5.2 Alternative Disinfectants: Chloramines, Chlorine Dioxide, Ozone & UV

5.2 Alternative Disinfectants: Chloramines, Chlorine Dioxide, Ozone & UV

While free chlorine remains the most common municipal disinfectant, stringent regulatory limits on disinfection byproducts (DBPs)—such as trihalomethanes and haloacetic acids—along with chlorine's relative ineffectiveness against protozoan pathogens like Cryptosporidium, have led many utilities to implement alternative disinfection technologies.

Water utilities frequently employ a multi-barrier disinfection strategy, combining a high-potency primary disinfectant (such as ozone, UV, or free chlorine) at the treatment plant to achieve rapid pathogen log-inactivation, followed by a persistent secondary disinfectant (such as chloramines or free chlorine) to safeguard the distribution network.

---

Chloramination: Chemistry, Dosing Ratios & Distribution Persistence

Chloramines are chemical compounds formed by intentionally reacting chlorine with ammonia in treated water. Chloramines serve almost exclusively as a secondary disinfectant in long, complex distribution systems.

The Chloramination Reaction & Dosing Ratios

Chlorine and ammonia are added under controlled conditions to favor monochloramine (NH2Cl):

NH3 + HOCl → NH2Cl + H2O

• Control target: Many utilities operate near 4.5:1 to 5:1 Cl2:NH3-N by weight, but this is a common operating range, not a universal setpoint. Feed strengths, pH, temperature, demand, contact sequence, distribution behavior, and the approved operating plan determine the target.
• Too little chlorine: Can leave excess free ammonia that supports nitrification and weakens residual control.
• Too much chlorine: Can move the reaction toward breakpoint and free-chlorine conditions; off-ratio operation may also increase taste-and-odor complaints.
• Operator method: Verify both chlorine and ammonia feeds, calculate the ratio on an ammonia-nitrogen basis, and trend monochloramine, free ammonia, nitrite, nitrate, pH, alkalinity, dissolved oxygen, temperature, and water age.

Advantages and Limitations of Chloramines

---

Distribution System Nitrification: Mechanics & Control

The greatest operational hazard in chloraminated distribution networks is nitrification—a two-step biological oxidation of excess unreacted free ammonia carried out by autotrophic nitrifying bacteria:

1. Step 1: Ammonia Oxidation (by Nitrosomonas bacteria): $2\text{NH}_3 + 3\text{O}_2 \xrightarrow{\text{Nitrosomonas}} 2\text{NO}_2^- \text{ (Nitrite)} + 2\text{H}^+ + 2\text{H}_2\text{O}$
2. Step 2: Nitrite Oxidation (by Nitrobacter bacteria): $2\text{NO}_2^- + \text{O}_2 \xrightarrow{\text{Nitrobacter}} 2\text{NO}_3^- \text{ (Nitrate)}$

Warning Signs of a Nitrification Episode

No single measurement proves nitrification. Operators look for a pattern: falling total-chlorine or monochloramine residual, rising nitrite or nitrate above the system baseline, decreasing dissolved oxygen, pH or alkalinity loss, warmer water, and increased biological activity. Each utility establishes action levels from its own history and approved nitrification-control plan.

Nitrification Mitigation Strategies

• Reduce water age through approved storage operation and targeted flushing.
• Verify chlorine and ammonia feed calibration and reduce uncontrolled free-ammonia carryover.
• Increase monitoring frequency and map the affected area before choosing a response.
• If the approved plan calls for a temporary free-chlorine conversion, coordinate dose, duration, DBP monitoring, public communication, sensitive-customer protection and return to chloramines with the primacy agency. A fixed annual frequency or duration is not universal.

---

Chlorine Dioxide (ClO₂): Generation, Oxidation & Chlorite Control

Chlorine dioxide ($\text{ClO}_2$) is a synthetic, dark reddish-yellow gas with a sharp, pungent odor. Unlike chlorine gas, chlorine dioxide does not hydrolyze in water; it dissolves purely as a dissolved neutral gas molecule, maintaining full oxidation strength across a wide pH range (pH 6.0 to 10.0).

┌───────────────────────────────────────────────────────────────┐
│               CHLORINE DIOXIDE ONSITE GENERATION              │
│           2 NaClO2 + Cl2 (g) ──► 2 ClO2 (aq) + 2 NaCl         │
│    Sodium Chlorite + Chlorine Gas ──► Chlorine Dioxide        │
└───────────────────────────────────────────────────────────────┘

Chemical Properties and Generation

Because concentrated chlorine dioxide gas is unstable and violently explosive at partial pressures above $10%\text{ in air}$ ($>76\text{ mm Hg}$), $\text{ClO}_2$ cannot be compressed, liquefied, or shipped in commercial cylinders. It must always be generated on-site at the water plant at the moment of application.

• Generation Method: Generated by reacting sodium chlorite solution ($\text{NaClO}_2$, $25%–31%$) with chlorine gas or hydrochloric acid ($\text{HCl}$) inside a specialized vacuum-driven generation chamber.
• Oxidation Characteristics: Highly selective oxidant. Extremely effective at oxidizing dissolved iron ($\text{Fe}^{2+}$) and manganese ($\text{Mn}^{2+}$), destroying phenolic taste and odor compounds, and neutralizing geosmin and 2-methylisoborneol (MIB) produced by cyanobacteria.
• DBP Advantage: Does not chlorinate organic matter. It does not form regulated trihalomethanes (TTHMs) or haloacetic acids (HAA5).

Regulatory Byproduct Limits & Monitoring

When chlorine dioxide oxidizes contaminants, it is reduced primarily to chlorite ($\text{ClO}_2^-$) and to a lesser extent chlorate ($\text{ClO}_3^-$):

$\text{ClO}_2 + e^- \longrightarrow \text{ClO}_2^- \quad (\text{Chlorite Ion Formation})$

• Chlorite Maximum Contaminant Level (MCL): $1.0\text{ mg/L}$. Chlorite induces hemolytic anemia and methemoglobinemia in sensitive populations.
• Maximum Residual Disinfectant Level (MRDL): $0.8\text{ mg/L}$ for $\text{ClO}_2$.
• Monitoring Rules: Operators must test for chlorine dioxide and chlorite daily at the plant point of entry (POE). If entry chlorite exceeds $1.0\text{ mg/L}$, the utility must collect a 3-sample distribution set (near first customer, middle, and maximum residence point) within 24 hours.

---

Ozone (O₃): Strong Oxidant, Corona Discharge & Bromate Formation

Ozone ($\text{O}_3$) is an allotrope of oxygen consisting of three oxygen atoms. It is one of the most powerful chemical oxidants known in water treatment (Oxidation Potential $E^\circ = 2.07\text{ V}$, compared to $1.36\text{ V}$ for chlorine gas).

                         OZONE GENERATION & OFF-GAS CYCLE
┌─────────────────┐       ┌─────────────────┐       ┌─────────────────┐
│  Clean, Dry Air │       │ High-Voltage    │       │ Ozone Contactor │
│  or Pure Oxygen │ ────► │ Corona Discharge│ ────► │ (Deep Bubble    │
│ (Dew Point <-60)│       │ Dielectric Cell │       │  Diffusion Tank)│
└─────────────────┘       └─────────────────┘       └────────┬────────┘
                                                             │ Off-Gas
                                                             ▼
                                                    ┌─────────────────┐
                                                    │ Thermal/Catalyt.│
                                                    │ Ozone Destructor│
                                                    │ (Converts O3->O2)│
                                                    └─────────────────┘

Generation and Contactor Architecture

• Site Generation via Corona Discharge: Ozone is unstable and is generated on site from conditioned dry air or oxygen using a high-voltage corona-discharge generator. The manufacturer and approved design establish feed-gas quality and electrical operating limits.
• Contactor and credit: Diffusers or injectors transfer ozone into a contactor. Pathogen inactivation credit depends on the approved contactor hydraulics, ozone residual profile, temperature and demonstrated operating conditions, not on one universal basin depth or CT claim.
• Off-gas and worker safety: Collected off-gas is routed through an approved thermal, catalytic or thermal-catalytic destruction system and ambient ozone is monitored. OSHA’s permissible exposure limit is 0.1 ppm as an 8-hour time-weighted average, not an absolute instantaneous ceiling; the facility alarm and response plan may use a more protective setpoint.

The Bromate Hazard

When ozone is applied to raw water containing naturally occurring dissolved bromide ($\text{Br}^-$), ozone oxidizes bromide to bromate ($\text{BrO}_3^-$), a potent human carcinogen:

$\text{Br}^- + \text{O}_3 \longrightarrow \text{OBr}^- \xrightarrow{\text{O}_3} \text{BrO}_3^- \quad (\text{Bromate Formation})$

• Bromate MCL: $0.010\text{ mg/L}$ ($10\ \mu\text{g/L}$).
• Bromate control: Dose, contact conditions, pH, ammonia addition and other controls can affect formation; the plant selects controls from source-water testing, treatment objectives and its approved process.
• No lasting distribution residual: Ozone decays rapidly and is not relied on to protect the distribution system. A system must maintain the secondary disinfectant required by its approved treatment and distribution plan.

---

Ultraviolet (UV) Disinfection: Photochemical Inactivation & Maintenance

Ultraviolet (UV) disinfection is a physical, non-chemical treatment process that exposes water to electromagnetic radiation in the UV-C spectrum (wavelengths $200–280\text{ nm}$).

Microbial DNA Strands:   ... — [ Thymine ] ════ [ Thymine ] — ...
                                      ▲          ▲
                                      │  UV-C    │
                                      │ (254 nm) │
                                      │          │
Irradiated DNA Strand:   ... — [ Thymine ─── Thymine ] — ...  (Thymine Dimer Formed)
                                (Replication Blocked -> Pathogen Inactivated)

Germicidal Mechanism: Thymine Dimerization

• Peak Germicidal Wavelength: Maximum absorption by microbial nucleic acids occurs at $254–260\text{ nm}$.
• Photochemical Disruption: UV photons absorbed by microbial DNA and RNA create pyrimidine photoproducts that disrupt replication and transcription. Cyclobutane thymine dimers are a familiar DNA example; RNA contains uracil rather than thymine.
• Loss of Infectivity: The nucleic-acid damage prevents successful replication while the organism may remain physically intact.
• Protozoan efficacy and validation: UV can be highly effective against chlorine-resistant protozoa, but regulatory log-inactivation credit comes from the validated reactor dose and the approved operating envelope rather than from a single generic dose.

Key Operational Parameters & Maintenance

1. UV Transmittance (%UVT): The percentage of 254-nm light transmitted through the stated path length. Lower UVT means less light reaches organisms; the validated operating limit, not a universal 80% cutoff, controls reactor operation.
2. Lamp Sleeve Fouling: Low-Pressure High-Output (LPHO) and Medium-Pressure (MP) mercury lamps are housed in transparent fused quartz sleeves. Over time, calcium, magnesium hardness, and iron precipitate onto the hot quartz surface, creating an opaque mineral scale that blocks UV transmission.
3. Sleeve Cleaning Protocols: Systems operate automated mechanical wipers coupled with periodic offline chemical soaking using mild acid solutions (typically citric acid or phosphoric acid) to strip mineral scale.
4. Duty Sensors: Calibrated germicidal UV sensors continuously monitor irradiance ($W/m^2$) to verify delivered UV dose ($\text{mJ/cm}^2 = \text{Irradiance} \times \text{Exposure Time}$).