bleaching powder strength pct available Cl: 30 - 35
sodium hypochlorite strength pct: 10 (commercial) / 5 (field diluted)
chlorine gas purity pct: 99.5
chlorine gas cylinder size kg: 100 - 900
UV dose mJ per cm2: 30 - 40
ozone dose mg per L: 0.5 - 2.0
ozone contact time minutes: 10

Clause-level requirements

Chlorination is mandatory for all public water supply in India. Residual chlorine 0.2 mg/L at farthest consumer point ensures bacterial protection during distribution.
Chlorine forms: (a) chlorine gas (99.5% purity, most economical for large WTPs > 5 MLD), (b) bleaching powder (calcium hypochlorite 30-35% available Cl, used in small/rural schemes), (c) sodium hypochlorite (5-10% commercial; stable, safer than gas), (d) on-site hypochlorite generation (from salt electrolysis).
Chlorine dose = Chlorine demand + Residual. Chlorine demand satisfied by reducing substances (Fe²⁺, Mn²⁺, organics, ammonia). Residual provides continued protection.
Contact time: minimum 30 minutes after chlorine addition and before distribution. Achieved via chlorine contact tank or clear water reservoir.
Breakpoint chlorination (for ammonia removal): dose until residual rises beyond the breakpoint — ammonia is destroyed and free chlorine persists.
UV disinfection (alternative): dose 30-40 mJ/cm² inactivates bacteria and viruses; no residual protection so must be combined with small chlorine residual for distribution system.

Practitioner notes — what goes wrong in the field

Chlorine gas: most economical for large WTPs but requires safety infrastructure — chlorine containment room, emergency scrubber, gas detector, training. CPCB licensing required.
Sodium hypochlorite: most common for medium WTPs (5-50 MLD). Safer than gas, easier to dose, but deteriorates in storage (10% loss per month).
Bleaching powder: for rural piped schemes (< 5 MLD) and small communities. Solution prepared daily; unstable.
On-site hypochlorite generation: from salt brine electrolysis. Eliminates chlorine transport/storage. Cost ₹5-10 lakh per MLD capacity. Growing adoption.
UV disinfection: no chemical handling, but no residual protection. Suitable as primary disinfection at treatment plant exit; supplement with small chlorine dose (0.2 mg/L) for distribution.
Ozone: highly effective for taste/odor removal and pathogen inactivation. Cost 3-5× chlorination. Used at Delhi Sonia Vihar, Chennai Veeranam WTPs.
CT value concept: higher CT = better disinfection. Cryptosporidium requires CT 100+ (not achieved by chlorine alone in typical contact times); UV or ozone for full Cryptosporidium removal.
Chlorine dose calculation: 100 MLD plant with 2 mg/L dose requires 200 kg/day chlorine = 2-4 × 100 kg cylinders per day. Annual 70-150 tonnes.
Chlorine residual monitoring: DPD test (lab-accurate, 5 min), orthotolidine (field, quick), online chlorine analyzer (continuous, $2000-10,000 equipment). Sample at plant exit, reservoirs, consumer taps.
Free vs combined chlorine: free chlorine (HOCl, OCl⁻) is effective; combined (chloramines from ammonia) is weaker but longer-lasting. Breakpoint chlorination destroys combined and produces free chlorine.
THM (trihalomethanes): disinfection byproducts from chlorine + organic matter. IS 10500 limit 100 μg/L. Reduce by removing organic matter before chlorination (coagulation) or switch to UV/ozone primary.
Safety: chlorine gas IDLH (Immediately Dangerous to Life or Health) 10 ppm. Proper ventilation, PPE, emergency response plan, evacuation procedure. Personnel training mandatory.

FAQs

What is typical chlorine dose?

1.0-2.5 mg/L for treated water. Dose = chlorine demand (0.5-1.5 mg/L) + required residual (0.5-1.0 mg/L at plant outlet, 0.2 mg/L at consumer). Varies with raw water quality.

What is minimum contact time?

30 minutes after chlorine addition and before distribution. Achieved via chlorine contact tank or clear water reservoir retention. CT value (concentration × time) ≥ 15 mg.min/L for E. coli inactivation.

What residual chlorine at consumer tap?

Minimum 0.2 mg/L at farthest consumer point (per IS 10500:2012). Higher residual 0.5 mg/L near WTP, declining through distribution. Below 0.2 mg/L indicates contamination risk — investigate and increase dose.

Can I use bleaching powder instead of chlorine gas?

Yes, for small schemes (< 5 MLD). Bleaching powder 30-35% available chlorine. Dose: for 1 MLD plant with 2 mg/L dose, need 2 kg chlorine/day = 6-7 kg bleaching powder/day. Prepare daily solution; unstable in storage.

Is UV disinfection sufficient?

UV inactivates bacteria, viruses, Cryptosporidium but provides NO residual — cannot protect distribution system. Use UV as primary at treatment plant outlet + small chlorine dose (0.2 mg/L) for distribution residual.

What is breakpoint chlorination?

Process of adding chlorine in excess of chlorine demand + ammonia demand, such that all ammonia is destroyed and free chlorine appears. Beyond the breakpoint, residual chlorine accumulates as free chlorine (more effective than chloramines).

What are THMs (trihalomethanes)?

Disinfection byproducts from chlorine reacting with organic matter (humic acids). IS 10500 limit 100 μg/L. Risk: carcinogenic at high exposure. Reduce via (a) remove organics before chlorination via coagulation, (b) switch to UV/ozone primary disinfection.

Calculator

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Chlorine Dose — Gas · Bleaching Powder · NaOCl

↓ Download Excel

Compute daily chlorine requirement for any plant capacity and convert to the three common chlorine forms used in Indian water supply.

Inputs

FlowMLD

Chlorine dosemg/L

Demand 0.5-1.5 mg/L + Residual 0.5-1.0 mg/L

Bleaching powder strength%

Available chlorine; 30-35% standard

NaOCl solution strength%

5% typical field-diluted, 10% commercial bulk

Contact timemin

Outputs

Chlorine (gas) required

20.00kg/day

kg/day = MLD × dose(mg/L)

Bleaching powder required

66.67kg/day

kg/day = Cl₂ / (strength%/100)

NaOCl solution required

400.00L/day

L/day = Cl₂ / (strength% × 10) [10 g/L per 1% approx]

Contact tank volume

57.9m³

V = Q × t / 60

↪ Ensures minimum 30 min contact time

CT value (residual × time)

18.0mg.min/L

CT = residual × contact (residual ≈ dose × 0.3 after demand satisfied)

↪ Target ≥ 15 for E.coli, ≥ 30 for virus

CPHEEO Reference Values

Dose typical1.0 – 2.5 mg/L

Residual at WTP outlet0.5 – 1.0 mg/L

Residual at consumer0.2 – 0.5 mg/L

Contact time minimum30 min

Bleaching powder strength30 – 35% available Cl

Download the Excel version to keep a local copy with live formulas — change inputs in the sheet and outputs recompute automatically.

Cross-references

CPHEEO WS Chapter 8IS 10500WHO disinfection guidelines