Tertiary Disinfection

Pathogen inactivation processes for public health protection, achieving 4-6 log reduction of bacteria, viruses, and protozoa to meet discharge and water reuse standards.

Overview

Disinfection is the final barrier against waterborne pathogens, inactivating bacteria, viruses, and protozoa to protect public health. The choice of disinfectant depends on target organisms, effluent quality requirements, regulatory standards, and site-specific constraints such as discharge sensitivity (chlorine toxicity) or energy availability.

Available Disinfection Technologies

UV-Disinfection

Non-chemical photoinactivation

• Technology: UV-C light (254 nm wavelength)
• Dose: 40-100 mJ/cm² (wastewater), 200-400 mJ/cm² (water reuse)
• Mechanism: DNA/RNA damage, prevents replication
• Contact time: 10-30 seconds (very rapid)
• No chemical addition, no DBPs, no residual

Key features:

• Excellent for Cryptosporidium (4+ log at 40 mJ/cm²)
• Effective across all pathogens (bacteria, viruses, protozoa)
• UV transmittance (UVT) dependent: Requires low turbidity (<2 NTU, preferably <1 NTU)
• Energy: 0.02-0.05 kWh/m³ (very low)
• No persistent residual (not suitable for distribution systems)
• Lamp replacement: 8,000-12,000 hours (annual for continuous operation)

Best for:

• Discharge to sensitive waters (fish protection, no chlorine)
• Cryptosporidium barrier (most effective at practical doses)
• Water reuse (no DBP formation)
• Sites with low operational complexity (no chemical handling)

Ozone-Disinfection

On-site generated powerful oxidant

• Chemical formula: O₃ (triatomic oxygen)
• Dose: 5-15 mg/L (wastewater)
• CT requirement: 0.5-4 mg·min/L (bacteria, viruses)
• Mechanism: Cell membrane oxidation, DNA/RNA damage
• Contact time: 10-20 minutes

Key features:

• Effective for bacteria and viruses (4-6 log)
• Moderate for protozoa (2-3 log Giardia/Crypto, less effective than UV)
• No DBPs except bromate (if bromide present)
• Decomposes rapidly (no persistent residual, 10-20 min half-life)
• On-site generation from air or oxygen (no chemical storage)
• Energy intensive: 0.10-0.20 kWh/m³ (ozone generation + contact tank)
• Co-benefit: Oxidizes organics, color, taste/odor

Best for:

• Discharge requiring no residual
• Color and taste/odor control (phenols, tannins)
• Virus inactivation (more effective than chlorine at neutral pH)
• Sites with power availability (energy-intensive)

Chlorination

Traditional chemical disinfection

• Chemical: Cl₂ gas or NaOCl (sodium hypochlorite)
• Dose: 5-15 mg/L (wastewater, accounts for demand + residual)
• CT requirement: 15-450 mg·min/L (organism and pH dependent)
• Residual: 0.5-2.0 mg/L free chlorine (at end of contact tank)
• Contact time: 15-30 minutes (peak flow)

Key features:

• Effective for bacteria (4-6 log), viruses (2-4 log)
• Poor for Cryptosporidium (>7,000 mg·min/L for 3-log, impractical)
• Persistent residual (advantage for distribution, disadvantage for discharge)
• Requires dechlorination for aquatic life protection (<0.01 mg/L)
• pH-dependent: HOCl (hypochlorous acid, effective) vs. OCl⁻ (hypochlorite, less effective)
• DBP formation: THMs, HAAs (regulatory concern, especially for reuse)
• Low cost, simple operation

Best for:

• Rural discharge (large rivers with dilution capacity)
• Combined sewer overflow (CSO) treatment (simple, rapid deployment)
• Process control (odor/slime control in collection systems)
• Backup disinfection (when UV fails or high turbidity)

Chlorine-Dioxide

Advanced chlorine alternative

• Chemical formula: ClO₂
• Generation: On-site from NaClO₂ + Cl₂ or NaClO₂ + HCl
• Dose: 0.5-3.0 mg/L
• CT requirement: 5-30 mg·min/L (bacteria, viruses)
• Residual: 0.1-0.5 mg/L (non-persistent, decays in hours)

Key features:

• pH-independent (effective pH 6-9, unlike chlorine)
• No THM/HAA formation (major advantage over chlorine)
• More effective for Giardia/Crypto than chlorine (but still less than UV)
• Byproducts: Chlorite (ClO₂⁻) and chlorate (ClO₃⁻), both limited to <0.7 mg/L
• On-site generation required (unstable, cannot transport)
• Higher cost than chlorine (generation equipment + chemicals)

Best for:

• Water reuse (no THMs, lower DBP risk)
• High-pH waters (chlorine ineffective at pH >8)
• Taste and odor control (phenol oxidation without chlorophenol formation)
• Virus barrier (more effective than chlorine at practical CT)

Peracetic-Acid

Emerging biodegradable disinfectant

• Chemical formula: CH₃COOOH (PAA)
• Dose: 2-8 mg/L (wastewater)
• CT requirement: 30-150 mg·min/L (E. coli 4-log)
• Contact time: 10-30 minutes
• No persistent residual (degrades to acetic acid + oxygen in 30-60 min)

Key features:

• No DBP formation (no THMs, HAAs, chlorite, chlorate)
• Effective bacteria and moderate viruses (3-5 log bacteria, 2-4 log viruses)
• Moderate protozoa (2-3 log Giardia, less than UV)
• pH and temperature independent (works at pH 5-8, effective at 1-30°C)
• No dechlorination required (biodegradable)
• Acetic acid byproduct adds minor BOD (2-4 mg/L, usually acceptable)
• High chemical cost (3-5× chlorine)

Best for:

• Discharge to very sensitive waters (no residual, no DBPs)
• Water reuse with strict DBP limits
• Cold-water applications (winter operation, effective at low T)
• Sites without dechlorination infrastructure

Technology Comparison

Disinfectant	Bacteria (log)	Virus (log)	Crypto (log)	Residual	DBPs	Turbidity Sensitivity	Energy/Chemical Cost	Complexity
UV	4-6	4-6	4+ (best)	None	None	High (UVT-dependent)	Low energy, no chemicals	Low
Ozone	4-6	4-6	2-3	None (decays)	Minimal (bromate)	Low	High energy	High (generation)
Chlorine	4-6	2-4	<1 (poor)	Yes (requires dechlorination)	Yes (THMs, HAAs)	Low	Low cost	Low (but needs dechlorination)
Chlorine Dioxide	4-6	4-5	1-2	Minimal (hours)	No THMs (chlorite/chlorate)	Low	Moderate	Moderate (on-site generation)
Peracetic Acid	3-5	2-4	2-3	None (biodegrades)	None	Low	High chemical cost	Low

Selection Guide

By Target Organism

Cryptosporidium Barrier (drinking water, water reuse):

• Primary: UV (4-6 log at 40-100 mJ/cm²)
• Alternative: Ozone (2-3 log, less effective), UF/MF (physical barrier)
• Ineffective: Chlorine (impractical CT >7,000 mg·min/L)

Virus Inactivation (4+ log):

• Primary: UV (reliable), Ozone (very effective), Chlorine Dioxide (effective)
• Alternative: Chlorine (2-4 log, lower), PAA (2-4 log, variable)
• Best: UV + Chlorine (multi-barrier, additive effect)

Bacteria Only (E. coli, fecal coliforms):

• All disinfectants effective (4-6 log achievable)
• Simplest: Chlorination (lowest cost, proven)
• No DBPs: UV or PAA

Multi-Barrier (belt-and-suspenders approach):

• UV + Chlorine (UV for Crypto, chlorine for backup and residual)
• UF + UV (physical + photochemical barrier)
• Ozone + UV (oxidation + photoinactivation)

By Discharge/Reuse Requirement

Discharge to Sensitive Waters (fish/aquatic life):

Requirement: No chlorine residual (<0.01 mg/L)
Options:
1. UV (no residual, no chemicals)
2. Ozone (no residual after 30-60 min)
3. PAA (biodegrades, no residual)
4. Chlorination + Dechlorination (if must use chlorine)

Discharge to Large River (dilution available):

Requirement: 4-log fecal coliform reduction
Option: Chlorination (simplest, lowest cost)
- Dechlorination often not required (dilution factor)
- Residual dissipates in receiving water

Water Reuse (non-potable irrigation, cooling towers):

Requirement: 4-log bacteria, minimal DBPs
Options:
1. UV (no DBPs, reliable)
2. PAA (no DBPs, no residual for distribution storage)
3. Chlorination (if residual desired for biofilm control in distribution)

Water Reuse (indirect potable reuse, IPR):

Requirement: 12+ log virus, 10+ log Crypto, minimal DBPs
Multi-barrier:
UF (6 log virus + physical barrier) → RO (TDS removal) → UV (4 log final barrier) → Chlorination (distribution residual)

Water Reuse (direct potable reuse, DPR):

Requirement: Extreme multi-barrier, extensive monitoring
Treatment train:
UF → RO → UV + AOP (H₂O₂) → GAC → Chlorination → Distribution
- UV-AOP: Advanced oxidation (hydroxyl radicals for trace organics)
- Multiple barriers, redundancy, real-time monitoring

By Site Constraints

Limited Operational Staff:

1. UV (automated, minimal maintenance except lamp replacement)
2. PAA (simple dosing, no generation equipment)
3. Disk Filter + UV (fully automated treatment train)

No Chemical Storage/Handling:

1. UV (only electricity required)
2. Ozone (on-site generation from air, no chemical storage)

Energy-Limited Sites (remote, off-grid):

1. Chlorination (no energy, chemical only)
2. PAA (no energy, chemical dosing only)
3. Avoid: UV (continuous power), Ozone (high energy)

Extreme Cold Climates:

1. PAA (effective at 1-5°C, temperature-independent)
2. UV (indoor installation, freeze protection)
3. Chlorine (increased CT required at low T, but functional)

High Turbidity Effluent (poor secondary treatment):

1. Chlorination (least sensitive to turbidity, but higher dose)
2. Ozone (moderate sensitivity)
3. Avoid: UV (very sensitive, UVT drops with turbidity >2 NTU)

Design Considerations

Pretreatment Requirements

For UV:

• Critical: Turbidity <2 NTU (preferably <1 NTU)
• UVT >65% (preferably >75%)
• TSS <10 mg/L (prevents UV shading)
• Upstream filtration essential (rapid sand, dual media, disk filter)

For Ozone:

• Important: Remove ozone demand substances (reduce dose)
• TSS <30 mg/L acceptable (less sensitive than UV)
• pH control optional (ozone effective pH 6-9)

For Chlorine:

• Important: Reduce chlorine demand (ammonia, organics, reduced metals)
• TSS <30 mg/L acceptable
• Ammonia consideration: Breakpoint chlorination (Cl₂:N = 10:1 for free Cl₂)

For ClO₂ and PAA:

• Less demanding, TSS <30 mg/L acceptable
• No specific pretreatment beyond secondary clarification

Process Sequencing

Typical sequence:

1. Filtration → Disinfection (80% of plants)

   • Filter reduces turbidity (for UV) or demand (for chemical disinfection)
   • Disinfection as final barrier

2. Membrane (UF/MF) → UV (water reuse)

   • Membrane provides low turbidity (<0.1 NTU, excellent for UV)
   • UV as final pathogen barrier

3. Ozone → Filtration → UV (advanced treatment)

   • Ozone for primary disinfection + organics oxidation
   • Filtration removes ozone-oxidized particles
   • UV as final Cryptosporidium barrier

4. Chlorination → Dechlorination (traditional with discharge)

   • Chlorination in contact tank (15-90 min)
   • Dechlorination (sodium bisulfite) before discharge

Contact Tank Design (for chemical disinfection)

Hydraulic efficiency:

• Serpentine baffling (baffle factor BF = 0.5-0.7)
• L:W ratio ≥ 20:1 (with baffles) for plug-flow approximation
• Minimize dead zones and short-circuiting

CT calculation:

$$CT_{actual} = C_{residual} \times t_{10} \times BF$$

Where:

• $C_{residual}$ = disinfectant concentration at end of tank (mg/L)
• $t_{10}$ = time for 10% of tracer to pass through (min)
• $BF$ = baffle factor (t₁₀ / t_theoretical)

Tracer studies:

• Rhodamine dye test to determine actual t₁₀
• Required for regulatory compliance (demonstrate actual CT)
• Frequency: Initial commissioning, after modifications, every 5 years

Performance Monitoring

Key Performance Indicators (KPIs)

Microbial Quality:

• Fecal coliforms or E. coli: <200 cfu/100mL (UK), <1000 cfu/100mL (US secondary)
• Enterococci: <100 cfu/100mL (recreational water contact)
• Virus testing: Coliphage (surrogate, if required for reuse)
• Cryptosporidium/Giardia: Monitoring (if potable reuse)

Disinfectant Delivery:

• UV: Dose delivered (mJ/cm²), lamp intensity, UVT
• Chlorine: Residual (0.5-2.0 mg/L), CT value (≥required CT)
• Ozone: Residual (0.1-0.5 mg/L), CT value
• ClO₂: Residual (0.1-0.5 mg/L), chlorite (<0.7 mg/L)
• PAA: Dose delivered (mg/L), residual (target ~0 mg/L at discharge)

Operational Efficiency:

• Chemical consumption vs. design (dose optimization)
• Energy consumption (UV lamps, ozone generator)
• Equipment uptime (>95% for critical disinfection systems)

Troubleshooting Common Issues

Problem	Likely Cause	Solution
High fecal coliforms	Inadequate dose/CT	Increase dose or contact time
	Short-circuiting in contact tank	Tracer study, improve baffling
	Upstream TSS spikes	Improve filtration upstream
	Disinfectant decay (chlorine)	Check residual throughout tank
UV: Low dose delivered	Lamp aging (low intensity)	Replace lamps (typically annual)
	Fouling (quartz sleeve scaling)	Increase cleaning frequency, check water chemistry
	Low UVT (high turbidity)	Improve upstream filtration, check TSS
Chlorine: Variable residual	Flow variation	Flow-paced dosing control
	Ammonia variation (breakpoint)	Monitor NH₃-N, adjust dose
Ozone: Low residual	High ozone demand	Improve upstream treatment, increase dose
	Generator malfunction	Check feed gas, corona discharge
DBP formation (THMs, HAAs)	High TOC in feed	Reduce contact time, lower dose, consider UV/PAA
	High temperature	Seasonal adjustment (lower dose in summer)
Residual in effluent (chlorine)	Dechlorination failure	Check SO₂ feed, increase dose, improve mixing
	Flow surge	Flow pacing, buffer tank

Multi-Barrier Disinfection

Rationale for Multi-Barrier

Regulatory drivers:

• Water reuse: 12+ log virus reduction required (no single process achieves this)
• Cryptosporidium: Chlorine ineffective, UV or filtration required
• Redundancy: Backup if primary system fails

Example multi-barrier credit:

UF membrane: 6 log virus removal (validated LRV)
UV disinfection: 4 log virus inactivation (at 100 mJ/cm²)
Chlorination: 2 log virus inactivation (residual in distribution)
Total: 12 log virus reduction ✓

Common Multi-Barrier Combinations

UV + Chlorine (most common):

• UV: Primary disinfection (bacteria, viruses, Crypto)
• Chlorine: Backup + residual for distribution system
• No DBPs from UV, minimal chlorine dose (residual only)

Ozone + UV:

• Ozone: Primary disinfection + organics oxidation
• UV: Crypto barrier + final disinfection
• Synergistic effect (ozone oxidizes UV-absorbing compounds, improves UVT)

UF + UV:

• UF: Physical barrier (6 log virus, 6 log bacteria, 6 log Crypto)
• UV: Additional disinfection (additive log reduction)
• Total >10 log pathogen removal (drinking water/reuse)

Ozone + Filtration + UV:

• Ozone: Primary disinfection + TOC oxidation
• Filtration: Remove ozone-oxidized particles (improve UVT)
• UV: Final Crypto barrier
• Used in advanced water reuse

Process Unit Links

• UV-Disinfection - Photoinactivation, no chemicals
• Ozone-Disinfection - On-site generated oxidant
• Chlorination - Traditional chemical disinfection
• Chlorine-Dioxide - pH-independent, no THMs
• Peracetic-Acid - Biodegradable, no DBPs

Related Processes

• Rapid-Sand-Filter - Upstream filtration (reduce turbidity for UV, demand for chlorine)
• Ultrafiltration - Membrane barrier (physical removal, multi-barrier with disinfection)
• Chemical-Dosing - Disinfectant feed systems, dechlorination
• Contact-Tank - Hydraulic design for CT achievement

References

1. WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 18: Disinfection.
2. White, G.C. (2010). White's Handbook of Chlorination and Alternative Disinfectants, 5th Ed., Wiley.
3. US EPA (1999). Disinfection Profiling and Benchmarking Guidance Manual, EPA 815-R-99-013.
4. AWWA (2011). Water Quality and Treatment, 6th Ed., Ch. 14: Disinfection.
5. US EPA (1999). Alternative Disinfectants and Oxidants Guidance Manual, EPA 815-R-99-014.
6. US EPA (2006). Ultraviolet Disinfection Guidance Manual, EPA 815-R-06-007.
7. NWRI (2012). Ultraviolet Disinfection Guidelines for Drinking Water and Water Reuse, 3rd Ed.