Appearance
Ozone Disinfection
Oxidative disinfection using dissolved ozone gas (O₃) to inactivate pathogens and oxidize micropollutants through powerful oxidation reactions, achieving 4+ log virus reduction with CT values of 0.5-1.0 mg·min/L while avoiding THM formation.
Overview
Ozone (O₃) is one of the strongest oxidizing disinfectants available (oxidation potential 2.07 V), providing rapid pathogen inactivation and simultaneous oxidation of color, taste/odor compounds, and trace organics. Ozone is generated on-site by electrical discharge through oxygen or air, dissolved into water via contact chambers, and residual ozone decomposes rapidly to oxygen. While highly effective, ozone requires significant capital investment and careful control to avoid bromate formation in bromide-containing waters.
When to Use
- Water reuse (potable and non-potable)
- Virus and protozoa inactivation (excellent for Cryptosporidium)
- Color, taste, and odor removal (geosmin, MIB, phenols)
- Micropollutant oxidation (pharmaceuticals, pesticides)
- Pre-treatment for biological filtration (biodegradable organics)
- No chlorinated DBP formation concerns (no THM/HAA)
- When chlorine-resistant pathogens present
- Advanced wastewater treatment
Key Characteristics
- Pathogen inactivation: 4+ log virus, 3+ log Cryptosporidium (excellent)
- CT value: 0.5-1.0 mg·min/L for 4-log virus (low contact time)
- Rapid reaction (seconds to minutes)
- No persistent residual (decays to O₂ in 10-30 min)
- Strong oxidant: Removes color, oxidizes organics
- Bromate (BrO₃⁻) formation concern if bromide >50 μg/L
- High capital and operating cost
- Energy: 8-15 kWh/kg O₃ produced
Operating Modes
Basic Mode
- Fixed ozone dose
- Simple CT calculation (C × T)
- Constant residual target
- Single contact chamber
Advanced Mode
- CT validation with decay kinetics
- Real-time ozone residual monitoring
- Bromate formation control (pH, dose optimization)
- Multi-stage contacting (optimize CT)
- Advanced oxidation (ozone + H₂O₂ or UV)
- Off-gas destruction control
Mathematical Model
Governing Equations
1. Ozone Disinfection - CT Concept
Pathogen inactivation by ozone follows CT (concentration × time):
$$\log\left(\frac{N}{N_0}\right) = -k_{O3} \times C \times T$$
Or:
$$CT = \frac{\log(N_0/N)}{k_{O3}}$$
Where:
- $C$ = ozone residual concentration (mg/L)
- $T$ = contact time (min)
- $CT$ = disinfection parameter (mg·min/L)
- $k_{O3}$ = ozone inactivation rate constant (L/mg·min), organism-specific
- $N/N_0$ = surviving fraction
Required CT values for wastewater (typical):
- 4-log virus: 0.5-1.0 mg·min/L
- 3-log Giardia: 0.5-1.5 mg·min/L
- 3-log Cryptosporidium: 5-10 mg·min/L (ozone excellent for Crypto!)
Example: For 4-log virus reduction (CT = 0.8 mg·min/L):
- If C = 0.4 mg/L residual, T = 0.8 / 0.4 = 2 minutes
- If C = 0.2 mg/L residual, T = 0.8 / 0.2 = 4 minutes
2. Ozone Demand and Dosing
Ozone demand is the ozone consumed by reactions before establishing residual:
$$D_{O3} = D_{demand} + C_{residual}$$
Where:
- $D_{O3}$ = applied ozone dose (mg/L)
- $D_{demand}$ = ozone demand (mg/L), consumed by organics, reduced species
- $C_{residual}$ = target residual (mg/L)
Ozone demand varies with water quality:
- DOC (dissolved organic carbon): 0.5-1.5 mg O₃ per mg DOC
- Reduced iron (Fe²⁺): 0.43 mg O₃ per mg Fe²⁺
- Reduced manganese (Mn²⁺): 0.88 mg O₃ per mg Mn²⁺
- Nitrite (NO₂⁻): 1.1 mg O₃ per mg NO₂⁻-N
- Sulfide (H₂S): 3.0 mg O₃ per mg H₂S-S
Total demand: $$D_{demand} = 1.0 \times DOC + 0.43 \times Fe + 0.88 \times Mn + \text{...}$$
Typical doses for tertiary effluent:
- Low DOC (<5 mg/L): 3-6 mg/L O₃
- Medium DOC (5-10 mg/L): 6-10 mg/L O₃
- High DOC (>10 mg/L): 10-15 mg/L O₃
3. Ozone Decay Kinetics
Ozone residual decays over time:
$$C(t) = C_0 \times e^{-k_{decay} \times t}$$
Where:
- $C(t)$ = ozone concentration at time t (mg/L)
- $C_0$ = initial ozone concentration (mg/L)
- $k_{decay}$ = decay rate constant (min⁻¹), water-specific
Decay rate depends on:
- pH (faster at high pH)
- Temperature (faster at high T)
- Organic matter (scavenges ozone)
- Catalysts (metals, hydroxide)
Typical half-life: 10-30 minutes in treated wastewater
4. Contact Time (T₁₀)
Use T₁₀ (time for 10% of flow to exit) for CT calculation to account for short-circuiting:
$$T_{10} = \frac{V_{contact}}{Q} \times f_{baffle}$$
Where:
- $V_{contact}$ = contact chamber volume (L)
- $Q$ = flow rate (L/min)
- $f_{baffle}$ = baffling factor (0.3-0.7 for plug flow, 0.1-0.3 for mixed)
Design for plug flow: Baffled serpentine chambers, f = 0.5-0.7
5. Bromate Formation
Ozone oxidizes bromide (Br⁻) to bromate (BrO₃⁻), a potential carcinogen:
$$Br^- + O_3 \rightarrow BrO_3^-$$
Regulatory limit: BrO₃⁻ <10 μg/L (WHO, EU, UK)
Bromate formation depends on:
- Bromide concentration (>50 μg/L problematic)
- Ozone dose (higher dose → more bromate)
- pH (lower pH reduces formation)
- Contact time (longer time → more bromate)
Bromate control strategies:
- Lower pH to 6.5-7.5 during ozonation
- Minimize ozone dose (optimize for CT only)
- Ammonia addition (scavenges ozone after disinfection)
- Shorter contact time with higher ozone residual
6. Ozone Generation
Ozone produced by corona discharge:
$$3 O_2 + \text{energy} \rightarrow 2 O_3$$
Energy consumption:
- Air-fed: 12-20 kWh/kg O₃
- Oxygen-fed (LOX or PSA): 8-12 kWh/kg O₃
Ozone concentration in gas:
- Air-fed: 1-3% O₃ by weight
- Oxygen-fed: 6-12% O₃ by weight
Ozone transfer efficiency:
- Venturi injector: 70-85%
- Fine bubble diffuser: 85-95%
- Sidestream injection: 90-98%
Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Flow rate | $Q$ | m³/h | 10-5000 | 500 | Design flow |
| Ozone dose | $D_{O3}$ | mg/L | 3-15 | 8 | Applied dose |
| Target residual | $C_{res}$ | mg/L | 0.2-0.5 | 0.3 | After demand satisfied |
| Contact time | $T_{10}$ | min | 5-20 | 10 | T₁₀ in contact chamber |
| Target CT | CT | mg·min/L | 0.5-2.0 | 1.0 | For 4-log virus |
| DOC | - | mg/L | 3-15 | 8 | Dissolved organic carbon |
| Bromide | Br⁻ | μg/L | 10-500 | 50 | Bromate precursor |
| pH | - | - | 6.5-8.0 | 7.5 | Operating pH |
| Temperature | T | °C | 10-25 | 15 | Water temperature |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Ozone generator capacity | kg O₃/h | Maximum production rate |
| Contact chamber volume | m³ | Volume for CT |
| Transfer efficiency | % | Ozone dissolution efficiency |
| Off-gas ozone | mg/L | Residual O₃ in off-gas (destroy) |
| Power consumption | kW | Generator electrical power |
| Specific energy | kWh/m³ | Energy per volume treated |
| Bromate concentration | μg/L | Predicted BrO₃⁻ formation |
| Annual operating cost | £/year | Electricity + O₂ + maintenance |
Configuration Guide
Step-by-Step Setup
Determine Ozone Dose
Based on water quality and target:
Demand calculation (example): DOC = 8 mg/L → 8 × 1.0 = 8.0 mg/L O₃ Fe²⁺ = 0.5 mg/L → 0.5 × 0.43 = 0.2 mg/L O₃ Total demand = 8.2 mg/L Target residual = 0.3 mg/L (for CT) Applied dose = 8.2 + 0.3 = 8.5 mg/L O₃Jar testing recommended to measure actual demand.
Select Target CT Value
For 4-log virus: CT = 0.8 mg·min/L (typical) With C_residual = 0.3 mg/L: T_10 required = 0.8 / 0.3 = 2.67 min Add safety factor (1.5×): T_10 design = 2.67 × 1.5 = 4.0 minDesign Contact Chamber
Plug flow serpentine chamber (preferred for CT):
$$V_{contact} = Q \times T_{10} / f_{baffle}$$
Q = 10,000 m³/d = 417 m³/h = 6,944 L/min T_10 = 4.0 min f_baffle = 0.6 (well-baffled) V_contact = 6,944 × 4.0 / 0.6 = 46,293 L = 46.3 m³Chamber configuration:
- Serpentine channels with 180° bends
- Number of baffles: 8-12 (maximize plug flow)
- Depth: 3-5 m (maximize ozone residence, minimize off-gas)
Select Ozone Generation Technology
Technology Ozone Conc. Energy (kWh/kg) Capital Cost Operating Cost Air-fed corona 1-3% 12-20 Low Medium (air dryer) PSA + corona 6-10% 8-12 Medium Medium (electricity) LOX + corona 10-12% 8-10 High High (LOX supply) Recommendation: PSA (pressure swing adsorption) oxygen + corona for municipal systems (balance of cost and efficiency).
Size Ozone Generator
Ozone mass required = Q × D_O3 / transfer_efficiency Q = 10,000 m³/d = 417 m³/h D_O3 = 8.5 mg/L = 8.5 g/m³ Transfer efficiency = 90% (sidestream injection) Ozone required = 417 m³/h × 8.5 g/m³ / 0.90 = 3,939 g/h = 3.94 kg/h Generator capacity (with 20% spare): 3.94 × 1.2 = 4.7 kg O₃/h Use 5 kg/h generator ✓Select Contacting Method
Method Transfer Efficiency Capital Cost Maintenance Headloss Venturi injector 70-85% Low Low High (0.3-0.5 bar) Fine bubble diffuser 85-95% Medium Medium (diffuser fouling) Low Sidestream injection 90-98% High Low Low Recommendation: Sidestream injection for large systems (high efficiency, low headloss).
Design Off-Gas Destruction
Ozone off-gas must be destroyed before venting:
Methods:
- Thermal destruction: Heat to 300°C (expensive)
- Catalytic destruction: Manganese dioxide catalyst @ 60°C (common)
- Activated carbon: Carbon bed adsorbs/decomposes O₃
Design: Off-gas ozone concentration: 0.5-2.0% by weight (if 90% transfer) Catalytic destroyer efficiency: >99% (outlet <0.1 ppm O₃)
Bromate Control
If bromide >50 μg/L:
Option 1: Lower pH to 6.8-7.2 during ozonation (Reduces bromate formation by 50-70%) Option 2: Add ammonia after ozone contact NH₃ dose: 0.5-1.0 mg/L (quenches residual ozone, prevents further bromate formation) Option 3: Minimize ozone dose (optimize CT) Use higher residual (0.4-0.5 mg/L) with shorter contact timeCalculate Power and Operating Cost
Generator capacity: 5 kg O₃/h Energy consumption: 10 kWh/kg O₃ (oxygen-fed) Power = 5 kg/h × 10 kWh/kg = 50 kW Daily energy = 50 kW × 24 h = 1,200 kWh/day Specific energy = 1,200 / 10,000 = 0.12 kWh/m³ ✓ Annual costs: Electricity (@ £0.12/kWh): 1,200 × 365 × £0.12 = £52,560/year Oxygen (PSA maintenance): £5,000/year Maintenance (generator): £10,000/year Total: £67,560/year = £0.0068/m³ = 0.68 p/m³
Typical Design Values
| Application | Ozone Dose (mg/L) | CT (mg·min/L) | Contact Time (min) | Specific Energy (kWh/m³) |
|---|---|---|---|---|
| Tertiary disinfection | 5-8 | 0.5-1.0 | 5-10 | 0.08-0.12 |
| Color/odor removal | 3-6 | 0.3-0.6 | 5-10 | 0.05-0.10 |
| Micropollutant oxidation | 8-12 | 1.0-2.0 | 10-15 | 0.12-0.18 |
| Pre-treatment for BAC | 2-5 | 0.2-0.5 | 3-5 | 0.03-0.08 |
| Potable reuse | 5-10 | 1.0-2.0 | 10-15 | 0.10-0.15 |
Performance Criteria
Pathogen Inactivation
| Organism | CT for 4-log Reduction (mg·min/L) |
|---|---|
| E. coli | 0.02-0.05 |
| Virus (poliovirus) | 0.5-1.0 |
| Giardia cysts | 0.5-1.5 |
| Cryptosporidium oocysts | 5-10 |
Note: Ozone excellent for Cryptosporidium (much better than chlorine).
Micropollutant Removal
Ozone oxidizes trace organics:
| Compound | Removal (%) @ 8 mg/L O₃ |
|---|---|
| Pharmaceuticals (general) | 50-90 |
| Carbamazepine | 80-95 |
| Diclofenac | 90-99 |
| Estrogens | 85-98 |
| Geosmin/MIB (odor) | 90-99 |
| Atrazine (pesticide) | 70-85 |
Bromate Formation
Predicted bromate based on bromide:
| Bromide (μg/L) | Ozone Dose (mg/L) | pH | Predicted BrO₃⁻ (μg/L) |
|---|---|---|---|
| 50 | 5 | 7.5 | 3-5 |
| 50 | 10 | 7.5 | 8-12 |
| 100 | 5 | 7.5 | 8-12 |
| 100 | 10 | 7.5 | 15-25 (exceeds limit!) |
| 100 | 10 | 6.8 | 5-10 (pH control) |
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Insufficient CT (low residual) | High ozone demand | Increase dose, improve pre-treatment |
| Ozone generator capacity insufficient | Add generator capacity | |
| Poor transfer efficiency | Check injector/diffuser, increase gas flow | |
| High bromate formation | High bromide in influent | Lower pH, minimize dose, add NH₃ quench |
| Excessive ozone dose | Optimize dose for CT only | |
| Generator low output | Air/oxygen feed contaminated | Check desiccant dryer, replace filters |
| Corona tubes degraded | Replace corona cells | |
| Cooling water temperature high | Improve cooling system | |
| Off-gas ozone high | Catalyst saturated or degraded | Replace catalyst bed |
| Low temperature (catalyst inactive) | Heat off-gas to 60°C | |
| Poor disinfection performance | Short-circuiting in contact chamber | Improve baffling, check T₁₀ |
| Rapid ozone decay | Check pH, temperature, organic content | |
| Corrosion of equipment | Ozone attack on materials | Use ozone-resistant materials (SS 316, PTFE, PVC) |
| High operating cost | Inefficient generator | Service generator, check energy consumption |
| Excessive ozone dose | Optimize dose via jar testing |
Operation & Maintenance
Daily Checks
- Monitor ozone residual (online analyzer)
- Verify CT value achieved
- Check generator output (kg O₃/h)
- Record flow rate and dose
- Inspect off-gas destroyer (ensure <0.1 ppm outlet)
- Review effluent bacteria data
Weekly Maintenance
- Measure ozone demand (jar test)
- Calibrate ozone analyzers
- Check desiccant dryer (air-fed systems)
- Inspect contact chamber for algae/biofilm
- Verify pH control system (if bromate control)
Monthly Tasks
- Clean ozone analyzers (sensor fouling)
- Inspect diffusers or injectors
- Measure bromate concentration (lab analysis)
- Check generator cooling system
- Review power consumption trends
Annual Tasks
- Service ozone generator (replace corona cells if needed)
- Replace desiccant bed (air-fed)
- Clean contact chamber (drain, inspect baffles)
- Replace off-gas catalyst (if degraded)
- CT validation study
- Review performance vs. regulatory requirements
Generator Maintenance
Corona cells:
- Life: 15,000-25,000 hours
- Replace when output drops >20%
Desiccant dryer (air-fed):
- Replace desiccant annually
- Critical for generator efficiency
PSA oxygen system:
- Zeolite bed life: 5-10 years
- Annual inspection and testing
Advanced Features
Advanced Oxidation Process (AOP)
Ozone + H₂O₂ or Ozone + UV for enhanced oxidation:
$$O_3 + H_2O_2 \rightarrow \cdot OH + O_2$$
$$O_3 + UV \rightarrow O_2 + O \rightarrow \cdot OH$$
- Hydroxyl radicals (·OH) are even more powerful than ozone (E₀ = 2.8 V)
- Enhanced removal of recalcitrant organics
- Application: Pharmaceuticals, industrial organics, PFAS precursors
Ozone + Biological Filtration
Ozone pre-treatment for biological activated carbon (BAC):
- Ozone breaks down large organics into smaller, biodegradable molecules
- Improved DOC removal in downstream GAC filter
- Extended GAC bed life
- Synergistic effect
Multi-Stage Ozonation
Optimize for both disinfection and oxidation:
- Stage 1: Low dose (3-5 mg/L) for disinfection and initial oxidation
- Stage 2: Higher dose (5-10 mg/L) for micropollutant oxidation
- Minimize bromate by distributing dose
Real-Time Control
- Ozone residual feedback control (PID loop)
- CT monitoring and alarm
- Bromate prediction model (dose optimization)
- Generator output modulation based on demand
Worked Example
Given
- Flow: 8,000 m³/d (average)
- DOC: 10 mg/L
- Bromide: 75 μg/L
- Target: 4-log virus (CT = 0.8 mg·min/L)
- Application: Indirect potable reuse
Solution
Step 1: Determine Ozone Demand
Ozone demand (empirical): 1.0 mg O₃ per mg DOC
D_demand = 10 × 1.0 = 10 mg/L O₃
Target residual: 0.4 mg/L (for CT)
Applied dose = 10 + 0.4 = 10.4 mg/L O₃Step 2: Calculate Contact Time
CT_target = 0.8 mg·min/L
C_residual = 0.4 mg/L
T_10 = 0.8 / 0.4 = 2.0 min
With safety factor 1.5:
T_10 design = 2.0 × 1.5 = 3.0 minStep 3: Design Contact Chamber
Q = 8,000 m³/d = 333 m³/h = 5,556 L/min
f_baffle = 0.6 (serpentine, well-baffled)
V_contact = 5,556 × 3.0 / 0.6 = 27,778 L = 27.8 m³
Chamber: 2m wide × 2m deep × 7m long (serpentine with baffles)Step 4: Size Ozone Generator
Ozone required = 333 m³/h × 10.4 g/m³ / 0.92 (transfer eff.) = 3,766 g/h = 3.77 kg/h
With 25% spare capacity: 3.77 × 1.25 = 4.7 kg/h
Use 5 kg/h oxygen-fed generator ✓Step 5: Assess Bromate Risk
Bromide = 75 μg/L (moderate risk)
Ozone dose = 10.4 mg/L (high)
Predicted bromate (empirical model):
BrO₃⁻ ≈ 0.10 × Br⁻(μg/L) × O₃(mg/L) / 10 = 0.10 × 75 × 10.4 / 10 = 7.8 μg/L
(Close to 10 μg/L limit—implement control)Step 6: Bromate Control
Option: Lower pH from 7.5 to 7.0 during ozonation
Expected reduction: 40-50%
Predicted BrO₃⁻ with pH control: 7.8 × 0.55 = 4.3 μg/L ✓
(Below 10 μg/L limit)Step 7: Calculate Operating Cost
Generator: 5 kg O₃/h × 10 kWh/kg = 50 kW
Daily energy = 50 × 24 = 1,200 kWh/day
Specific energy = 1,200 / 8,000 = 0.15 kWh/m³
Annual costs:
Electricity: 1,200 × 365 × £0.12 = £52,560/year
PSA maintenance: £5,000/year
Generator maintenance: £8,000/year
Total: £65,560/year = £0.0082/m³ = 0.82 p/m³Step 8: Capital Cost Estimate
Ozone system (5 kg/h, oxygen-fed, installed): £400,000
- Generator: £150,000
- PSA oxygen system: £80,000
- Contact chamber: £100,000
- Off-gas destruction: £40,000
- Controls: £30,000Related Units
- UV-Disinfection - Alternative disinfection (no bromate)
- GAC-Filter - Downstream for ozonation byproduct removal
- Chlorination - Alternative chemical disinfection
- Rapid-Sand-Filter - Upstream TSS removal
- pH-Adjustment - For bromate control
References
- Langlais, B., Reckhow, D.A., and Brink, D.R. (1991). Ozone in Water Treatment: Application and Engineering, Lewis Publishers.
- AWWA (2011). Water Quality and Treatment: A Handbook on Drinking Water, 6th Ed., McGraw-Hill, Ch. 7.
- WEF (2020). Advanced Wastewater Treatment Systems, WEFMOP 35, Ch. 8.
- von Gunten, U. (2003). "Ozonation of Drinking Water: Part II. Disinfection and By-Product Formation." Water Research 37(7): 1469-1487.
- EPA (2006). Ultraviolet Disinfection Guidance Manual, EPA 815-R-06-007 (includes ozone comparison).
- Haag, W.R. and Hoigné, J. (1983). "Ozonation of Bromide-Containing Waters: Kinetics of Formation of Hypobromous Acid and Bromate." Environ. Sci. Technol. 17(5): 261-267.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1480-1520.