5.1 Chlorination & Chemical Disinfection

3.2 Chlorination & Chemical Disinfection

Disinfection is the selective destruction or inactivation of disease-causing pathogenic organisms (bacteria, viruses, and protozoan cysts) in drinking water. Under the Safe Drinking Water Act (SDWA), drinking water utilities must satisfy two primary disinfection objectives: primary disinfection to achieve required pathogen log-inactivation at the treatment plant, and secondary disinfection to maintain a persistent disinfectant residual throughout the distribution network to prevent microbial regrowth.

---

Chemical Disinfectants & Properties

Water utilities utilize several chemical disinfectants, each possessing distinct chemical properties, handling requirements, and operational characteristics.

---

Chlorine Aqueous Chemistry: $ ext{HOCl}$ vs. $ ext{OCl}^-$ and the Role of pH

When chlorine gas or hypochlorite salts are added to water, they undergo hydrolysis and ionization reactions that determine the disinfectant's germicidal power.

Hydrolysis Reaction

Chlorine gas reacts instantaneously with water to form hypochlorous acid ($ ext{HOCl}$) and hydrochloric acid ($ ext{HCl}$): $\text{Cl}_2 + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{H}^+ + \text{Cl}^-$

Sodium hypochlorite hydrolyzes to yield hypochlorous acid and sodium hydroxide: $\text{NaOCl} + \text{H}_2\text{O} \rightleftharpoons \text{HOCl} + \text{Na}^+ + \text{OH}^-$

Ionization Equilibrium & Germicidal Potency

Hypochlorous acid ($ ext{HOCl}$) is a weak acid that partially dissociates into hydrogen ions ($ ext{H}^+$) and hypochlorite ions ($ ext{OCl}^-$): $\text{HOCl} \rightleftharpoons \text{H}^+ + \text{OCl}^- \quad (pK_a \approx 7.5 \text{ at } 25^\circ\text{C})$

Critical Exam Fact: Hypochlorous acid ($ ext{HOCl}$) is 80 to 100 times more effective at destroying bacteria and inactivating viruses than the hypochlorite ion ($ ext{OCl}^-$) because $ ext{HOCl}$ is electrically neutral and easily penetrates negatively charged cell walls.

Because the dissociation equilibrium is governed strictly by water pH, maintaining optimal pH is critical:

• At pH 6.0: $>97%$ of free chlorine exists as highly active $ ext{HOCl}$.
• At pH 7.5: Exactly $50%$ exists as $ ext{HOCl}$ and $50%$ as $ ext{OCl}^-$.
• At pH 8.5: $<10%$ exists as $ ext{HOCl}$, requiring significantly higher doses or longer contact times to achieve equivalent disinfection.

$\text{Free Available Chlorine (FAC)} = [\text{HOCl}] + [\text{OCl}^-]$

---

The Breakpoint Chlorination Curve

When chlorine is added to raw water containing inorganic reducing agents (iron, manganese, hydrogen sulfide, nitrites) and organic nitrogen/ammonia compounds, it reacts in predictable stages defined by the breakpoint chlorination curve.

The Four Zones of Breakpoint Chlorination

1. Zone 1 (Initial Chlorine Demand): Chlorine reacts with inorganic reducing compounds ($\text{Fe}^{2+}, \text{Mn}^{2+}, \text{H}_2\text{S}, \text{NO}_2^-$). All added chlorine is consumed; zero residual is measured.
2. Zone 2 (Combined Chlorine Formation): Chlorine reacts with free ammonia ($\text{NH}_3$) and organic nitrogen to form chloramines:
   • Monochloramine ($\text{NH}_2\text{Cl}$): Formed at $\text{Cl}_2:\text{NH}_3\text{-N}$ weight ratios up to 5:1.
   • Dichloramine ($\text{NHCl}_2$): Formed as ratio increases above 5:1 (causes swimming pool odors).
   • Total chlorine residual increases to a peak value comprised entirely of combined chlorine residual.
3. Zone 3 (Chloramine Destruction / Dip): As chlorine dosage increases beyond the peak (ratios between 5:1 and 10:1), excess chlorine oxidizes chloramines into nitrogen gas ($\text{N}_2$), nitrous oxide ($\text{N}_2\text{O}$), and nitrogen trichloride ($\text{NCl}_3$). Total chlorine residual decreases to the breakpoint.
4. Zone 4 (Free Available Chlorine Residual): Past the breakpoint, all ammonia and combined compounds have been destroyed. Any additional chlorine added results in a direct 1:1 linear increase in Free Available Chlorine (FAC) residual.

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

---

Chloramination Practice & Operational Ratios

To minimize disinfection byproducts in extended distribution systems, utilities feed ammonia in conjunction with chlorine to form monochloramine ($\text{NH}_2\text{Cl}$).

• Target Weight Ratio: The ideal weight ratio is 4.5:1 to 5.0:1 ($\text{Cl}_2$ to $\text{NH}_3\text{-N}$).
• Uncombined Ammonia Hazards: If the ratio drops below 4:1, excess free ammonia enters the distribution system, triggering nitrification (microbial conversion of ammonia to nitrite $\text{NO}_2^-$ and nitrate $\text{NO}_3^-$ by Nitrosomonas and Nitrobacter bacteria), which consumes disinfectant residual and degrades water quality.

---

Gaseous Chlorine Safety & Emergency Protocols

Chlorine gas ($\text{Cl}_2$) is extremely hazardous. OSHA, EPA Risk Management Program (RMP), and SC DES enforce strict safety standards for chlorine gas facilities.

Physical Properties & Container Storage

• Chlorine gas is yellow-green, $2.5$ times heavier than air (settles in low areas, basements, and pits), and toxic at low concentrations ($10\text{ ppm}$ IDLH - Immediately Dangerous to Life or Health).
• 150-lb Cylinders: Stored upright and secured with safety chains. Valves feature a single fusible plug designed to melt at 158°F to 165°F (70°C to 74°C) to prevent catastrophic cylinder rupture during a fire.
• 1-Ton Containers: Stored horizontally on trunnions. Equipped with two valves (top valve delivers gas, bottom valve delivers liquid chlorine) and six fusible plugs (three on each concave end hood).

Leak Detection & Repair Kits

• Leak Testing: Commercial $10%$ aqua ammonia solution ($\text{NH}_4\text{OH}$) is squirted near suspected leaks. Ammonia vapor reacts with escaping chlorine gas to produce a dense white smoke of ammonium chloride ($\text{NH}_4\text{Cl}$): $\text{NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl} \quad \text{(Dense White Smoke)}$

Exam Warning: NEVER use water on a chlorine gas leak. Water reacts with gaseous chlorine to form highly corrosive hydrochloric and hypochlorous acids, rapidly enlarging the leak opening.

• Emergency Repair Kits (Chlorine Institute):
  • Kit A: Designed for 100-lb and 150-lb gas cylinders.
  • Kit B: Designed for 1-ton containers.
  • Kit C: Designed for bulk tank cars and tank trucks.
• Respiratory Protection: A positive-pressure Self-Contained Breathing Apparatus (SCBA) is mandatory when entering a chlorine room during a leak alarm or when changing gas cylinders.