6.1 Disinfection Chemistry, Chlorination Dynamics & CT Calculations

3.4 Disinfection Chemistry, Chlorination Dynamics & CT Calculations

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Disinfection Objectives & Pathogen Targets

Drinking water disinfection serves two distinct public health objectives:

1. Primary Disinfection: The in-plant inactivation or killing of pathogenic organisms within contact basins and clearwells to achieve regulatory log-reduction credits.
2. Secondary Disinfection: The maintenance of a persistent biocidal chemical residual throughout the distribution piping network and finished water storage tanks to suppress bacterial regrowth, inhibit biofilm development, and protect against contamination from low-pressure back-siphonage events.

Primary Microbial Targets

• Giardia lamblia Cysts: A flagellated protozoan parasite causing human giardiasis. The infectious cyst has a tough, fibrous outer wall that makes it moderately resistant to chemical disinfection. Under the Surface Water Treatment Rule (SWTR), treatment plants must achieve a minimum $3.0\text{-log}$ ($99.9\%$) removal and/or inactivation of Giardia cysts.
• Cryptosporidium parvum Oocysts: A protozoan parasite whose thick outer shell is virtually impervious to standard free chlorine concentrations ($CT > 7,000 \text{ mg}\cdot\text{min/L}$ required). Inactivation requires physical filtration, Ultraviolet (UV) irradiation, or ozone.
• Enteric Viruses: Small viral particles (including rotavirus, hepatitis A, norovirus, and enteroviruses) responsible for severe gastrointestinal illnesses. SWTR requires a minimum $4.0\text{-log}$ ($99.99\%$) removal and/or inactivation of enteric viruses.
• Residual Requirements: For surface water and GWUDI systems, the disinfectant residual entering the distribution system may not be below $0.2 \text{ mg/L}$ for more than 4 hours. The residual in the distribution system may not be undetectable in more than 5 percent of monthly samples for two consecutive months. Any Oregon system that adds a disinfectant must keep a detectable residual throughout distribution, measured at least twice a week and with every coliform sample (OAR 333-061-0036(9)).

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Disinfectant Types & Chemical Properties

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Aqueous Chlorine Chemistry & pH Equilibrium

When chlorine gas ($\text{Cl}_2$) is dissolved in water, it undergoes an instantaneous hydrolysis reaction (completing in milliseconds):

$$\text{Cl}_2\text{(g)} + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{H}^+ + \text{Cl}^-$$

This reaction produces hypochlorous acid ($\text{HOCl}$), hydrogen ions (which lower pH), and chloride ions. When sodium hypochlorite ($\text{NaOCl}$) is used, it dissociates:

$$\text{NaOCl} + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{Na}^+ + \text{OH}^-$$

producing hypochlorous acid and hydroxide ions (which raise pH).

+-----------------------------------------------------------------------------+
|               CHLORINE pH EQUILIBRIUM: HOCl vs. OCl-                        |
+-----------------------------------------------------------------------------+
|                                                                             |
|   HOCl <=====================> H+  +  OCl-      (pKa = 7.54 at 20°C)        |
|                                                                             |
|   100% |  *** HOCl (Potent Germicide, Neutral Molecule)                     |
|        |     *                                                              |
|    75% |      **                                                            |
|        |        *                                                           |
|    50% |---------*----------------------- (pH 7.5: 50% HOCl / 50% OCl-)      |
|        |          *                                                         |
|    25% |           **                                                       |
|        |             *                                                      |
|     0% |______________***________________ OCl- (Weak Germicide, Anion)      |
|        4.0   5.0   6.0   7.0   7.5   8.0   9.0   10.0  (pH Units)           |
+-----------------------------------------------------------------------------+

The Acid-Base Dissociation Equilibrium

Hypochlorous acid is a weak acid that partially dissociates into hydrogen ions and hypochlorite ions ($\text{OCl}^-$):

$$\text{HOCl} \rightleftharpoons \text{H}^+ + \text{OCl}^-$$

• Biocidal Potency: Hypochlorous acid ($\text{HOCl}$) is an electrically neutral, uncharged molecule of small molecular size. Because it possesses no electrical charge, it rapidly penetrates the negatively charged lipid bilayer membrane of bacterial cells and protozoan cysts, inactivating vital intracellular enzymes. In contrast, the hypochlorite ion ($\text{OCl}^-$) carries a negative electrical charge and is electrostatically repelled by bacterial cell walls. Consequently, $\text{HOCl}$ is 80 to 100 times more effective as a disinfectant than $\text{OCl}^-$.
• pH Dependence ($pK_a \approx 7.54 \text{ at } 20^\circ\text{C}$):
  • At $\text{pH } 6.0$: $\approx 97\% \text{ HOCl}$ and $3\% \text{ OCl}^-$
  • At $\text{pH } 7.0$: $\approx 78\% \text{ HOCl}$ and $22\% \text{ OCl}^-$
  • At $\text{pH } 7.5$: $\approx 50\% \text{ HOCl}$ and $50\% \text{ OCl}^-$
  • At $\text{pH } 8.0$: $\approx 26\% \text{ HOCl}$ and $74\% \text{ OCl}^-$
  • At $\text{pH } 8.5$: $\approx 9\% \text{ HOCl}$ and $91\% \text{ OCl}^-$
• Operational Takeaway: Operating a disinfection clearwell at $\text{pH } 8.5$ rather than $\text{pH } 7.0$ reduces the concentration of active $\text{HOCl}$ by nearly $90\%$. To achieve equivalent pathogen inactivation at elevated pH, an operator must drastically increase chlorine dosage or substantially extend contact time.

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Breakpoint Chlorination Dynamics

When chlorine is dosed into water containing reducing inorganic minerals and ammonia nitrogen ($\text{NH}_3$), it proceeds across four distinct operational zones along the breakpoint chlorination curve:

+-----------------------------------------------------------------------------+
|                     THE BREAKPOINT CHLORINATION CURVE                       |
+-----------------------------------------------------------------------------+
| Total Chlorine                                                              |
| Residual                                                                    |
| (mg/L)   ^                                             Zone 4:              |
|          |                                             Free Available       |
|          |                           Zone 2:           Chlorine (1:1 Slope) |
|          |                        Combined Residual    /                    |
|          |                           (Chloramines)    /                     |
|          |                                /\         /                      |
|          |                               /  \       /                       |
|          |                              /    \     /                        |
|          |      Zone 1:                /      \   /                         |
|          |    Initial Demand          /   Zone 3: v Breakpoint Nadir        |
|          |   (Inorganics: Fe, Mn, H2S)    Chloramine Destruction            |
|        0 +---------------------------+----------+------------------------>  |
|          0                           5:1       7.6:1 - 10:1                 |
|                                    Chlorine-to-Ammonia Ratio (by weight)    |
+-----------------------------------------------------------------------------+

The Four Zones

1. Zone 1: Initial Chlorine Demand: Chlorine reacts immediately with easily oxidizable inorganic substances (ferrous iron $\text{Fe}^{2+}$, manganous manganese $\text{Mn}^{2+}$, nitrite $\text{NO}_2^-$, and hydrogen sulfide $\text{H}_2\text{S}$). Chlorine is reduced to inert chloride ($\text{Cl}^-$); no chlorine residual is established.
2. Zone 2: Formation of Chloramines (Combined Chlorine): Once inorganic demand is met, chlorine reacts with ammonia nitrogen ($\text{NH}_3$) to form inorganic chloramines: $$\text{NH}_3 + \text{HOCl} \rightarrow \text{NH}_2\text{Cl (Monochloramine)} + \text{H}_2\text{O}$$ $$\text{NH}_2\text{Cl} + \text{HOCl} \rightarrow \text{NHCl}_2\text{ (Dichloramine)} + \text{H}_2\text{O}$$ $$\text{NHCl}_2 + \text{HOCl} \rightarrow \text{NCl}_3\text{ (Trichloramine)} + \text{H}_2\text{O}$$ Measured total chlorine residual rises in direct proportion to chemical dosage. The residual in Zone 2 exists entirely as combined chlorine.
3. Zone 3: Destruction of Chloramines (The Breakpoint Transition): As the chlorine-to-ammonia nitrogen ratio increases beyond $5:1$ toward $7.6:1 \text{ to } 10:1$ by weight, free hypochlorous acid begins oxidizing chloramines, converting combined nitrogen into inert nitrogen gas ($\text{N}_2\uparrow$), nitrous oxide ($\text{N}_2\text{O}$), and hydrochloric acid: $$2\text{NH}_2\text{Cl} + \text{HOCl} \rightarrow \text{N}_2\uparrow + 3\text{HCl} + \text{H}_2\text{O}$$ During this phase, additional chlorine feed destroys existing chloramines, causing the measured residual to plunge sharply toward zero. The lowest point on this curve is the breakpoint nadir. (Operators who observe falling chlorine residuals despite increasing feed pumps are caught in Zone 3!)
4. Zone 4: Free Available Residual Chlorination: Once all ammonia is fully oxidized at the breakpoint, all further chlorine added remains uncombined as Free Available Chlorine ($\text{HOCl} + \text{OCl}^-$). The residual rises linearly on a $1:1$ ratio ($45^\circ$ slope) with applied dosage.

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Disinfection Byproducts (DBPs) & Regulatory Standards

When free chlorine contacts naturally occurring humic and fulvic organic matter (TOC), it reacts to form carcinogenic Disinfection Byproducts (DBPs) governed under OAR 333-061-0036 (Stage 1 & Stage 2 DBPR):

1. Total Trihalomethanes (TTHMs) — Maximum Contaminant Level: $0.080 \text{ mg/L}$ ($80 \ \mu\text{g/L}$)

TTHMs comprise four halogenated methanes: chloroform ($\text{CHCl}_3$), bromodichloromethane ($\text{CHBrCl}_2$), dibromochloromethane ($\text{CHBr}_2\text{Cl}$), and bromoform ($\text{CHBr}_3$). TTHM formation increases with high TOC, high water temperature, long water age, and elevated pH.

2. Five Haloacetic Acids (HAA5) — Maximum Contaminant Level: $0.060 \text{ mg/L}$ ($60 \ \mu\text{g/L}$)

HAA5 represents the sum of monochloroacetic, dichloroacetic, trichloroacetic, monobromoacetic, and dibromoacetic acids. Unlike TTHMs, HAA5 formation is favored at lower (acidic) pH.

3. Maximum Residual Disinfectant Levels (MRDLs)

To prevent systemic health risks and excessive DBP creation, the EPA and OHA enforce maximum residual disinfectant levels:

• Free Chlorine & Chloramines: MRDL = $4.0 \text{ mg/L}$ (running annual average).
• Chlorine Dioxide: MRDL = $0.8 \text{ mg/L}$.

Compliance Methodology: Locational Running Annual Average (LRAA)

Compliance is not evaluated against system-wide averages. Under Stage 2 DBPR, utilities must calculate a Locational Running Annual Average (LRAA) quarterly for each individual monitoring location in the distribution system. If any single monitoring point exceeds $0.080 \text{ mg/L}$ for TTHMs or $0.060 \text{ mg/L}$ for HAA5 across four consecutive quarters, the system is in violation.

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The $CT$ Concept, Baffling Factors & Step-by-Step Calculations

Pathogen inactivation kinetics are governed by Chick's Law and the $CT$ concept:

$$CT = C \times T_{10}$$

• $C$: Disinfectant residual concentration ($\text{mg/L}$) measured at the exit of the contact chamber.
• $T_{10}$: Effective contact time (minutes) required for $10\%$ of the entering water to pass through the basin (meaning $90\%$ of the water remains in the basin for at least time $T_{10}$).

Baffling Factors ($BF$)

Because tanks experience hydraulic short-circuiting, effective contact time ($T_{10}$) is significantly less than theoretical hydraulic detention time ($DT$). It is calculated using a Baffling Factor ($BF$) determined by empirical tracer studies:

$$DT = \frac{\text{Clearwell Volume (gal)}}{\text{Peak Flow Rate (gpm)}} \qquad T_{10} = DT \times BF$$

Treatment Technique Log-Credits

Under the Surface Water Treatment Rule, conventional filtration plants receive physical removal credits of $2.5\text{-log}$ for Giardia and $2.0\text{-log}$ for viruses if Combined Filter Effluent turbidity standards are met. Chemical disinfection must supply the remaining inactivation:

• Remaining Disinfection Target for Conventional Plants: $0.5\text{-log}$ Giardia and $2.0\text{-log}$ virus inactivation.
• Direct Filtration Plants: Receive $2.0\text{-log}$ Giardia credit, requiring chemical disinfection to achieve $1.0\text{-log}$ Giardia and $3.0\text{-log}$ virus inactivation.

Comprehensive Step-by-Step Worked $CT$ Problem

Facility Operational Data:

• Clearwell Volume: $600,000 \text{ gallons}$
• Peak Plant Flow: $4,000 \text{ gpm}$
• Baffling Classification: Superior serpentine baffling ($BF = 0.7$)
• Water Quality Conditions: $\text{pH} = 7.5$, $\text{Temperature} = 10^\circ\text{C}$
• Measured Effluent Free Chlorine Residual ($C$): $1.5 \text{ mg/L}$

Step 1: Calculate Theoretical Detention Time ($DT$)

$$DT = \frac{\text{Volume}}{\text{Peak Flow}} = \frac{600,000 \text{ gal}}{4,000 \text{ gpm}} = 150 \text{ minutes}$$

Step 2: Calculate Effective Contact Time ($T_{10}$)

$$T_{10} = DT \times BF = 150 \text{ min} \times 0.7 = 105 \text{ minutes}$$

Step 3: Calculate Actual Achieved $CT$ ($CT_{\text{actual}}$)

$$CT_{\text{actual}} = C \times T_{10} = 1.5 \text{ mg/L} \times 105 \text{ min} = 157.5 \text{ mg}\cdot\text{min/L}$$

Step 4: Compare with EPA SWTR Lookup Tables for Giardia

The EPA table for 3-log Giardia inactivation by free chlorine at $10^\circ\text{C}$ lists residuals in 0.2 mg/L steps. Round the measured 1.5 mg/L up to 1.6 mg/L, the conservative choice. At pH 7.5:

• Required $CT$ for $3.0\text{-log}$ Giardia inactivation = $144 \text{ mg}\cdot\text{min/L}$
• Required $CT$ for the $0.5\text{-log}$ Giardia still needed after conventional filtration = $144 \div 6 = 24 \text{ mg}\cdot\text{min/L}$

$$\text{Inactivation Ratio} = \frac{CT_{\text{actual}}}{CT_{\text{required}}} = \frac{157.5}{24} \approx 6.6$$

Because the inactivation ratio is well above 1.0, the clearwell easily provides the required 0.5-log credit. It in fact achieves more than 3-log Giardia inactivation by chemical disinfection alone:

$$\text{Log inactivation} = 3 \times \frac{157.5}{144} \approx 3.3\text{-log}$$

Step 5: Verify Enteric Virus Inactivation

EPA tables specify that at $10^\circ\text{C}$ and $\text{pH } 6.0-9.0$, a free chlorine $CT$ of only $6.0 \text{ mg}\cdot\text{min/L}$ is required for $4.0\text{-log}$ virus inactivation.

$$\text{Virus Inactivation Ratio} = \frac{157.5 \text{ mg}\cdot\text{min/L}}{6.0 \text{ mg}\cdot\text{min/L}} = 26.25 \ge 1.0$$

Virus compliance is overwhelmingly satisfied.