Disinfection

Inactivation of pathogenic microorganisms in treated wastewater effluent

Overview

Disinfection is the selective destruction of disease-causing (pathogenic) organisms to reduce public health risks from wastewater discharge or reuse. Common methods include ultraviolet (UV) radiation, chlorination, ozonation, and peracetic acid. Disinfection differs from sterilization in that it does not destroy all microorganisms, only those that pose health hazards.

UV Disinfection uses ultraviolet light at 254 nm wavelength to damage microbial DNA, preventing reproduction. It is effective, chemical-free, has no residual, and leaves no disinfection byproducts (DBPs). However, it requires clear effluent (low TSS and turbidity) and provides no residual protection.

Chlorination uses chlorine gas (Cl₂), sodium hypochlorite (NaOCl), or calcium hypochlorite to oxidize microbial cell components. It provides residual disinfection but forms DBPs (trihalomethanes, haloacetic acids) and requires dechlorination before discharge to sensitive waters. Common in municipal WWTPs due to effectiveness and low cost.

Both methods are evaluated using the CT concept (Concentration × Time), which relates disinfectant dose and contact time to log inactivation of target organisms.

Theory and Equations

Basic Mode - UV Disinfection

Simple dose-based inactivation:

$$\text{Log removal} = f(\text{UV dose})$$

$$\text{Effluent count} = \text{Influent count} \times 10^{-\text{Log removal}}$$

Typical UV doses for different log removal:

• 1-log (90%): 20-30 mJ/cm²
• 2-log (99%): 40-60 mJ/cm²
• 3-log (99.9%): 80-100 mJ/cm²
• 4-log (99.99%): 120-150 mJ/cm²

Basic Mode - Chlorination

Simple residual-based approach:

$$\text{Chlorine dose} = \text{Chlorine demand} + \text{Residual}$$

Where:

• Chlorine demand = BOD/COD oxidation + ammonia chlorination + other reactions
• Typical demand: 5-20 mg/L
• Target residual: 0.5-1.0 mg/L (free) or 1.0-2.0 mg/L (combined)

Advanced Mode - UV Disinfection

Chick-Watson Law:

$$\frac{dN}{dt} = -k \times I \times N$$

Integrating:

$$\ln\left(\frac{N}{N_0}\right) = -k \times I \times t = -k \times \text{Dose}$$

$$\text{Log removal} = \frac{k \times \text{Dose}}{2.303}$$

Where:

• $N$ = microbial concentration (CFU/100mL)
• $N_0$ = initial concentration
• $k$ = UV inactivation rate constant (cm²/mJ)
  • E. coli: k ≈ 0.35 cm²/mJ
  • Total coliform: k ≈ 0.30 cm²/mJ
  • Viruses: k ≈ 0.10-0.25 cm²/mJ
• $I$ = UV intensity (mW/cm²)
• $t$ = contact time (s)
• Dose = $I \times t$ (mJ/cm²)

Dose Calculation:

$$\text{Applied Dose} = \frac{P \times UVT \times \eta}{Q}$$

Where:

• $P$ = lamp power (W)
• $UVT$ = UV transmittance (%)
• $\eta$ = system efficiency (typically 0.30-0.40)
• $Q$ = flow rate (m³/s)

Advanced Mode - Chlorination

CT Concept:

$$CT = C \times t$$

Where:

• $C$ = disinfectant concentration (mg/L)
• $t$ = contact time (min)
• $CT$ = concentration-time product (mg·min/L)

Required CT for target log removal:

$$\text{Log removal} = f(CT, \text{organism}, \text{pH}, \text{temp})$$

Chlorine Demand:

$$\text{Demand} = K_{BOD} \times \text{BOD} + K_{NH3} \times \text{NH}3\text{-N} + K$$

Where:

• $K_{BOD}$ ≈ 1.0-1.5 mg Cl₂/mg BOD
• $K_{NH3}$ ≈ 7.6 mg Cl₂/mg NH₃-N (chloramine formation)
• $K_{NH3}$ ≈ 10 mg Cl₂/mg NH₃-N (breakpoint chlorination)

Decay Kinetics:

$$\frac{dC}{dt} = -k_d \times C$$

$$C(t) = C_0 \times e^{-k_d \times t}$$

Where:

• $k_d$ = decay rate constant (1/min)

Model Parameters

UV Disinfection Parameters

Parameter	Symbol	Unit	Default	Range	Description
Target Log Removal	-	log	3.0	1.0-5.0	Pathogen inactivation
UV Dose	Dose	mJ/cm²	80	20-150	Applied UV dose
Inactivation Rate (E. coli)	k	cm²/mJ	0.35	0.30-0.40	E. coli rate constant
UV Transmittance	UVT	%	65	50-85	At 254 nm, 1 cm path
TSS	TSS	mg/L	10	<20	Effluent suspended solids
Lamp Efficiency	η	-	0.35	0.30-0.40	System UV efficiency

Chlorination Parameters

Parameter	Symbol	Unit	Default	Range	Description
Target Log Removal	-	log	4.0	2.0-6.0	Pathogen inactivation
Chlorine Dose	Dose	mg/L	8.0	5.0-20.0	Applied chlorine
Contact Time	t	min	30	15-120	Detention time
Target Residual	C_res	mg/L	0.5	0.2-2.0	Free or combined Cl₂
Chlorine Demand	-	mg/L	6.0	3.0-15.0	Chlorine consumed
pH	pH	-	7.0	6.5-8.5	Affects HOCl/OCl⁻ ratio
Temperature	T	°C	20	10-30	Affects reaction rates

Design Criteria

UV Disinfection

Parameter	Typical Range	Notes
Peak UV Dose	80-150 mJ/cm²	For 3-4 log removal
Average UV Dose	60-100 mJ/cm²	Average conditions
UV Transmittance	>60%	At 254 nm, 1 cm path
TSS	<20 mg/L	<30 mg/L maximum
Channel Velocity	0.3-0.6 m/s	Prevents settling
Headloss	0.1-0.3 m	Across reactor
Lamp Replacement	12-18 months	Or 9,000-12,000 hours

Chlorination

Parameter	Typical Range	Notes
Chlorine Dose	5-20 mg/L	Depends on demand
Contact Time	15-120 min	Based on CT tables
Peak Hour CT	Higher	EPA CT tables by organism
Free Chlorine Residual	0.5-1.0 mg/L	After contact time
Combined Chlorine Residual	1.0-2.0 mg/L	Chloramines
Channel Configuration	Serpentine	Maximize plug flow
Baffling Factor	>0.7	Minimize short-circuiting

Worked Examples

Example 1: UV Disinfection - Required Dose

Given:

• Influent E. coli: 1,000,000 CFU/100mL (10⁶)
• Target effluent: 100 CFU/100mL (10²)
• Inactivation rate constant (k): 0.35 cm²/mJ

Calculate: Required UV dose

1. Required log removal:

   • $\text{Log removal} = \log_{10}\left(\frac{10^6}{10^2}\right) = \log_{10}(10^4) = 4\text{-log}$

2. Required UV dose:

   • From Chick-Watson: $\text{Log removal} = \frac{k \times \text{Dose}}{2.303}$
   • $\text{Dose} = \frac{\text{Log removal} \times 2.303}{k} = \frac{4 \times 2.303}{0.35}$
   • $\text{Dose} = \frac{9.212}{0.35} = 26.3 \text{ mJ/cm²}$

3. Design dose (with safety factor 2.0):

   • $\text{Design dose} = 26.3 \times 2.0 = 52.6 \text{ mJ/cm²}$
   • Use 60 mJ/cm² (standard dose for 4-log E. coli)

Solution:

• Theoretical dose: 26.3 mJ/cm²
• Design dose with safety factor: 60 mJ/cm²

Example 2: Chlorination - Dose Calculation

Given:

• Flow: 10,000 m³/d
• Effluent BOD: 10 mg/L
• Effluent NH₃-N: 2.0 mg/L
• Target residual: 0.5 mg/L (free chlorine)
• Contact time: 30 min
• Temperature: 20°C, pH: 7.5

Calculate: Required chlorine dose

1. Chlorine demand:
   • BOD demand: $10 \times 1.2 = 12 \text{ mg/L}$ (using K = 1.2)
   • Ammonia demand: $2.0 \times 7.6 = 15.2 \text{ mg/L}$ (chloramine formation)
   • Other demand: $2.0 \text{ mg/L}$ (iron, manganese, etc.)
   • Total demand: $12 + 15.2 + 2.0 = 29.2 \text{ mg/L}$

Note: This is high demand due to ammonia. In practice, nitrification before chlorination reduces demand.

2. Required chlorine dose:

   • $\text{Dose} = \text{Demand} + \text{Residual} = 29.2 + 0.5 = 29.7 \text{ mg/L}$

3. Daily chlorine usage:

   • $\text{Usage} = 10,000 \times 29.7 / 1000 = 297 \text{ kg/d}$

Solution:

• Chlorine dose: 29.7 mg/L (high due to ammonia)
• Daily usage: 297 kg Cl₂/d
• Recommendation: Achieve nitrification upstream to reduce dose to ~8-10 mg/L

Example 3: CT Calculation for Chlorination

Given:

• Chlorine residual: 1.0 mg/L (free chlorine)
• Contact time: 45 min
• Temperature: 20°C
• pH: 7.5
• Target: 4-log Giardia inactivation

Calculate: Verify if CT is adequate

1. Applied CT:

   • $CT = 1.0 \times 45 = 45 \text{ mg·min/L}$

2. Required CT (from EPA tables):

   • For 4-log Giardia at 20°C, pH 7.5, free chlorine:
   • Required CT ≈ 141 mg·min/L

3. Comparison:

   • Applied CT (45) < Required CT (141)
   • Inadequate disinfection for 4-log Giardia

4. Options to achieve 4-log:

   • Increase contact time: $t = 141 / 1.0 = 141 \text{ min}$
   • Increase residual: $C = 141 / 45 = 3.1 \text{ mg/L}$
   • Or combination: $C = 2.0 \text{ mg/L}, t = 71 \text{ min}$

Solution:

• Applied CT: 45 mg·min/L
• Required CT: 141 mg·min/L (4-log Giardia)
• Not adequate - need longer contact time or higher residual

Assumptions and Limitations

• UV models assume ideal lamp performance and uniform intensity distribution
• UV transmittance must be measured; varies with water quality
• Chlorination CT values are organism-specific (bacteria vs. viruses vs. protozoa)
• CT concept assumes ideal plug flow; baffling factor accounts for short-circuiting
• Does not model disinfection byproduct (DBP) formation from chlorination
• Temperature and pH significantly affect chlorine efficacy
• UV provides no residual protection in distribution system
• Particle-associated microorganisms are more resistant to both UV and chlorine
• Does not account for lamp fouling or aging in UV systems

Troubleshooting

Issue	Possible Cause	Solution
UV Disinfection
High effluent bacteria	Low UVT, high TSS, lamp fouling	Improve upstream treatment, clean lamps
High power consumption	Fouled lamps, low UVT	Increase cleaning frequency, check water quality
Lamp failure	End of lamp life, power issues	Replace lamps per schedule, check ballasts
Low UVT	High TSS, color, organics	Improve secondary/tertiary treatment
Chlorination
High chlorine demand	High BOD/COD, ammonia present	Improve biological treatment, nitrification
Unable to maintain residual	Insufficient dose, low contact time	Increase dose, repair leaks
DBP formation	High organics, long contact time	Reduce contact time, move chlorination point
Ammonia breakthrough	Insufficient chlorine for breakpoint	Increase dose or accept chloramines
Low pH	Insufficient alkalinity	Add alkalinity before chlorination

Related Process Units

• Secondary Clarifier - Upstream solids removal
• Filtration - Improves TSS removal before UV
• Activated Sludge - Nitrification reduces chlorine demand
• Dechlorination - Required after chlorination for discharge

References

1. EPA (1999). Wastewater Technology Fact Sheet: Ultraviolet Disinfection, EPA 832-F-99-064.
2. EPA (1999). Wastewater Technology Fact Sheet: Chlorine Disinfection, EPA 832-F-99-062.
3. EPA (1991). Guidance Manual for Compliance with the Filtration and Disinfection Requirements for Public Water Systems Using Surface Water Sources, EPA 815-R-91-013.
4. Metcalf & Eddy (2014). Wastewater Engineering: Treatment and Resource Recovery, 5th Edition.
5. WEF (2019). Disinfection, Manual of Practice No. 29.
6. Bolton, J.R. and Linden, K.G. (2003). "Standardization of Methods for Fluence (UV Dose) Determination in Bench-Scale UV Experiments," Journal of Environmental Engineering, 129(3):209-215.
7. White, G.C. (2010). White's Handbook of Chlorination and Alternative Disinfectants, 5th Edition, Wiley.

Model Implementation

TypeScript Model Location: backend/models/processes/Disinfection.ts

Key Methods:

• calculateUVDose() - UV dose from target log removal
• calculateChlorineD ose() - Chlorine dose from demand and residual
• calculateCT() - CT product for chlorination
• calculateLogRemoval() - Pathogen inactivation
• validate() - Parameter range validation

Change Log

Date	Version	Changes
2025-12-30	1.0	Initial documentation covering UV and chlorination methods