3.4 Alternative Disinfection: Chloramination, Ultraviolet (UV) Light & Chlorine Dioxide

Alternative Disinfection: Chloramination, Ultraviolet (UV) Light & Chlorine Dioxide

To balance strict microbial pathogen inactivation with rigorous Disinfection Byproduct (DBP) regulations, water utilities frequently implement alternative disinfection technologies. The three most common alternatives are chloramination, ultraviolet (UV) irradiation, and chlorine dioxide.

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1. Chloramination (Secondary Disinfection)

Chloramination is the intentional formation of monochloramine ($\text{NH}_2\text{Cl}$) by combining chlorine with ammonia. It is widely utilized as a secondary disinfectant in extensive distribution networks.

Advantages and Limitations

• Advantages: Produces virtually no trihalomethanes (TTHMs) or haloacetic acids (HAA5s); maintains an exceptionally persistent, durable residual across high-water-age networks; penetrates microbial biofilms more effectively than free chlorine.
• Limitations: Weak oxidant for primary disinfection (requires massive CT values for Giardia and virus inactivation); toxic to kidney dialysis patients and aquatic life in aquariums; prone to biological nitrification in storage reservoirs.

Stoichiometric Feed Control: The 5:1 Cl₂:NH₃-N Ratio

To produce monochloramine while preventing nuisance species, utilities feed chlorine and ammonia at an exact weight ratio of 4.5:1 to 5:0 Cl₂ to NH₃-N (ammonia measured as nitrogen):

Cl2 : NH3-N Ratio < 4.0:1  --> Excess Unreacted Free Ammonia (Triggers Nitrification)
Cl2 : NH3-N Ratio = 4.5:1 - 5.0:1 --> Optimal Monochloramine Formation (Target Zone)
Cl2 : NH3-N Ratio > 5.5:1  --> Dichloramine Formation (Pungent Taste & Odor Complaints)
Cl2 : NH3-N Ratio >= 7.6:1 --> Breakpoint Chlorination (Oxidation of Chloramines)

Nitrification in Distribution Systems

When unreacted free ammonia is present in warm, low-flow distribution pipes or stratified storage tanks, naturally occurring ammonia-oxidizing bacteria (AOB) and nitrite-oxidizing bacteria (NOB) initiate a two-step biological oxidation process:

1. Ammonia to Nitrite (AOB): $2\text{NH}_3 + 3\text{O}_2 \xrightarrow{\text{AOB}} 2\text{NO}_2^- \text{ (Nitrite)} + 2\text{H}^+ + 2\text{H}_2\text{O}$
2. Nitrite to Nitrate (NOB): $2\text{NO}_2^- + \text{O}_2 \xrightarrow{\text{NOB}} 2\text{NO}_3^- \text{ (Nitrate)}$

Operational Warning Signs of Nitrification:

• Sharp drop in total chlorine residual.
• Increase in nitrite ($\text{NO}_2^-$) levels above 0.05 mg/L.
• Increase in free ammonia concentration.
• Decrease in pH and alkalinity.
• Increase in Heterotrophic Plate Counts (HPC).

Nitrification Control Tactics:

• Adjust chlorine-to-ammonia ratio back to 5:1 to eliminate excess free ammonia.
• Deep-cycle storage tanks to reduce water age (< 5–7 days).
• Conduct aggressive unidirectional main flushing.
• Implement a temporary "Free Chlorine Burn" (suspending ammonia feed and maintaining a free chlorine residual for 2 to 4 weeks to kill nitrifying biofilm).

2. Ultraviolet (UV) Light Disinfection

Ultraviolet disinfection utilizes electromagnetic radiation in the germicidal C-band (UV-C), specifically wavelengths between 200 and 300 nm, with peak germicidal efficiency occurring at 254 nm.

Mechanism of Pathogen Inactivation

Unlike chemical oxidants that rupture cell walls or denature proteins, UV light penetrates the microbial cell wall and is absorbed by the nucleic acids (DNA and RNA). The photon energy causes adjacent thymine bases (in DNA) or uracil bases (in RNA) to form covalent double bonds (thymine dimers). This cross-linking prevents the organism from unzipping its DNA for replication and transcription, rendering the pathogen non-infectious and harmless without chemical lysis.

Exceptional Protozoan Efficacy

Chlorine is notoriously ineffective against Cryptosporidium oocysts (requiring a prohibitive CT of > 7,000 mg·min/L). In stark contrast, UV achieves high-level protozoan inactivation at very low doses:

Note: Because Adenovirus is highly UV-resistant, EPA mandates a high UV dose of 186 mJ/cm² if UV is credited for 4-log virus inactivation, or utilities combine UV with a chemical disinfectant (free chlorine) to satisfy virus credits.

Fundamental UV Operating Parameters

• UV Dose (Fluence): Expressed in millijoules per square centimeter ($\text{mJ/cm}^2$) or milliwatt-seconds per square centimeter ($\text{mWs/cm}^2$): $\text{UV Dose } (\text{mJ/cm}^2) = \text{UV Intensity } (\text{mW/cm}^2) \times \text{Exposure Time (seconds)}$
• UV Transmittance (% UVT): The percentage of 254 nm light that penetrates a 1-cm path of water. High UVT (> 90–95%) ensures deep light penetration; low UVT (< 80%) caused by iron, manganese, humic color, or suspended solids severely limits disinfection.
• Lamp Types:
  • Low-Pressure Low-Intensity (LPLI) / Low-Pressure High-Output (LPHO): Monochromatic output at exactly 253.7 nm; high electrical efficiency; ideal for small-to-medium systems.
  • Medium-Pressure High-Output (MPHO): Polychromatic broad-spectrum output (200–300 nm); intense light output in a compact footprint; higher power draw.
• Quartz Sleeve Maintenance: Hardness scale and iron foul the protective quartz sleeves housing the UV lamps. Automated mechanical wipers utilizing citric acid or dilute phosphoric acid clean sleeves periodically without taking reactors offline.
• No Residual: UV provides zero chemical residual in treated water. Utilities must feed chlorine or chloramines downstream of UV reactors before water enters the distribution network.

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3. Chlorine Dioxide ($\text{ClO}_2$)

Chlorine dioxide is a neutral, synthetic gas that exists in water as a true dissolved gas without hydrolyzing into acids.

Generation Chemistry

Because chlorine dioxide gas is unstable and explosive under pressure, it cannot be compressed or shipped in cylinders; it must be generated on-site immediately prior to injection. The most common municipal generation method reacts liquid sodium chlorite ($\text{NaClO}_2$) with chlorine gas ($\text{Cl}_2$):

$2\text{NaClO}_2 + \text{Cl}_2 \longrightarrow 2\text{ClO}_2\uparrow + 2\text{NaCl}$

Alternative generators utilize sodium chlorite and hydrochloric acid ($\text{HCl}$) or sodium chlorate electrochemical systems.

Disinfection & Oxidation Characteristics

• Broad Oxidation Utility: Rapidly oxidizes soluble iron ($\text{Fe}^{2+}$) and manganese ($\text{Mn}^{2+}$), destroys phenolic taste/odor compounds, and degrades geosmin and MIB.
• pH Independence: Unlike chlorine, $\text{ClO}_2$ disinfection potency is stable and effective across a broad pH range of 4.0 to 10.0.
• No Halogenated DBPs: Chlorine dioxide does not react with humic substances to form TTHMs or HAA5s.

Regulatory Constraints & Byproduct Limits

• Maximum Residual Disinfectant Level (MRDL): $\mathbf{0.8\text{ mg/L}}$ for $\text{ClO}_2$.
• Chlorite ($\text{ClO}_2^-$) MCL: $\mathbf{1.0\text{ mg/L}}$. Chlorine dioxide reduces to chlorite ion in water. Utilities must monitor daily for chlorite at the entry point and collect monthly 3-sample distribution sets (near first customer, average residence, and max residence time).