7.3 Alternative Disinfectants: Ozone, Chlorine Dioxide & Ultraviolet

7.3 Alternative Disinfectants: Ozone, Chlorine Dioxide & Ultraviolet

While chlorination and chloramination remain the workhorses of drinking water treatment, the regulatory promulgation of the Long Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) and the Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 D/DBPR) revealed critical vulnerabilities in chlorine-based disinfection. Specifically, free chlorine is notoriously ineffective against Cryptosporidium parvum oocysts at practical municipal dosages, and excessive chlorine dosing in surface waters rich in natural organic matter generates elevated levels of carcinogenic trihalomethanes and haloacetic acids. To overcome these limitations, modern water utilities deploy three major alternative disinfection technologies: Ozone ($O_3$), Chlorine Dioxide ($ClO_2$), and Ultraviolet Light (UV).

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Ozone Disinfection ($O_3$)

Ozone is a bluish, highly unstable gas comprising three oxygen atoms. It is one of the most powerful chemical oxidants and primary disinfectants utilized in water treatment, possessing an oxidation-reduction potential of 2.07 volts (approximately 1.5 times the oxidizing potential of hypochlorous acid).

                                    [ Ozone Generation and Contact Flowsheet ]

     Feed Gas Source                   Corona Discharge Generator              Baffled Contact Basin
   [ Ambient Air or O2 ] ---> [ Desiccant Air Dryer ] ---> [ High-Voltage Arc ] ---> [ Deep Contactor (18-22 ft) ] ---> Filtered Water
                              (Dew Point < -60°C)          (6,000 to 20,000 V)       * Ceramic Porous Diffusers
                                                                                     * 10 to 15 min detention
                                                                                                |
                                                                                                v Off-Gas
                                                                                   [ Ozone Destruct Unit ] ---> Safe O2 Discharge
                                                                                   (Catalytic MnO2 or >300°C)

Generation Chemistry and Equipment Design

Because ozone is thermodynamically unstable, it cannot be packaged, compressed, or transported in cylinders; it must be generated on-site and introduced into the process immediately. Ozone is produced by passing dried air or pure oxygen feed gas through a corona discharge generator:

$3O_2 + \text{Electrical Energy} \rightarrow 2O_3$

• Corona Discharge Mechanism: Feed gas flows through a narrow discharge gap between two high-voltage electrodes separated by a dielectric barrier (borosilicate glass or ceramic). An alternating current electrical potential (6,000 to 20,000 volts at medium to high frequency) discharges across the gap, splitting molecular oxygen ($O_2$) into free oxygen radicals that recombine with adjacent oxygen molecules to form ozone ($O_3$).
• Crucial Feed Gas Dew Point Requirement: When using ambient air as the feed gas, the air must pass through desiccant dryers to achieve a dew point of $-60^\circ\text{C}$ ($-76^\circ\text{F}$) or lower. If moisture enters the corona discharge chamber, the plasma arc causes water vapor and nitrogen gas to react, synthesizing nitric acid ($HNO_3$): $\text{Moisture} + N_2 + O_2 \xrightarrow{\text{Electrical Arc}} 2HNO_3$ Nitric acid rapidly corrodes dielectric tubes, stainless steel piping, and electrodes, resulting in catastrophic generator failure.
• Contactor Design: Ozone gas is injected into the bottom of deep (18 to 22 feet), covered, multi-stage concrete contact basins through fine-bubble ceramic porous diffusers. The deep hydraulic water column maximizes gas transfer efficiency (>90% dissolution).
• Ozone Off-Gas Destruct System: Ozone is a severe respiratory toxin and pulmonary irritant. The Occupational Safety and Health Administration (OSHA) Permissible Exposure Limit (PEL) for ozone is 0.1 ppm in breathing air. Residual ozone gas collecting in the contactor headspace cannot be vented directly to the atmosphere; it must be drawn under vacuum through an ozone destruct unit. The destruct unit utilizes a metal oxide catalyst (manganese dioxide/copper oxide) or high-temperature thermal heating (>300°C / 572°F) to instantly convert residual $O_3$ back into harmless oxygen gas ($O_2$).

Pathogen Inactivation and Regulated Byproducts

• Pathogen Efficacy: Ozone provides exceptional primary disinfection against Cryptosporidium oocysts, Giardia cysts, viruses, and bacteria at very low contact times ($CT$). For example, achieving 2-log (99%) Cryptosporidium inactivation at 15°C requires an ozone $CT$ of approximately 6.0 mg·min/L, whereas free chlorine would require an unachievable $CT$ exceeding 1,000 to 3,000 mg·min/L.
• Regulated DBP (Bromate): If raw source water contains naturally occurring bromide ions ($Br^-$), ozone oxidizes bromide into bromate ($BrO_3^-$), a potent human carcinogen: $Br^- + O_3 \rightarrow OBr^- \xrightarrow{2O_3} BrO_3^- + 2O_2$ Under the Stage 1 and Stage 2 D/DBPR, bromate is strictly regulated with a Maximum Contaminant Level (MCL) of $0.010\text{ mg/L}$ ($10,\mu\text{g/L}$). Plants treating bromide-bearing waters must implement bromate control strategies, including lowering contactor pH to 6.0–6.5, dosing ammonia to form non-reactive bromamines, or optimizing ozone dose.
• Residual Limitation: Ozone decomposes rapidly in water, exhibiting an operational half-life of only 10 to 30 minutes. It leaves zero persistent secondary residual in the distribution system. Downstream chemical addition of free chlorine or chloramines is mandatory to maintain distribution sanitary integrity.

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Chlorine Dioxide Disinfection ($ClO_2$)

Chlorine dioxide is a stable dissolved neutral gas that functions as a selective, high-potency chemical oxidant. Unlike chlorine gas, chlorine dioxide does not hydrolyze in water; it exists entirely as a dissolved, uncharged gas molecule ($ClO_2$).

Generation Chemistry and Operational Safety

Chlorine dioxide gas is unstable and violently explosive in air at concentrations exceeding 10% by volume. Therefore, it cannot be shipped or stored as a compressed gas. It must be synthesized on-site as an aqueous solution (1,000 to 3,000 mg/L) and immediately injected into the water flow. The dominant municipal generation reactions utilize sodium chlorite ($NaClO_2$):

1. Two-Chemical System (Chlorite + Chlorine Gas): $2NaClO_2 + Cl_2(g) \rightarrow 2ClO_2(aq) + 2NaCl$
2. Three-Chemical Acid System (Chlorite + Hypochlorite + Hydrochloric Acid): $2NaClO_2 + NaOCl + 2HCl \rightarrow 2ClO_2 + 3NaCl + H_2O$
3. Acid-Chlorite System (Chlorite + Hydrochloric Acid): $5NaClO_2 + 4HCl \rightarrow 4ClO_2 + 5NaCl + 2H_2O$

Generator Efficiency Requirement: Municipal generation equipment must achieve a chemical conversion efficiency of $>95%$. Operating below 95% efficiency allows unreacted chlorine and raw chlorite precursors to enter the finished water stream, triggering regulatory violations.

Chemical Selectivity and DBP Standards

• Absence of Chlorinated DBPs: Chlorine dioxide reacts with organic matter predominantly through one-electron oxidation transfers, rather than electrophilic substitution or addition. As a result, $ClO_2$ does not form trihalomethanes (TTHMs) or haloacetic acids (HAA5s). Furthermore, it does not react with ammonia, avoiding chloramine formation.
• Inorganic DBPs (Chlorite and Chlorate): The primary limitation of chlorine dioxide is the formation of inorganic oxyhalide byproducts: chlorite ($ClO_2^-$) and chlorate ($ClO_3^-$). Chlorite is formed when $ClO_2$ accepts an electron during oxidation reactions. Under the Stage 1/2 D/DBPR:
  • Chlorite MCL: $1.0\text{ mg/L}$ (daily monitoring at the plant entrance and three-sample distribution monitoring).
  • Chlorine Dioxide MRDL: $0.8\text{ mg/L}$. Because approximately 50% to 70% of dosed $ClO_2$ converts directly into chlorite ion, municipal plants are practically restricted to maximum applied $ClO_2$ doses of 1.2 to 1.4 mg/L to prevent chlorite MCL exceedances.

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Ultraviolet (UV) Light Disinfection

Ultraviolet disinfection is a physical, non-chemical process that inactivates microorganisms through photochemical alteration of cellular nucleic acids.

                                [ Photochemical Thymine Dimerization ]

       Normal Microbial DNA Strand                           UV-C Irradiated DNA Strand (254 nm)
       ... A --- T --- T --- G ...                           ... A --- [ T == T ] --- G ...
           |     |     |     |      +   UV-C Photons   --->      |       |||             |
       ... T --- A --- A --- C ...                           ... T --- A --- A --- C ...
                                                               (Dimer Distorts Helix: Blocks Replication!)

Photochemical Mechanism: Thymine Dimer Formation

Microbial inactivation occurs in the UV-C germicidal spectrum, with peak DNA absorption occurring between 254 and 265 nanometers (nm). When UV-C photons penetrate a microbial cell wall, they are absorbed by the purine and pyrimidine bases of DNA and RNA. Specifically, UV photons break carbon-carbon double bonds in adjacent thymine bases on the DNA strand, fusing them into covalent cyclobutane thymine dimers:

• The formation of thymine dimers distorts the DNA double helix structure.
• When the organism attempts to replicate inside a human host, DNA polymerase cannot transcribe past the dimer lesion.
• The pathogen is rendered reproductively sterile and non-infectious, eliminating its ability to cause waterborne disease.

Pathogen Inactivation: The Cryptosporidium Revolution vs. Adenovirus Vulnerability

• Cryptosporidium and Giardia: Protozoan cysts and oocysts possess large genomic targets and minimal DNA repair capabilities against UV. Under the LT2ESWTR Disinfection Guidance Manual, a very low UV dose of $3.0\text{ to }4.0\text{ mJ/cm}^2$ achieves 2-log (99%) inactivation of Cryptosporidium parvum, and $12\text{ mJ/cm}^2$ achieves 3-log (99.9%) inactivation. UV completely conquered the Cryptosporidium challenge that plagued conventional chlorination.
• The Adenovirus Blind Spot: In stark contrast, enteric adenoviruses (double-stranded DNA viruses responsible for childhood respiratory and gastrointestinal illness) possess dense structural protein capsids and robust host-cell DNA repair mechanisms. Under the LT2ESWTR, achieving 4-log (99.99%) inactivation of adenovirus requires an enormous UV dose of $186\text{ mJ/cm}^2$.
• The Multi-Barrier Strategy: Because designing a municipal UV reactor for 186 mJ/cm² requires massive power consumption and oversized lamp banks, surface water utilities universally implement a multi-barrier approach: UV reactors are sized to achieve required Cryptosporidium and Giardia credits (10 to 40 mJ/cm²), while downstream free chlorination in the clearwell easily satisfies regulatory 4-log virus inactivation CT requirements.

UV Reactor Architecture and Lamp Technology

• Low-Pressure Low-Output (LPLO): Emits monochromatic light at 253.7 nm. Operates at low lamp temperatures (40°C–60°C). High electrical efficiency, but low power output; suited for small plants (<1 MGD).
• Low-Pressure High-Output (LPHO): Emits monochromatic light at 253.7 nm with 2 to 4 times the UV intensity of LPLO lamps. High electrical efficiency and compact footprint; the modern standard for municipal water treatment.
• Medium-Pressure (MP): Operates at extreme internal mercury pressures and high operating temperatures (600°C–900°C), emitting broad polychromatic radiation across 200 to 320 nm. High power density requires fewer lamps, but consumes significantly more electrical energy per million gallons treated.
• Quartz Sleeves and Automatic Wipers: Mercury lamps are housed in transparent high-purity quartz sleeves that isolate the hot electrical lamps from water while transmitting UV-C light. Dissolved calcium, iron, and manganese precipitate onto the hot outer sleeve surface, creating scale that blinds UV transmission. Modern reactors utilize automatic mechanical wiper rings (equipped with elastomeric wipers and periodic citric or phosphoric acid washes) that clean the quartz sleeves every few hours without interrupting treatment flow.

Key UV Operating Parameters

• UV Transmittance (UVT, %): The percentage of 254 nm light that penetrates a 1.0-cm path length of water. Clean drinking water typically exhibits 85% to 95% UVT. Dissolved organic matter, natural tannins, iron, and turbidity absorb UV photons, depressing UVT and requiring the reactor to increase lamp power to maintain compliance.
• UV Dose (Fluence): The total UV energy delivered per unit area, expressed in millijoules per square centimeter ($ ext{mJ/cm}^2$) or Joules per square meter ($ ext{J/m}^2$), where $1\text{ mJ/cm}^2 = 10\text{ J/m}^2$: $\text{UV Dose} = \text{UV Intensity } (I, \text{mW/cm}^2) \times \text{Exposure Time } (t, \text{seconds})$
• Zero Residual: UV irradiation adds no chemicals and leaves zero secondary residual in finished water. Downstream chemical chlorination or chloramination is mandatory.

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Master Disinfectant Comparative Matrix

Table 1: Multi-Disinfectant Performance and Regulatory Comparison

Table 2: Alternative Disinfectant Operating Parameters and Regulatory Thresholds