5.3 Alternative Disinfection: Ozone, UV & Chlorine Dioxide

Alternative Disinfection: Ozone, UV & Chlorine Dioxide

While chlorination remains the dominant disinfection technology in North America, stringent regulatory limits on halogenated Disinfection Byproducts (DBPs)—specifically Total Trihalomethanes (TTHMs) and Five Haloacetic Acids (HAA5) under the EPA Stage 1 and Stage 2 Disinfectants and Disinfection Byproducts Rules—alongside the discovery of chlorine-resistant pathogens like Cryptosporidium parvum, have driven the widespread adoption of advanced alternative disinfectants.

Water and wastewater treatment facilities increasingly deploy Ozonation ($O_3$), Ultraviolet (UV) Disinfection, and Chlorine Dioxide ($ClO_2$) as primary disinfectants and advanced oxidants.

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1. Ozonation ($O_3$) Systems & Process Chemistry

Ozone ($O_3$) is an unstable, pale blue gas with a sharp, pungent odor. Possessing a standard oxidation potential of 2.07 V, ozone is one of the most powerful oxidants utilized in water treatment.

Ozone Generation Physics

Because ozone is thermodynamically unstable and rapidly decomposes into diatomic oxygen ($O_2$), it cannot be shipped or stored and must be generated on-site continuously:

1. Feed Gas Preparation: Ozone is produced from either clean, ambient air compressed and dried to a severe dew point ($< -60^\circ\text{C} \text{ or } -76^\circ\text{F}$) to prevent corrosive nitric acid ($HNO_3$) formation, or high-purity Liquid Oxygen (LOX, >93% purity).
2. Dielectric Barrier Corona Discharge: The prepared oxygen gas passes through a narrow discharge gap between two high-voltage electrodes separated by a dielectric barrier (glass or ceramic). A high-voltage AC electric field (6,000 to 20,000 volts at frequencies up to several kilohertz) dissociates diatomic oxygen molecules ($O_2$) into oxygen radicals, which collide with intact oxygen molecules to form ozone:

$\text{O}_2 + e^- \longrightarrow 2\text{O}^\bullet$

$\text{O}^\bullet + \text{O}_2 + M \longrightarrow \text{O}_3 + M$

• Air-fed generators produce 1% to 3% ozone by weight.
• LOX-fed systems achieve 8% to 14% ozone by weight with significantly lower energy consumption per pound of ozone generated.

┌──────────────────────────────────────────────────────────────────────────┐
│                     Ozone Contactor & Destruct Train                     │
├──────────────────────────────────────────────────────────────────────────┤
│  [LOX / Air Prep] ──► [Corona Discharge Generator]                      │
│                                │                                         │
│                                ▼ Ozone Gas Injection                     │
│  [Raw/Clarified Water] ──► [Multi-Stage Deep Contactor (18-22 ft)]       │
│                                │                                         │
│       ┌────────────────────────┴────────────────────────┐                │
│       ▼ Off-Gas (Contactor Headspace)                   ▼ Ozone Residual │
│  [Demister & Preheater]                    [Ozonated Water to Biological │
│       │                                     Activated Carbon / Filters]  │
│       ▼                                                                  │
│  [Thermal (300°C) or Catalytic (MnO₂) Destruct Unit]                     │
│       │                                                                  │
│       ▼ Safe Ambient Exhaust (< 0.1 ppm O₃)                              │
└──────────────────────────────────────────────────────────────────────────┘

Ozone Contact Basins & Off-Gas Destruction

• Contact Chambers: Ozone contactors are deep concrete structures (typically 18 to 22 feet deep to maximize gas dissolution) featuring multiple over-under baffle chambers. Ozone is transferred into the water column via fine-bubble porous ceramic diffusers or pressurized side-stream venturi injection systems.
• Off-Gas Destruction: Ozone is toxic to human respiratory systems, with an OSHA Permissible Exposure Limit (PEL) of 0.1 ppm (8-hr TWA) and an Immediately Dangerous to Life or Health (IDLH) threshold of 5.0 ppm. Undissolved ozone gas collecting in the contactor headspace must be routed through ozone destruct units—using high-temperature thermal destruction (300°C to 350°C) or catalytic manganese dioxide ($MnO_2$) beds—to convert residual $O_3$ back into diatomic oxygen ($O_2$) before atmospheric discharge.

Ozone Applications & Disinfection Byproduct Control

• Pathogen Inactivation: Ozone is hundreds of times more effective than free chlorine against Giardia cysts and Cryptosporidium oocysts, achieving 2-log to 3-log Cryptosporidium inactivation at modest $CT$ values ($CT \approx 5\text{ to } 15\text{ mg}\cdot\text{min/L}$ depending on temperature).
• Oxidation of Micropollutants: Ozone rapidly cleaves aromatic rings and carbon-carbon double bonds, destroying taste-and-odor compounds (2-Methylisoborneol [MIB] and geosmin), cyanotoxins (microcystins), iron, manganese, color, and pharmaceutical residues.
• Bromate ($BrO_3^-$) Formation & Mitigation: When raw water contains natural bromide ions ($Br^-$), ozone oxidizes bromide to hypobromous acid ($HOBr/OBr^-$) and subsequently to bromate ($BrO_3^-$), a potent human carcinogen regulated with a strict MCL of 10 µg/L (0.010 mg/L):

$\text{Br}^- + \text{O}_3 \longrightarrow \text{OBr}^- + \text{O}_2$

$\text{OBr}^- + 2\text{O}_3 \longrightarrow \text{BrO}_3^- + 2\text{O}_2$

[!WARNING] Bromate Control Strategies:

1. Acid Addition (pH Depression): Lowering contactor water pH to 6.0–6.5 shifts the equilibrium from hypobromite ion ($OBr^-$) to hypobromous acid ($HOBr$), suppressing the rate of bromate formation.
2. Ammonia Addition: Adding low doses of ammonia ($0.1\text{–}0.3\text{ mg/L}$) binds with hypobromous acid to form bromamines ($NH_2Br$), sequestering bromine away from the bromate pathway.
3. Hydrogen Peroxide Addition: $H_2O_2$ reduces hypobromite back to bromide ($Br^-$).

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2. Ultraviolet (UV) Disinfection Mechanics

Ultraviolet (UV) disinfection is a physical, chemical-free process that inactivates pathogens without creating regulated halogenated DBPs or adding chemical mass to the water stream.

┌──────────────────────────────────────────────────────────────────────────┐
│                     Ultraviolet Germicidal Mechanism                     │
├──────────────────────────────────────────────────────────────────────────┤
│  UVC Photons (~254 nm) Strike Microbial Cell                             │
│                      │                                                   │
│                      ▼                                                   │
│  Absorbed by DNA / RNA Pyrimidine Bases (Thymine / Uracil)               │
│                      │                                                   │
│                      ▼                                                   │
│  Forms Cyclobutane Pyrimidine Dimers (Thymine Dimers C=C Bond)          │
│                      │                                                   │
│                      ▼                                                   │
│  Blocks DNA Polymerase Transcription & Cell Replication                  │
│                      │                                                   │
│                      ▼                                                   │
│  Microorganism Rendered Completely Non-Infectious (Sterilized)           │
└──────────────────────────────────────────────────────────────────────────┘

Photochemical Germicidal Mechanism

UV light occupies the electromagnetic spectrum between 100 nm and 400 nm. Germicidal UV (UVC band, specifically 200 to 280 nm, with peak absorption at 254 to 265 nm) is absorbed by microbial nucleic acids (DNA and RNA). The photon energy induces adjacent thymine bases (in DNA) or uracil bases (in RNA) to form covalent carbon-carbon bonds, creating cyclobutane pyrimidine dimers (thymine dimers). These dimers distort the DNA double helix, preventing DNA polymerase from transcribing or replicating the genetic code. The pathogen is not physically ruptured or lysed, but is permanently sterilized and incapable of causing infection.

UV Lamp Technologies

Operating Parameters & Sizing Formulas

1. UV Dose (Fluence): $\text{UV Dose } (\text{mJ/cm}^2) = \text{UV Intensity } (\text{mW/cm}^2) \times \text{Exposure Time } (\text{seconds})$ (Note: $1\text{ mJ/cm}^2 = 1\text{ mWs/cm}^2 = 10\text{ J/m}^2$)
2. UV Transmittance (UVT %): The percentage of light at 254 nm that passes through a 1-cm quartz water cell relative to pure deionized water. UVT is reduced by dissolved organic carbon (DOC), iron ($Fe^{2+}/Fe^{3+}$), manganese, and suspended turbidity particles. Typical surface waters have a UVT of 85% to 95%; tertiary recycled water ranges from 65% to 80%.
3. Quartz Sleeve Fouling & Cleaning: Submerged mercury arc lamps are encased within high-purity fused quartz sleeves. Mineral foulants (calcium, magnesium, iron) and organic biofilms precipitate onto the sleeve surface, reducing UV output. Automated mechanical wipers equipped with Teflon rings, paired with periodic chemical flushes using citric acid or phosphoric acid, clean the sleeves without taking the reactor offline.
4. Bioassay Validation: Per the EPA UV Disinfection Guidance Manual (UVDGM), UV reactors cannot be rated solely by theoretical modeling. Systems must undergo physical biodosimetry validation testing using non-pathogenic challenge microorganisms (such as MS2 bacteriophage or Bacillus subtilis spores) across various flow rates and UVT conditions to determine the Reduction Equivalent Dose (RED).

[!NOTE] UV Pathogen Vulnerability Comparison: Notice that protozoan parasites (Cryptosporidium and Giardia) are extraordinarily sensitive to UV irradiation, requiring only 12 mJ/cm² for 3-log inactivation. In contrast, enteric viruses (such as Adenovirus) possess double-stranded DNA or compact protein capsids that require 186 mJ/cm² for 4-log inactivation. Consequently, UV is typically paired with downstream free chlorine or chloramines in multi-barrier systems.

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3. Chlorine Dioxide ($ClO_2$) Systems

Chlorine dioxide ($ClO_2$) is a synthetic dissolved gas oxidant that exists as a true dissolved gas in water without hydrolyzing into hypochlorous acid or hypochlorite ions.

Oxidation Chemistry & Generation

• Oxidizing Capacity: Chlorine dioxide has an oxidation state of $+4$. In water, it undergoes a 5-electron reduction to chloride ($Cl^-$): $\text{ClO}_2 + 5e^- + 4\text{H}^+ \longrightarrow \text{Cl}^- + 2\text{H}_2\text{O}$ Because of this 5-electron transfer, $ClO_2$ has 2.63 times the oxidizing capacity of chlorine on an equivalent weight basis.
• On-Site Generation: Gaseous chlorine dioxide is unstable and explosive at air concentrations above 10% by volume. It is generated on site as an aqueous solution using specialized vacuum-driven reactors:
  1. Chlorine Gas - Sodium Chlorite Method: $2\text{NaClO}_2 + \text{Cl}_2 \longrightarrow 2\text{ClO}_2 + 2\text{NaCl}$
  2. Hydrochloric Acid - Sodium Chlorite Method (Acid-Chlorite): $5\text{NaClO}_2 + 4\text{HCl} \longrightarrow 4\text{ClO}_2 + 5\text{NaCl} + 2\text{H}_2\text{O}$

Operational Benefits & Byproduct Regulations

• No Halogenated DBP Formation: Chlorine dioxide does not react with humic and fulvic organic substances via electrophilic substitution, preventing the formation of trihalomethanes (TTHM) and haloacetic acids (HAA5).
• Broad pH Independence: Unlike free chlorine, the disinfection efficacy of $ClO_2$ remains largely stable across a broad pH range of 6.0 to 10.0.
• Inorganic DBPs & Compliance Limits:
  1. Chlorite ($ClO_2^-$): In water, chlorine dioxide is partially reduced to inorganic chlorite ($ClO_2 + e^- \rightarrow ClO_2^-$). Chlorite is a regulated DBP with an enforceable MCL of 1.0 mg/L.
  2. Chlorine Dioxide Residual: Regulated with a Maximum Residual Disinfectant Level (MRDL) of 0.8 mg/L.
  3. Chlorite Suppression: Utilities feed ferrous iron ($Fe^{2+}$) or sulfur-based reducing agents (sodium sulfite, sodium bisulfite) to reduce chlorite back to harmless chloride ($Cl^-$) before finished water enters the distribution system.