2.4 Disinfection Chemistry, CT Calculations & Disinfection Byproducts

Primary Chemical Disinfectants & Properties

Disinfection is the selective destruction or inactivation of pathogenic microorganisms (bacteria, viruses, and protozoan cysts). Disinfection must achieve primary inactivation at the treatment facility and maintain a secondary protective residual throughout the distribution network.

Disinfectant Types & Operational Characteristics

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Aqueous Chlorine Chemistry & pH Equilibria

When chlorine gas ($Cl_2$) or hypochlorite ($NaOCl / Ca(OCl)_2$) dissolves in water, it undergoes two fundamental chemical reactions:

1. Hydrolysis to Hypochlorous Acid

$$Cl_2 + H_2O \rightleftharpoons HOCl + H^+ + Cl^- \quad (\text{Instantaneous, } < 0.1\text{ second})$$

2. Temperature- and pH-Dependent Dissociation

Hypochlorous acid ($HOCl$) is a weak acid that partially dissociates into the hypochlorite ion ($OCl^-$):

$$HOCl \rightleftharpoons H^+ + OCl^- \quad (pK_a = 7.53 \text{ at } 25^\circ\text{C})$$

The sum of $[HOCl] + [OCl^-]$ is defined as Free Available Chlorine (FAC).

   100% HOCl |********************\                                  
             |                     \                                 
    75% HOCl |                      \                                
             |                       \  <-- pH 7.5 (50% HOCl / 50% OCl-)
    50% HOCl |                        \                              
             |                         \                             
    25% HOCl |                          \                            
             |                           \******************** 100% OCl-
      0%     +-------+-------+-------+-------+-------+-------+-------
            pH 5    pH 6    pH 7   pH 7.5   pH 8    pH 9    pH 10

• At pH 6.0: $\sim 97\%\;HOCl$ and $3\%\;OCl^-$
• At pH 7.5: $\sim 50\%\;HOCl$ and $50\%\;OCl^-$
• At pH 8.0: $\sim 22\%\;HOCl$ and $78\%\;OCl^-$
• At pH 9.0: $\sim 3\%\;HOCl$ and $97\%\;OCl^-$

Germicidal Kinetic Significance: Hypochlorous acid ($HOCl$) is an uncharged, neutral molecule structurally similar to water ($H_2O$), allowing it to rapidly penetrate the lipid cell membranes of bacteria and viruses to oxidize internal enzyme systems. The hypochlorite ion ($OCl^-$) carries a negative electrical charge and is electrostatically repelled by the negative cell wall. Consequently, $HOCl$ is 80 to 100 times more effective as a germicide than $OCl^-$. As treated water pH rises above $7.5$, chlorine disinfection potency drops exponentially, requiring higher doses or longer contact times.

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Breakpoint Chlorination Dynamics

When chlorine is added to raw water containing reducing inorganics and ammonia ($NH_3$), it reacts along the classic Breakpoint Chlorination Curve:

  Residual Chlorine (mg/L)
     |
     |                                      / (Zone 4: Free Available Residual)
     |              C (Peak Combined)      /  (HOCl + OCl-)
     |             /\                     /
     |            /  \                   /
     |           /    \ (Zone 3)        /
     |   (Zone 2)/     \               /
     |          /       \             / 
     | (Zone 1)/         \           /  
     +--------+-----------+---------+----------------------------->
     0        A           B         D (BREAKPOINT)      Chlorine Dose (mg/L)

The Four Zones of Breakpoint Chlorination

1. Zone 1: Inorganic Demand (0 to Point A): Chlorine reacts immediately with reducing inorganic compounds: ferrous iron ($Fe^{2+}$), manganous manganese ($Mn^{2+}$), hydrogen sulfide ($H_2S$), and nitrite ($NO_2^-$). Chlorine is reduced to chloride ($Cl^-$); zero chlorine residual is established.
2. Zone 2: Combined Chlorine Formation (Point A to Point C): Chlorine reacts with ammonia-nitrogen ($NH_3\text{-}N$) to form inorganic chloramines:

$$NH_3 + HOCl \rightarrow NH_2Cl \text{ (Monochloramine)} + H_2O$$ $$NH_2Cl + HOCl \rightarrow NHCl_2 \text{ (Dichloramine)} + H_2O$$

The residual measured is entirely Combined Available Chlorine. Peak combined residual occurs at Point C ($Cl_2:NH_3\text{-}N$ weight ratio $\approx 5:1$). 3. Zone 3: Chloramine Destruction (Point C to Point D): Adding chlorine beyond the $5:1$ ratio causes chlorine to oxidize chloramines into nitrogen gas ($N_2$), nitrous oxide ($N_2O$), and hydrochloric acid:

$$2NH_2Cl + HOCl \rightarrow N_2\uparrow + 3HCl + H_2O$$

During this reaction, intermediate nitrogen trichloride ($NCl_3$, trichloramine) forms at $pH < 7.5$, creating foul, pungent chemical odors. Total residual drops sharply to its minimum at Point D (The Breakpoint). 4. Zone 4: Free Available Residual (Past Point D): Once all ammonia is fully oxidized (at a stoichiometric $Cl_2:NH_3\text{-}N$ weight ratio of $7.6:1\text{ to }10:1$), the breakpoint is reached. Any additional chlorine added beyond Point D exists $100\%$ as Free Available Chlorine ($HOCl + OCl^-$), increasing on a direct $1:1$ ratio with added dose.

$$\text{Total Chlorine Residual} = \text{Free Chlorine Residual} + \text{Combined Chlorine Residual}$$ $$\text{Chlorine Demand} = \text{Chlorine Dose} - \text{Total Chlorine Residual}$$

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CT Calculations & Pathogen Inactivation Standards

Under the EPA Surface Water Treatment Rule (SWTR), disinfection adequacy is quantified by the $CT$ Concept:

$$CT_{\text{calc}} = C \times T_{10}$$

Where:

• $C$ = Free chlorine (or other disinfectant) residual concentration measured at the contact chamber effluent in $\text{mg/L}$.
• $T_{10}$ = The effective contact time in minutes during which exactly $90\%$ of the water remains in the basin at peak hourly flow ($Q_{\text{peak}}$), accounting for hydraulic short-circuiting.

Basin Baffling Factors ($BF$) & $T_{10}$ Determination

$$T_{10} = \theta \times BF = \left(\frac{\text{Basin Volume (gallons)}}{\text{Peak Hourly Flow (gpm)}}\right) \times BF$$

SWTR Log-Inactivation Compliance

Surface water systems must achieve total multi-barrier pathogen reduction credits:

• Giardia lamblia: $3.0\text{-log (}99.9\%\text{)}$ removal/inactivation.
• Viruses: $4.0\text{-log (}99.99\%\text{)}$ removal/inactivation.
• Cryptosporidium: $2.0\text{-log to }5.5\text{-log}$ reduction under LT2ESWTR.

In a conventional filtration plant, regulatory rules award $2.5\text{-log}$ Giardia credit and $2.0\text{-log}$ Virus credit for coagulation, sedimentation, and filtration. The remaining $0.5\text{-log}$ Giardia and $2.0\text{-log}$ Virus inactivation must be achieved through chemical disinfection $CT$.

$$\text{Inactivation Ratio (IR)} = \frac{CT_{\text{calculated}}}{CT_{\text{required (from EPA Tables)}}} \ge 1.00$$

Environmental Sensitivity: Required $CT$ values from EPA lookup tables increase dramatically when water temperature drops (cold water slows disinfection kinetics) and when pH rises (shifting $HOCl$ to $OCl^-$).

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Disinfection Byproducts (DBPs) & Stage 2 D/DBP Compliance

Disinfection Byproducts (DBPs) form when chemical disinfectants (especially free chlorine) react with Natural Organic Matter (NOM precursors: humic and fulvic acids) and bromide ($Br^-$).

   [Natural Organic Matter (TOC)] + [Free Chlorine (HOCl)] ===> [TTHM (80 ug/L)] + [HAA5 (60 ug/L)]

Primary Regulated DBP Classes & Maximum Contaminant Levels (MCLs)

1. Total Trihalomethanes (TTHM): $\text{MCL} = \mathbf{0.080\text{ mg/L}} \text{ (}80\,\mu\text{g/L)}$
   • Sum of: Chloroform ($CHCl_3$), Bromoform ($CHBr_3$), Bromodichloromethane ($CHBrCl_2$), Dibromochloromethane ($CHBr_2Cl$).
2. Five Haloacetic Acids (HAA5): $\text{MCL} = \mathbf{0.060\text{ mg/L}} \text{ (}60\,\mu\text{g/L)}$
   • Sum of: Monochloroacetic, Dichloroacetic, Trichloroacetic, Monobromoacetic, Dibromoacetic acids.
3. Bromate ($BrO_3^-$): $\text{MCL} = \mathbf{0.010\text{ mg/L}} \text{ (}10\,\mu\text{g/L)}$ (Formed when ozone reacts with raw water bromide $Br^-$).
4. Chlorite ($ClO_2^-$): $\text{MCL} = \mathbf{1.0\text{ mg/L}}$ (Byproduct of chlorine dioxide).
5. Maximum Residual Disinfectant Levels (MRDL): Free Chlorine = $4.0\text{ mg/L}$; Chloramines = $4.0\text{ mg/L}$; Chlorine Dioxide = $0.8\text{ mg/L}$.

Stage 1 vs. Stage 2 D/DBP Rule Compliance

• Stage 1 D/DBP Rule: Evaluated compliance based on a system-wide Running Annual Average (RAA) across all sampling points combined, which masked localized distribution hotspots.
• Stage 2 D/DBP Rule: Mandates compliance based on a Locational Running Annual Average (LRAA) calculated quarterly at each individual monitoring site. Every single sampling site must independently maintain an $\text{LRAA} \le \text{MCL}$.
• Operational Evaluation Level (OEL): An early warning metric calculated quarterly to identify impending exceedances before a violation occurs:

$$\text{OEL} = \frac{\text{Quarter } 1 + \text{Quarter } 2 + 2 \times (\text{Current Quarter } 3)}{4} > \text{MCL}$$

If the calculated $\text{OEL} > \text{MCL}$, the utility must submit an Operational Evaluation Report to VDH within 90 days identifying root causes (water age, storage tank turnover, precursor removal efficiency).

EPA Enhanced Coagulation Precursor Removal Matrix

Utilities with surface water sources must practice Enhanced Coagulation to achieve mandatory Step 1 Total Organic Carbon (TOC) removal percentages before chlorination: