4.2 Contact Time (CT) Calculations & Alternative Disinfectants

4.2 Contact Time (CT) Calculations & Alternative Disinfectants

Under the Safe Drinking Water Act (SDWA) Surface Water Treatment Rule (SWTR) and the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR), water treatment plants must demonstrate adequate disinfection to prevent waterborne disease outbreaks. Disinfection compliance is governed by the kinetic relationship between disinfectant concentration and contact time.

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1. The Concentration × Time (CT) Concept & Log Inactivation

Disinfection kinetics follow the Chick-Watson model, where microbial inactivation is proportional to disinfectant residual concentration ($C$) multiplied by exposure contact time ($T$):

$$CT = C \times T$$

Where $C$ is the disinfectant residual leaving the contact basin in mg/L, $T$ is effective contact time in minutes, and $CT$ is expressed in mg/L·min.

Regulatory Inactivation Standards & Filtration Credits

Federal and Illinois EPA regulations mandate minimum cumulative removal and inactivation targets for surface water and groundwater under direct influence (GWUDI):

• 3-log (99.9%) removal/inactivation of Giardia lamblia cysts
• 4-log (99.99%) removal/inactivation of enteric viruses
• 2-log (99%) removal/inactivation of Cryptosporidium parvum oocysts

Conventional filtration plants meeting turbidity standards (combined filter effluent $\le 0.3$ NTU) receive physical removal credits of 2.5-log Giardia, 2.0-log viruses, and 2.0-log Cryptosporidium. Chemical disinfection must achieve the remaining 0.5-log Giardia and 2.0-log virus inactivation. Direct filtration facilities receive credits of 2.0-log Giardia, 1.0-log viruses, and 2.0-log Cryptosporidium, requiring chemical disinfection to supply 1.0-log Giardia and 3.0-log virus inactivation.

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2. Baffling Factors (BF) & Effective Contact Time ($T_{10}$)

In treated water storage basins and clearwells, density stratification, inlet velocity jetting, and wall friction cause hydraulic short-circuiting. Consequently, regulations prohibit using the Theoretical Detention Time ($DT = V/Q$) for CT compliance. Instead, utilities must calculate $T_{10}$—the time required for 10% of a tracer dye slug to pass through the basin (i.e., 90% of water remains in the basin for at least $T_{10}$):

$$T_{10} = Theoretical\text{ Detention Time }(DT) \times Baffling\text{ Factor }(BF)$$ $$\text{Actual } CT = C_{\text{residual}} \times T_{10}$$

Worked CT Calculation Example

A plant operates a 450,000-gallon clearwell at a peak hourly flow rate of 3,000 gpm. The clearwell is fitted with serpentine baffles rated as Superior ($BF = 0.7$). The finished water free chlorine residual leaving the basin is 1.4 mg/L. Calculate the actual $CT$ achieved:

1. Theoretical Detention Time: $DT = \frac{450,000\text{ gal}}{3,000\text{ gpm}} = 150\text{ minutes}$
2. Effective Contact Time: $T_{10} = 150\text{ min} \times 0.7 = 105\text{ minutes}$
3. Actual Disinfection $CT$: $CT_{\text{actual}} = 1.4\text{ mg/L} \times 105\text{ min} = 147\text{ mg/L}\cdot \text{min}$

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3. Water Temperature & pH Effects on Required CT

Required CT values are obtained from EPA/IEPA CT Lookup Tables based on disinfectant concentration, log inactivation target, water temperature, and pH:

• Temperature Effects: Chemical disinfection reaction rates decrease by approximately half for every 10°C drop. In winter conditions (0.5°C), chlorine requires up to four times higher CT to achieve 3-log Giardia inactivation than in warm summer water (20°C). Operators must increase chlorine doses or storage levels during winter.
• pH Effects: Elevated pH shifts the equilibrium from hypochlorous acid ($HOCl$) to the less potent hypochlorite ion ($OCl^-$). At 10°C and $C = 1.4$ mg/L, the required CT for 3-log Giardia inactivation increases from 75 mg/L·min at pH 6.5 to 155 mg/L·min at pH 8.5 (more than doubling the required contact time).

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4. Chloramination: Chemistry, Benefits & Nitrification

Chloramination is the deliberate injection of ammonia into chlorinated water at a precise 4.5:1 to 5:1 $Cl_2:NH_3\text{-}N$ weight ratio to selectively form monochloramine ($NH_2Cl$):

• Advantages: Monochloramine persists much longer in distribution mains than free chlorine; produces minimal trihalomethanes (TTHMs, MCL 80 µg/L) and haloacetic acids (HAA5, MCL 60 µg/L); and eliminates chlorophenolic taste and odors.
• Disadvantages: Monochloramine is 50 to 100 times weaker than free chlorine against bacteria and viruses; is virtually ineffective against Cryptosporidium; is toxic to kidney dialysis patients and aquatic life; and introduces free ammonia that risks distribution nitrification.

Distribution Nitrification & Control

Autotrophic bacteria oxidize free ammonia in a two-step process:

1. Nitrosomonas oxidize ammonia to nitrite: $2NH_3 + 3O_2 \rightarrow 2NO_2^- + 2H^+ + 2H_2O$
2. Nitrobacter oxidize nitrite to nitrate: $2NO_2^- + O_2 \rightarrow 2NO_3^-$

Nitrification Warning Signs: Rapid drop in total chloramine residual, increase in free ammonia, spike in nitrite ($NO_2^-\text{-}N > 0.05$ mg/L), decrease in dissolved oxygen (DO), and drop in pH. Corrective Actions: Unidirectional main flushing, cycling storage tanks to reduce water age, maintaining tight 5:1 dosing ratios, and conducting an annual temporary free chlorine burn (2 to 4 weeks).

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5. Ultraviolet (UV) Disinfection Technology

UV disinfection is a physical process irradiating water with electromagnetic energy at the 254 nm germicidal wavelength:

• Mechanism: UV light penetrates microbial cells and is absorbed by DNA/RNA, forming cyclobutane pyrimidine dimers between adjacent thymine bases. This disrupts DNA replication and transcription, rendering pathogens non-infectious.
• Pathogen Efficacy: Exceptionally potent against Cryptosporidium oocysts and Giardia cysts, achieving 3-log to 4-log inactivation at low doses of 5 to 12 mJ/cm². However, viruses (Adenovirus) are highly resistant, requiring up to 186 mJ/cm² for 4-log inactivation.
• Operating Parameters: $\text{UV Dose } (mJ/cm^2) = \text{Intensity } (mW/cm^2) \times \text{Time } (s)$. Performance depends on UV Transmittance (% UVT, target > 85–90%) and preventing quartz sleeve fouling from calcium, iron, or biofilms via automated mechanical wipers.
• Limitation: UV provides zero chemical residual; secondary chlorination is mandatory for distribution network biostability.

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6. Ozone ($O_3$) Oxidation & Disinfection

Ozone is an unstable triatomic gas with exceptional oxidation potential ($E^0 = +2.07\text{ V}$):

• Generation & Contacting: Generated on-site by passing dry air or liquid oxygen through high-voltage corona discharge fields (6,000–20,000 V): $3O_2 \rightarrow 2O_3$. Contactors utilize deep concrete basins (18–22 ft) with fine bubble diffusers and over-under baffles providing 10–15 minutes of contact.
• Off-Gas Destruction: Ozone gas is highly toxic (OSHA PEL 0.1 ppm). Contactor off-gas must be routed through thermal or metal oxide catalytic destruct units converting residual $O_3$ back to $O_2$.
• Bromate Byproduct Hazard: In raw waters containing bromide ($Br^-$), ozone oxidizes bromide to form bromate ($BrO_3^-$), a regulated human carcinogen with an MCL of 0.010 mg/L (10 ppb). Control measures include lowering contactor pH to 6.0–6.5, adding ammonia, or optimizing upstream organic precursor removal.