Appearance
Peracetic Acid Disinfection
CH₃COOOH-based disinfection providing rapid pathogen inactivation with no persistent residual or disinfection byproducts, biodegrading to acetic acid and oxygen for sensitive receiving waters.
Overview
Peracetic acid (PAA, CH₃COOOH) is an emerging disinfectant in wastewater treatment offering a unique combination of effective pathogen inactivation without the formation of harmful disinfection byproducts (DBPs). Unlike chlorine, PAA does not produce trihalomethanes (THMs), haloacetic acids (HAAs), or chlorite/chlorate, making it ideal for discharge to sensitive aquatic environments and water reuse applications. PAA degrades rapidly to acetic acid and oxygen, leaving no persistent toxic residual.
When to Use
- Discharge to sensitive waters (no chlorinated residual)
- Water reuse applications (no THM/HAA formation)
- Cold-water disinfection (effective at low temperature)
- High-pH effluent (pH 5-8 effectiveness)
- Facilities without dechlorination infrastructure
- Emergency disinfection (rapid deployment)
- Biofilm control in treatment processes
- Combined with UV for multi-barrier treatment
Key Characteristics
- Log reduction: 3-5 log bacteria, 2-4 log viruses, 2-3 log protozoa
- No persistent residual (degrades in minutes)
- No THM, HAA, chlorite, or chlorate formation
- Effective across pH 5-8 (optimal pH 7-7.5)
- Temperature independent (works at 1-30°C)
- Contact time: 10-30 minutes typical
- Dose: 2-8 mg/L (wastewater), 0.5-3 mg/L (potable)
- Adds minor BOD (acetic acid product)
- Strong oxidizer (corrosion risk)
- Pungent vinegar odor
Operating Modes
Basic Mode
- Fixed dose control (mg/L PAA)
- Simple plug-flow or complete-mix contact tank
- Time-based CT calculation
- Default dose for E. coli (3 mg/L × 15 min)
- No residual monitoring required
Advanced Mode
- Chick-Watson kinetic model
- Variable dose based on TSS and COD (organic interference)
- Temperature-corrected kinetics
- Multi-species pathogen inactivation
- PAA decay modeling
- Acetic acid impact on effluent BOD
- Cost-benefit optimization
Mathematical Model
Chemical Properties
Molecular structure:
$$CH_3COOOH \leftrightarrow CH_3COOH + \frac{1}{2}O_2$$
PAA is an equilibrium mixture:
- Peracetic acid: 15-40% (active ingredient)
- Hydrogen peroxide: 5-20% (stabilizer, mild disinfectant)
- Acetic acid: 20-40% (carrier, reaction product)
- Water: 15-40%
Disinfection Kinetics
Chick-Watson model:
$$\ln\left(\frac{N}{N_0}\right) = -k \times C_{PAA} \times t$$
Where:
- $N/N_0$ = surviving fraction
- $k$ = specific lethality coefficient (L/(mg·min), organism-specific)
- $C_{PAA}$ = PAA concentration (mg/L)
- $t$ = contact time (min)
In log₁₀ form:
$$\log\left(\frac{N}{N_0}\right) = -\frac{k \times C_{PAA} \times t}{2.303}$$
Typical CT Values (Secondary Effluent)
E. coli (4-log reduction @ pH 7, 15°C):
- CT ≈ 30-50 mg·min/L
- Dose 3 mg/L × 15 min = 45 mg·min/L ✓
Enterococci:
- CT ≈ 40-70 mg·min/L (slightly more resistant)
Coliphage (viral surrogate):
- CT ≈ 60-120 mg·min/L
Giardia cysts:
- CT ≈ 100-200 mg·min/L
Cryptosporidium oocysts:
- CT ≈ 500-1000 mg·min/L (highly resistant, like chlorine)
Temperature Effect
PAA is less temperature-dependent than chlorine:
$$k_T = k_{20} \times \theta^{(T-20)}$$
Where $\theta$ ≈ 1.02-1.05 (weak temperature dependence)
Practical implication: PAA works well in cold weather (advantage over chlorine).
Degradation Kinetics
PAA degrades via:
$$CH_3COOOH \rightarrow CH_3COOH + \frac{1}{2}O_2$$
First-order decay:
$$\frac{dC}{dt} = -k_{decay} \times C$$
Half-life in wastewater: 10-30 minutes (depends on organics, metals)
Residual PAA:
- Immediate post-contact: 0.5-2 mg/L
- After 30 min: <0.1 mg/L
- After 60 min: ~0 mg/L (non-detect)
Advantage: No dechlorination needed.
Interference by Organics
High TSS/COD increases PAA demand:
$$Demand_{PAA} = k_{TSS} \times TSS + k_{COD} \times COD + k_{base}$$
Empirical:
- $k_{TSS}$ ≈ 0.1-0.3 mg PAA per mg TSS
- $k_{COD}$ ≈ 0.05-0.15 mg PAA per mg COD
- $k_{base}$ ≈ 1-2 mg/L (baseline)
Example:
TSS = 10 mg/L, COD = 40 mg/L
Demand = 0.2×10 + 0.1×40 + 1.5 = 2 + 4 + 1.5 = 7.5 mg/L
For residual 2 mg/L, dose = 7.5 + 2 = 9.5 mg/LAcetic Acid and BOD Impact
PAA degrades to acetic acid:
$$\text{Molar ratio: } 1 \text{ mol PAA} \rightarrow 1 \text{ mol CH}_3\text{COOH}$$
$$\frac{60 \text{ g/mol (acetic acid)}}{76 \text{ g/mol (PAA)}} = 0.79$$
BOD contribution:
$$BOD_{added} \approx 0.79 \times Dose_{PAA} \times f_{decay}$$
Where $f_{decay}$ = fraction of PAA that degrades (typically 0.7-0.9)
Example:
Dose = 5 mg/L PAA
Acetic acid = 0.79 × 5 × 0.8 = 3.16 mg/L
BOD₅ of acetic acid ≈ 0.7 × COD ≈ 0.7 × 1.07 × 3.16 ≈ 2.4 mg/L
Effluent BOD increase: ~2-3 mg/L (usually acceptable)Design Parameters
Input Parameters
| Parameter | Symbol | Unit | Range | Default | Description |
|---|---|---|---|---|---|
| Flow rate | $Q$ | m³/d | - | - | Design flow (peak hourly) |
| Contact time | $t$ | min | 10-30 | 15 | Hydraulic retention time |
| PAA dose | $D$ | mg/L | 2-10 | 5 | Applied PAA concentration |
| Target CT | $CT$ | mg·min/L | 30-150 | 50 | Design CT value |
| Influent TSS | $TSS$ | mg/L | 5-30 | 12 | Suspended solids |
| Influent COD | $COD$ | mg/L | 20-80 | 40 | Chemical oxygen demand |
| Temperature | $T$ | °C | 1-30 | 15 | Wastewater temperature |
| pH | - | - | 6.5-8.0 | 7.2 | Wastewater pH |
Output Parameters
| Parameter | Unit | Description |
|---|---|---|
| Tank volume | m³ | Contact tank volume |
| PAA consumption | kg/d | Daily PAA product use (15% active) |
| Actual CT | mg·min/L | Achieved disinfection dose |
| Residual PAA | mg/L | PAA at end of contact |
| BOD increase | mg/L | Acetic acid contribution |
| Log reduction | log₁₀ | Pathogen inactivation |
| Chemical cost | $/year | Annual PAA cost |
Configuration Guide
Step-by-Step Setup
Determine Target CT
Based on regulatory and pathogen requirements:
E. coli (4-log, secondary effluent): CT = 40-60 mg·min/L Enterococci (4-log): CT = 60-80 mg·min/L Water reuse (conservative): CT = 80-120 mg·min/L Typical selection: CT = 50 mg·min/LSelect Contact Time
Practical: t = 15-20 minutes @ peak flow Use t_theoretical = 20 minutes Assume complete-mix or plug-flow (BF not critical, no residual concern)Calculate PAA Dose
$$Dose_{PAA} = \frac{CT}{t}$$
Dose = 50 / 20 = 2.5 mg/L (active PAA) Add demand: TSS = 10 mg/L, COD = 40 mg/L Demand ≈ 0.2×10 + 0.1×40 = 2 + 4 = 6 mg/L Total dose = 6 + 2.5 = 8.5 mg/L active PAADesign Contact Tank
$$V = \frac{Q \times t}{60}$$
Q_peak = 12,000 m³/d = 500 m³/h t = 20 min V = 500 × (20/60) = 167 m³ Use V = 180 m³ (allow 8% safety) Tank design: - Simple rectangular tank (no extensive baffling needed) - Dimensions: 15m long × 4m wide × 3m deep - Mechanical mixing (optional, improves contact)Calculate PAA Product Consumption
PAA supplied as 15% solution (typical commercial product):
$$M_{product} = \frac{Q \times Dose_{active}}{0.15}$$
Q_avg = 8,000 m³/d Dose_active = 8.5 mg/L Active PAA = 8,000 × 8.5 × 10⁻³ = 68 kg/d Product (15% PAA) = 68 / 0.15 = 453 kg/d Annual = 165,345 kg/year = 165 tonnes/year Cost @ $2,500/tonne = $413,363/year (Higher than chlorine, but no dechlorination cost)Verify BOD Impact
Acetic acid formation = 0.79 × 8.5 × 0.8 = 5.4 mg/L BOD₅ increase ≈ 0.7 × 1.07 × 5.4 ≈ 4 mg/L If effluent BOD limit = 10 mg/L and current BOD = 6 mg/L: New BOD = 6 + 4 = 10 mg/L (just meets limit) ✓No Dechlorination Required
PAA degrades naturally:
- No chemical cost for dechlorination
- No dechlorination equipment
- Residual ~0 mg/L after 60 minutes
Typical Design Values
| Application | Contact Time (min) | PAA Dose (mg/L) | CT (mg·min/L) | BOD Increase (mg/L) |
|---|---|---|---|---|
| Secondary effluent (E. coli) | 15-20 | 3-5 | 45-75 | 2-4 |
| Advanced treatment (reuse) | 20-30 | 4-6 | 80-120 | 3-5 |
| Peak wet weather | 10-15 | 5-8 | 50-100 | 3-6 |
| Cold water (<5°C) | 20-25 | 4-6 | 80-120 | 3-5 |
Performance Criteria
Pathogen Inactivation
| Organism | CT for 99.9% (3-log) @ pH 7, 15°C | CT for 99.99% (4-log) |
|---|---|---|
| E. coli | 12-18 mg·min/L | 25-35 mg·min/L |
| Enterococci | 18-25 mg·min/L | 35-50 mg·min/L |
| Total coliforms | 15-22 mg·min/L | 30-45 mg·min/L |
| F-specific coliphage | 30-60 mg·min/L | 60-120 mg·min/L |
| MS2 phage (viral surrogate) | 40-80 mg·min/L | 80-160 mg·min/L |
| Giardia lamblia | 50-100 mg·min/L | 100-200 mg·min/L |
Note: Cryptosporidium highly resistant (CT >500 mg·min/L), UV required for effective control.
pH Effect (Less Sensitive than Chlorine)
| pH | Relative PAA Efficacy |
|---|---|
| 5.0 | 90% |
| 6.0 | 95% |
| 7.0 | 100% (optimum) |
| 7.5 | 98% |
| 8.0 | 85% |
| 9.0 | 60% (reduced) |
Practical range: pH 6.5-8.0 for consistent performance.
Comparison to Chlorine
| Factor | PAA | Free Chlorine |
|---|---|---|
| E. coli CT (4-log) | 30-50 mg·min/L | 0.08 mg·min/L |
| Virus efficacy | Moderate | Good |
| Cryptosporidium | Poor (like Cl₂) | Very poor |
| pH sensitivity | Low | High (pH >8 ineffective) |
| Temperature sensitivity | Low | High |
| THM formation | None | Yes |
| Persistent residual | No | Yes (requires dechlorination) |
| Cost | 3-5× higher | Baseline |
| BOD increase | 2-4 mg/L | None |
Troubleshooting
| Issue | Cause | Solution |
|---|---|---|
| Inadequate disinfection | Low PAA dose | Increase dose to meet CT requirement |
| High TSS/COD demand | Improve upstream treatment, increase dose | |
| Short-circuiting | Add baffles, improve mixing | |
| Low contact time | Increase tank volume or reduce flow | |
| High PAA consumption | High organic demand | Optimize secondary treatment (reduce BOD/TSS) |
| PAA product degradation | Fresh chemical, proper storage (cool, dark) | |
| Interfering substances (catalase) | Pre-oxidation, adjust pH | |
| BOD increase exceeds limit | Excessive PAA dose | Reduce dose to minimum effective |
| High decay fraction | Accept higher residual (less decay) | |
| Odor complaints (vinegar smell) | Residual acetic acid | Ensure complete degradation, aeration |
| PAA overdose | Reduce dose, improve contact time | |
| Variable disinfection | Flow variation | Flow-paced PAA dosing |
| Demand variation | Monitor TSS/COD, adjust dose | |
| Corrosion issues | PAA is strong oxidizer | Stainless steel or FRP tanks, check materials |
| High concentration at injection | Dilute PAA before addition, improve mixing | |
| PAA residual detection | Incomplete decay | Extend contact time, increase temperature |
| Overdosing | Reduce dose to stoichiometric requirement |
Operation & Maintenance
Daily Checks
- Verify PAA feed rate (pump stroke or flow meter)
- Check PAA storage level and order chemical
- Monitor effluent fecal coliform (if online)
- Inspect contact tank operation
- Verify no pungent odor at discharge
- Measure PAA residual (optional, should be near zero)
Weekly Maintenance
- Lab testing for fecal coliforms/E. coli
- Verify PAA product concentration (titration)
- Inspect chemical feed pumps and lines
- Check for corrosion at injection point
- Review PAA consumption trends
- Measure effluent BOD (verify acetic acid impact)
Monthly Tasks
- Calibrate PAA feed system
- Test contact time (tracer study if issues)
- Inspect tank mixing (if mechanical mixer)
- Review disinfection performance vs. CT target
- Optimize dose based on seasonal demand variation
- Check materials for corrosion damage
Annual Tasks
- Full system performance audit
- PAA product quality testing (% active, H₂O₂ content)
- Structural inspection of contact tank
- Review regulatory compliance (fecal coliform limits)
- Cost analysis and optimization
- Alternative disinfectant comparison (chlorine, UV)
Advanced Features
Flow-Paced Dosing
Automatically adjust PAA based on flow:
$$Dose(t) = \frac{Q(t)}{Q_{design}} \times Dose_{design}$$
Prevents overdosing at low flow, ensures adequate dose at peak flow.
Demand-Based Dosing
Monitor TSS or COD online and adjust dose:
$$Dose = k_{TSS} \times TSS + k_{COD} \times COD + Dose_{baseline}$$
Example:
If TSS increases from 10 to 20 mg/L:
Additional dose = 0.2 × (20-10) = 2 mg/L
Adjust dose from 5 → 7 mg/LPAA + UV Combination
Synergistic disinfection:
- PAA: Bacteria and viruses
- UV: Cryptosporidium and viruses
- Lower doses for both (cost savings)
- No DBP formation
- Robust multi-barrier
Design approach:
PAA: CT = 30 mg·min/L (reduced from 50)
UV: 40 mJ/cm² (4-log Cryptosporidium)
Combined log reduction: 5-6 log (better than either alone)Biofilm Control in Processes
PAA for periodic biofilm removal:
- Dose: 10-20 mg/L (higher than disinfection)
- Contact: 1-2 hours
- Application: MBR membranes, trickling filters, RBCs
- Frequency: Monthly or as needed
Worked Example
Given
- Flow: 5,000 m³/d average, 10,000 m³/d peak
- Target: E. coli <200 cfu/100mL (4-log reduction)
- Secondary effluent: TSS = 12 mg/L, COD = 45 mg/L, BOD₅ = 8 mg/L
- Temperature: 10°C (cold climate)
- Effluent BOD limit: 15 mg/L
- pH = 7.3
Solution
Step 1: Select Target CT
For E. coli 4-log @ 10°C: CT ≈ 50 mg·min/L (conservative)Step 2: Select Contact Time
Use t = 20 minutes @ peak flow (practical)Step 3: Calculate PAA Dose
Base dose = CT / t = 50 / 20 = 2.5 mg/L
PAA demand:
TSS = 12 mg/L → 0.2 × 12 = 2.4 mg/L
COD = 45 mg/L → 0.1 × 45 = 4.5 mg/L
Total demand = 2.4 + 4.5 = 6.9 mg/L
Total dose = 6.9 + 2.5 = 9.4 mg/L
Use 10 mg/L for design safetyStep 4: Design Contact Tank
Q_peak = 10,000 m³/d = 417 m³/h
V = 417 × (20/60) = 139 m³
Use V = 150 m³ (8% safety factor)
Tank: 12m × 4m × 3.2m deep = 153 m³ ✓Step 5: Chemical Consumption
PAA (active) @ average flow:
Daily = 5,000 × 10 × 10⁻³ = 50 kg/d
Annual = 18,250 kg/year
PAA product (15% active):
Product = 18,250 / 0.15 = 121,667 kg/year = 122 tonnes
Cost @ $2,500/tonne = $304,167/year
Compare to chlorine + dechlorination:
Chlorine: ~$80,000/year + Dechlorination: ~$20,000/year = $100,000
PAA premium = $204,167/year
(Trade-off: Higher cost, but no DBPs, no dechlorination equipment)Step 6: Verify BOD Impact
Acetic acid = 0.79 × 10 × 0.8 = 6.3 mg/L
BOD₅ increase ≈ 0.7 × 1.07 × 6.3 ≈ 4.7 mg/L
Effluent BOD = 8 + 4.7 = 12.7 mg/L ✓ (below 15 mg/L limit)Step 7: Verify CT Achievement
Actual CT = 10 mg/L × 20 min = 200 mg·min/L
Safety factor = 200 / 50 = 4× (very conservative design)
Could reduce dose to 5 mg/L for CT = 100 mg·min/L (still 2× safety factor)
Optimized design: Dose = 5 mg/L, saves 50% chemical costRelated Units
- UV-Disinfection - Synergistic combination
- Chlorination - Traditional alternative
- Ozone-Disinfection - Similar no-DBP profile
- Chlorine-Dioxide - Intermediate option
- Chemical-Dosing - PAA feed systems
References
- Kitis, M. (2004). "Disinfection of Wastewater with Peracetic Acid: A Review." Environ. Int. 30(1): 47-55.
- Koivunen, J. and Heinonen-Tanski, H. (2005). "Peracetic Acid (PAA) Disinfection of Primary, Secondary and Tertiary Treated Municipal Wastewaters." Water Res. 39(18): 4445-4453.
- WEF (2018). Design of Municipal Wastewater Treatment Plants, MOP 8, 6th Ed., Ch. 18.
- Luukkonen, T. and Pehkonen, S.O. (2017). "Peracids in Water Treatment: A Critical Review." Crit. Rev. Environ. Sci. Technol. 47(1): 1-39.
- Metcalf & Eddy (2014). Wastewater Engineering, 5th Ed., pp. 1402-1415.
- Veschetti, E. et al. (2003). "Pilot-Plant Comparative Study of Peracetic Acid and Sodium Hypochlorite Wastewater Disinfection." Water Res. 37(1): 78-94.
- US EPA (2012). Guidelines for Water Reuse, EPA/600/R-12/618.