Appearance
Chlorination
Chlorine gas or sodium hypochlorite disinfection achieving 4-log pathogen reduction through CT-based contact tank design with residual monitoring and chloramine formation control.
Overview
Chlorination is the most widely used disinfection method in wastewater treatment, providing reliable pathogen inactivation through oxidative damage to cell membranes and genetic material. While effective and economical, chlorination requires careful management of contact time, residual chlorine measurement, and dechlorination to protect aquatic life.
When to Use
- Primary disinfection for effluent discharge (regulatory compliance)
- Fecal coliform reduction to <200 cfu/100mL (UK) or <1000 cfu/100mL (US)
- Combined with UV for robust multi-barrier approach
- Process control (slime/odor control in collection systems)
- Peak wet weather disinfection (simpler than UV)
- Combined sewer overflow (CSO) treatment
Key Characteristics
- Log reduction: 4-6 log bacteria, 2-4 log viruses, 2-3 log protozoa (Giardia)
- Residual maintained: 0.5-2.0 mg/L free chlorine
- Contact time: 15-30 minutes @ peak flow
- CT requirement: 15-450 mg·min/L (species and pH dependent)
- Temperature sensitivity: Higher CT required at low T
- Dechlorination required: Sodium bisulfite or sulfur dioxide
- Trihalomethane (THM) formation risk
Operating Modes
Basic Mode
- Fixed dose control
- Simple plug-flow contact tank
- Time-based CT calculation
- Manual residual adjustment
- Default CT value for E. coli (450 mg·min/L @ pH 7, 10°C)
Advanced Mode
- Chick-Watson kinetic model
- Variable dose based on ammonia nitrogen (chloramine formation)
- Baffle factor correction (short-circuiting)
- Temperature-corrected CT values
- Multi-species pathogen inactivation
- DBP (disinfection byproduct) prediction
- Automatic dechlorination control
Mathematical Model
Chick-Watson Inactivation Kinetics
The fundamental equation for disinfection:
$$\ln\left(\frac{N}{N_0}\right) = -k' \times C^n \times t$$
Or in log₁₀ form:
$$\log\left(\frac{N}{N_0}\right) = -\frac{k' \times C^n \times t}{2.303}$$
Where:
- $N$ = pathogen concentration after disinfection (count/100mL)
- $N_0$ = initial pathogen concentration (count/100mL)
- $k'$ = specific lethality coefficient (L/(mg·min) for n=1)
- $C$ = disinfectant concentration (mg/L)
- $n$ = coefficient of dilution (typically 0.86-1.0 for chlorine)
- $t$ = contact time (min)
Simplified form (n=1):
$$\log\left(\frac{N}{N_0}\right) = -k \times CT$$
Where $CT$ = concentration × time (mg·min/L)
CT Concept (Concentration × Time)
The CT value represents disinfection "dose":
$$CT_{required} = C_{residual} \times t_{contact}$$
Actual CT (accounting for short-circuiting):
$$CT_{actual} = C_{residual} \times t_{10} \times BF$$
Where:
- $t_{10}$ = time for 10% of tracer to pass through tank (min)
- $BF$ = baffle factor = $t_{10} / t_{theoretical}$ (dimensionless, 0.3-0.7)
UK CT Values for Free Chlorine (DWI Standards)
4-log Giardia cyst inactivation at 10°C:
| pH | CT (mg·min/L) |
|---|---|
| 6.0 | 42 |
| 6.5 | 53 |
| 7.0 | 79 |
| 7.5 | 110 |
| 8.0 | 155 |
| 8.5 | 220 |
| 9.0 | 310 |
US EPA CT Tables (SWTR):
For 99.9% (3-log) Giardia inactivation @ pH 7.0:
| Temperature (°C) | CT (mg·min/L) |
|---|---|
| 0.5 | 165 |
| 5 | 117 |
| 10 | 88 |
| 15 | 70 |
| 20 | 58 |
| 25 | 48 |
For E. coli (typical wastewater target):
- 99.99% (4-log) inactivation: CT ≈ 15-30 mg·min/L @ pH 7, 15°C
Temperature Correction
$$CT_T = CT_{20} \times \theta^{(20-T)}$$
Where:
- $\theta$ = temperature coefficient (1.02-1.05 for chlorine, typically 1.03)
- $T$ = actual temperature (°C)
Chlorine Demand
Chlorine is consumed by:
- Ammonia (chloramine formation)
- Organic matter (oxidation)
- Reduced metals (Fe²⁺, Mn²⁺)
- Sulfides
$$C_{applied} = C_{demand} + C_{residual}$$
Chloramine formation (breakpoint chlorination):
$$NH_3 + HOCl \rightarrow NH_2Cl + H_2O \quad \text{(monochloramine)}$$
$$NH_2Cl + HOCl \rightarrow NHCl_2 + H_2O \quad \text{(dichloramine)}$$
Breakpoint:
$$2NH_3 + 3HOCl \rightarrow N_2 + 3HCl + 3H_2O$$
Cl₂:N mass ratio for breakpoint ≈ 7.6:1 to 10:1
Disinfection Byproduct Formation (DBPs)
Trihalomethanes (THMs):
$$\text{Organic precursors} + Cl_2 \rightarrow CHCl_3, CHBrCl_2, CHBr_2Cl, CHBr_3$$
Empirical model:
$$THM = k_{THM} \times C_{Cl_2} \times TOC \times t \times f(pH, T)$$
Where:
- $TOC$ = total organic carbon (mg/L)
- $k_{THM}$ = empirical rate constant
Haloacetic acids (HAAs):
Similar formation mechanism, typically 0.5-1.5 times THM concentration.
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Flow rate | $Q$ | m³/d | - | - | Design flow (peak hourly) |
| Contact time | $t$ | min | 15-90 | 30 | Hydraulic retention time |
| Target CT | $CT$ | mg·min/L | 15-450 | 80 | Design CT value |
| Baffle factor | $BF$ | - | 0.3-0.7 | 0.5 | Short-circuit correction |
| Chlorine dose | $D$ | mg/L | 2-15 | 5 | Applied chlorine dose |
| Target residual | $C_r$ | mg/L | 0.5-2.0 | 1.0 | Free chlorine residual |
| Influent NH₃-N | $NH_3$ | mg/L | 0-30 | 2 | Ammonia nitrogen |
| Influent BOD₅ | $BOD$ | mg/L | 5-30 | 15 | Biochemical oxygen demand |
| Temperature | $T$ | °C | 5-25 | 15 | Wastewater temperature |
| pH | - | - | 6.5-8.5 | 7.2 | Wastewater pH |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Tank volume | m³ | Contact tank volume |
| Tank dimensions | m | Length × width × depth |
| Chlorine demand | kg Cl₂/d | Total chlorine consumption |
| t₁₀ time | min | 10% tracer passage time |
| Actual CT | mg·min/L | Achieved disinfection dose |
| Log reduction | log₁₀ | Pathogen inactivation |
| Dechlorination dose | kg SO₂/d | Sodium bisulfite required |
| Chemical cost | $/year | Annual chlorine + dechlorination |
Configuration Guide
Step-by-Step Setup
Determine Design CT Value
Based on regulatory requirements:
UK - Surface water discharge: CT ≥ 80 mg·min/L (Giardia 4-log @ pH 7, 10°C) US - Secondary treatment: 450 mg·min/L (E. coli limit, conservative) US - Advanced treatment: Based on state requirements Typical selection: - Urban river discharge: CT = 80-150 mg·min/L - Recreational water: CT = 150-300 mg·min/L - Water reuse: CT = 300-450 mg·min/L (often combined with UV)Select Contact Time
Minimum contact time from regulations:
- UK: t₁₀ ≥ 30 minutes @ peak flow
- US: Modal contact time ≥ 90 minutes (conservative)
Practical design: t_theoretical = 30 minutes (common) Assume BF = 0.5 (serpentine baffled tank) t_10 = BF × t_theoretical = 0.5 × 30 = 15 minutesCalculate Required Residual
$$C_{residual} = \frac{CT_{required}}{t_{10}}$$
Example: CT_required = 80 mg·min/L t_10 = 15 min C_residual = 80 / 15 = 5.3 mg/L (This is HIGH - need to increase contact time or accept lower CT) Better design: t_theoretical = 45 min, BF = 0.6 → t_10 = 27 min C_residual = 80 / 27 = 3.0 mg/L ✓Design Contact Tank Volume
$$V = \frac{Q \times t_{theoretical}}{60}$$
Q = 10,000 m³/d = 417 m³/h (average) Q_peak = 2.5 × Q_avg = 1042 m³/h (peak factor 2.5) Use peak flow for design: V = 1042 × (45/60) = 781 m³ Round up: V = 800 m³Design Tank Geometry
Serpentine baffle configuration for BF = 0.6:
- Length:Width ratio ≥ 20:1 (with baffles)
- Depth: 3-5 m (typically 4m)
- Number of passes: 6-10
V = 800 m³ Depth = 4m Horizontal area = 800 / 4 = 200 m² For L:W = 25:1 (effective with baffles): L × W = 200 m² L = 25W 25W² = 200 → W = 2.83 m L = 70.8 m Design: 71m long × 2.8m wide × 4m deep Number of passes: 8 (each pass 8.9m long)Calculate Chlorine Demand
Breakpoint curve method:
If NH₃-N = 2 mg/L, need Cl₂:N ≈ 10:1 for breakpoint:
$$Dose_{breakpoint} = 10 \times 2 = 20 \text{ mg/L Cl}_2$$
Practical approach (empirical):
$$Dose = Demand + Residual$$
Typical demand for secondary effluent: 3-8 mg/L Assume 5 mg/L demand Residual needed: 3.0 mg/L Total dose = 5 + 3 = 8 mg/L Cl₂Calculate Chlorine Consumption
$$M_{Cl_2} = Q \times Dose \times 10^{-3}$$
Q = 10,000 m³/d Dose = 8 mg/L M_Cl₂ = 10,000 × 8 × 10⁻³ = 80 kg Cl₂/day Annual = 80 × 365 = 29,200 kg/year If using 12% NaOCl: NaOCl consumption = 29,200 / 0.12 = 243,333 kg/year = 243 tonnes/yearDesign Dechlorination System
Residual after contact tank ≈ 3.0 mg/L, must reduce to <0.01 mg/L for discharge.
Sodium bisulfite (NaHSO₃):
$$NaHSO_3 + Cl_2 + H_2O \rightarrow NaHSO_4 + 2HCl$$
Stoichiometric ratio: 1.46 kg NaHSO₃ per kg Cl₂
Residual to remove = 3.0 mg/L Dose = 1.46 × 3.0 = 4.4 mg/L NaHSO₃ Daily consumption = 10,000 × 4.4 × 10⁻³ = 44 kg/d Annual = 16,060 kg/year
Typical Design Values
| Application | Contact Time (min) | Chlorine Dose (mg/L) | Residual (mg/L) | CT (mg·min/L) | Dechlorination |
|---|---|---|---|---|---|
| Secondary effluent (UK) | 30 (t₁₀=15) | 5-8 | 2-3 | 80-100 | Required |
| Secondary effluent (US) | 90 (modal) | 8-15 | 5-8 | 450+ | Required |
| Peak wet weather | 15 (t₁₀=8) | 10-15 | 3-5 | 40-60 | Sometimes waived |
| CSO treatment | 5-10 (flash mix) | 15-25 | 5-10 | 25-100 | Sometimes waived |
| Process control | Variable | 2-5 | 0.5-2 | N/A | Not required |
Performance Criteria
Pathogen Inactivation
| Organism | CT for 99.9% (3-log) @ pH 7, 15°C | CT for 99.99% (4-log) |
|---|---|---|
| E. coli | 0.05 mg·min/L | 0.08 mg·min/L |
| Fecal coliforms | 0.1 mg·min/L | 0.15 mg·min/L |
| Enteric viruses | 3-12 mg·min/L | 6-24 mg·min/L |
| Giardia cysts | 45-150 mg·min/L | 90-300 mg·min/L |
| Cryptosporidium | 7,200 mg·min/L (ineffective) | >15,000 mg·min/L |
Note: Cryptosporidium is highly resistant to chlorine. UV or ozone required for effective inactivation.
Effect of pH on Efficacy
Hypochlorous acid (HOCl) is 80-100× more effective than hypochlorite ion (OCl⁻).
Speciation:
$$HOCl \leftrightarrow H^+ + OCl^-$$
pKₐ ≈ 7.5 @ 20°C
| pH | % HOCl | % OCl⁻ | Relative Efficacy |
|---|---|---|---|
| 6.0 | 97 | 3 | 100% |
| 7.0 | 76 | 24 | 88% |
| 7.5 | 50 | 50 | 60% |
| 8.0 | 24 | 76 | 35% |
| 9.0 | 3 | 97 | 8% |
Practical implication: Disinfection at pH 8-9 requires 3-5× higher CT than pH 6-7.
Residual Chlorine Limits
Aquatic toxicity:
- Acute toxicity (96h LC₅₀): 0.01-0.5 mg/L for most fish species
- Chronic effects: <0.002 mg/L (salmonids very sensitive)
Regulatory limits (effluent discharge):
- UK (Freshwater Fish Directive): <0.01 mg/L total residual
- US EPA: Varies by state, typically <0.011 mg/L (4-day average)
- California: <0.02 mg/L (1-hour average)
Conclusion: Dechlorination is almost always required for surface water discharge.
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Inadequate disinfection (high fecal coliforms) | Low chlorine dose | Increase dose, verify residual |
| Short-circuiting in tank | Improve baffling, tracer study | |
| High ammonia (chloramine formation) | Increase dose to breakpoint | |
| pH too high (OCl⁻ predominates) | Adjust pH to 6.5-7.5 if possible | |
| Low contact time | Reduce flow or add tank volume | |
| Interfering substances (organics) | Improve upstream treatment | |
| High chlorine demand | Ammonia in effluent | Nitrify upstream, increase dose |
| High BOD (organic matter) | Improve secondary treatment | |
| Reduced metals (Fe²⁺, Mn²⁺) | Pre-oxidation or filtration | |
| Sulfides (septic conditions) | Aeration, odor control | |
| Residual breakthrough (in effluent) | Dechlorination system failure | Check SO₂ feed, increase dose |
| Inadequate mixing | Improve dechlorination mixing | |
| Flow surge | Add flow pacing, buffer tank | |
| High THM/HAA formation | High TOC in influent | Reduce contact time, lower dose |
| High temperature | Seasonal adjustment | |
| High pH (for HAAs) | pH adjustment | |
| Chlorine gas leak | Equipment failure | Emergency shutdown, evacuate, ventilate |
| Overpressure in storage | Inspect relief valves, temperature control | |
| Variable residual | Flow variation | Flow-paced dosing control |
| Demand variation | Ammonia monitoring, feedback control |
Operation & Maintenance
Daily Checks
- Measure chlorine residual (manual DPD test or online analyzer)
- Verify chlorine feed rate and storage level
- Check dechlorination system operation
- Monitor effluent fecal coliform (if online monitor available)
- Inspect contact tank for short-circuiting (surface observation)
- Verify no chlorine odor at discharge point
Weekly Maintenance
- Calibrate online chlorine analyzers (free vs. total)
- Lab confirmation of residual chlorine
- Collect fecal coliform samples for lab analysis
- Inspect chlorine injectors and diffusers
- Check dechlorination chemical storage
- Verify contact tank water level and flow distribution
Monthly Tasks
- Conduct tracer study (if issues suspected) to verify t₁₀
- Measure CT profile (residual at multiple points in tank)
- Inspect baffles for integrity
- Clean chlorine analyzer probe
- Review chlorine consumption trends
- Check for THM/HAA formation (if required)
- Inspect chlorine storage (gas cylinders or hypochlorite tank)
Annual Tasks
- Full tracer study (rhodamine dye) to determine baffle factor
- Structural inspection of contact tank
- Chlorine system safety audit
- Emergency response drill (chlorine leak)
- Review design vs. actual performance
- Optimize dosing strategy based on seasonal patterns
- THM/HAA compliance testing (if discharge to sensitive water)
Advanced Features
Flow-Paced Chlorine Dosing
Automatically adjust dose based on flow:
$$Dose_{Cl_2}(t) = \frac{Q(t) \times C_{setpoint}}{Q_{design}}$$
Prevents overdosing at low flow and underdosing at high flow.
Ammonia-Based Dosing (Breakpoint Control)
Monitor NH₃-N online and dose to breakpoint:
$$Dose = 10 \times NH_3\text{-N} + C_{residual}$$
Ensures free chlorine residual even with variable ammonia.
Compound Loop Control
- Measure residual at end of contact tank
- Compare to setpoint (e.g., 1.0 mg/L free Cl₂)
- Adjust dose using PID controller
- Account for lag time (contact time delay)
DBP Minimization Strategies
- Reduce contact time: Meet CT requirement but no excess
- Optimize dose: Lower dose + longer time vs. higher dose + shorter time
- pH control: Lower pH (6.5-7.0) reduces HAA formation
- Pre-filtration: Remove organic precursors before chlorination
- Alternative point of application: Chlorinate after filtration instead of before
Alternative Chlorine Sources
| Source | Available Cl₂ | Advantages | Disadvantages |
|---|---|---|---|
| Chlorine gas (Cl₂) | 100% | Low cost, highly effective | Safety risk, complex storage |
| Sodium hypochlorite (NaOCl) | 12-15% | Safe, easy handling | Higher cost, degrades in storage |
| Calcium hypochlorite (Ca(OCl)₂) | 65-70% | Stable solid, safe | Adds calcium, requires dissolution |
| On-site generation (electrochlorination) | 0.4-0.8% | Eliminates transport, safe | High capital, energy intensive |
Worked Example
Given
- Flow: 8,000 m³/d average, 20,000 m³/d peak (storm event)
- Design for peak flow
- Target: E. coli <1000 cfu/100mL (99.99% reduction from 10⁷)
- Secondary effluent: NH₃-N = 3 mg/L, BOD₅ = 15 mg/L, pH = 7.2, T = 12°C
- Regulatory requirement: CT ≥ 80 mg·min/L
Solution
Step 1: Select Contact Time
Use t_theoretical = 40 minutes @ peak flow
Assume well-baffled tank: BF = 0.6
t_10 = 0.6 × 40 = 24 minutesStep 2: Calculate Required Residual
CT_required = 80 mg·min/L
C_residual = CT / t_10 = 80 / 24 = 3.3 mg/L free Cl₂Step 3: Estimate Chlorine Demand
Ammonia demand (to breakpoint): 10 × 3 = 30 mg/L (too high)
Alternative: Accept chloramine formation, dose beyond breakpoint
Practical demand for secondary effluent @ 12°C: 6 mg/L
Total dose = Demand + Residual = 6 + 3.3 = 9.3 mg/L
Use 10 mg/L for design safety marginStep 4: Design Contact Tank
Q_peak = 20,000 m³/d = 833 m³/h
V = 833 × (40/60) = 555 m³
Use V = 600 m³ (allow 8% safety factor)
Tank geometry:
Depth = 4.5 m
Area = 600 / 4.5 = 133 m²
For L:W = 30:1 (serpentine):
L = 63 m, W = 2.1 m
Number of passes: 9 (each 7m long)Step 5: Verify Performance
Actual CT = 3.3 mg/L × 24 min = 79.2 mg·min/L ✓ (meets 80 target)
For E. coli (CT requirement ≈ 0.08 mg·min/L for 4-log):
Log reduction = 79.2 / 0.08 = 990× more than required
Expect >6-log reduction ✓Step 6: Chemical Consumption
Chlorine:
Daily (avg flow) = 8,000 × 10 × 10⁻³ = 80 kg Cl₂/d
Annual = 29,200 kg/year
Using 12% NaOCl:
NaOCl = 29,200 / 0.12 = 243,333 kg/year = 243 tonnes
Cost @ $500/tonne = $121,650/year
Dechlorination (assume 3 mg/L residual to remove):
NaHSO₃ dose = 1.46 × 3 = 4.4 mg/L
Daily = 8,000 × 4.4 × 10⁻³ = 35.2 kg/d
Annual = 12,848 kg/year
Cost @ $800/tonne = $10,278/year
Total chemical cost = $131,928/yearRelated Units
- UV-Disinfection - Alternative primary disinfection
- Ozone-Disinfection - More effective for protozoa
- Chlorine-Dioxide - No THM formation
- Peracetic-Acid - No residual, biodegradable
- Chemical-Dosing - Chlorine feed systems
References
- White, G.C. (2010). White's Handbook of Chlorination and Alternative Disinfectants, 5th Ed., Wiley.
- US EPA (1999). Disinfection Profiling and Benchmarking Guidance Manual, EPA 815-R-99-013.
- WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 18.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1347-1402.
- Drinking Water Inspectorate (UK) (2022). Guidance on the Water Supply (Water Quality) Regulations.
- AWWA (2011). Water Quality and Treatment, 6th Ed., Ch. 14: Disinfection.
- Crittenden, J.C. et al. (2012). MWH's Water Treatment, 3rd Ed., Ch. 19.