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

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

While free chlorine has historically served as the cornerstone of American water treatment, the promulgation of the EPA Disinfectants and Disinfection Byproducts Rules (Stage 1 and Stage 2 D/DBPR) has compelled water utilities to adopt alternative primary and secondary disinfection technologies. When free chlorine reacts with naturally occurring Total Organic Carbon (TOC, humic and fulvic acids) and bromide present in source water, it forms carcinogenic disinfection byproducts—principally Total Trihalomethanes (TTHM, MCL = 0.080 mg/L / 80 µg/L) and Five Haloacetic Acids (HAA5, MCL = 0.060 mg/L / 60 µg/L). Across Arizona's extensive distribution networks and warm surface water supplies, utilities leverage chloramines, chlorine dioxide, ozone, and ultraviolet (UV) irradiation to meet stringent pathogen inactivation standards while complying with Stage 2 D/DBPR Locational Running Annual Average (LRAA) limits.

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Chloramination Chemistry & Operational Process Control

Chloramination is the deliberate, controlled combination of ammonia and chlorine to produce inorganic chloramines for secondary distribution residual maintenance.

Synthesis Reactions & Species Distribution

When chlorine (as hypochlorous acid) is mixed with aqueous ammonia ($NH_3$ or ammonium ion $NH_4^+$), three sequential substitution reactions occur depending on the chlorine-to-ammonia ratio, pH, and temperature:

$NH_3 + HOCl \rightleftharpoons NH_2Cl + H_2O \quad (\text{Monochloramine})$ $NH_2Cl + HOCl \rightleftharpoons NHCl_2 + H_2O \quad (\text{Dichloramine})$ $NHCl_2 + HOCl \rightleftharpoons NCl_3 + H_2O \quad (\text{Nitrogen Trichloride})$

• Monochloramine ($NH_2Cl$): The exclusively desired species in drinking water distribution. It is stable, long-lasting, possesses minimal taste and odor, and does not react with dissolved natural organic matter to form THMs or HAAs.
• Dichloramine ($NHCl_2$): Forms as the chlorine-to-ammonia ratio increases or when pH drops below 7.0. It possesses a pungent, disagreeable "swimming pool" or "bleachy" odor and causes consumer taste complaints.
• Nitrogen Trichloride ($NCl_3$): Forms at high chlorine-to-ammonia ratios ($>8:1$) or highly acidic pH ($<5.0$). It is an unstable, highly volatile, noxious gas that causes severe eye irritation, coughing, and extreme taste and odor complaints at concentrations as low as 0.02 mg/L.

Controlling the $Cl_2 : NH_3\text{-N}$ Weight Ratio

The formation of specific chloramine species is governed by the weight ratio of chlorine to ammonia-nitrogen ($Cl_2 : NH_3\text{-N}$):

• Stoichiometric Optimum: Stoichiometrically, monochloramine forms at a weight ratio of 5.06 : 1 (based on molecular weights: 70.9 g/mol $Cl_2$ to 14.0 g/mol $N$).
• Target Operational Band: In municipal practice, operators maintain the feed ratio strictly between 4.5:1 and 5.0:1 (typically 4.8:1) at a finished water pH of 7.5 to 8.5.

   Cl2 : NH3-N Ratio       Operational Condition & Water Quality Outcome
  ------------------------------------------------------------------------------------
   < 4.0 : 1              Excess Free Ammonia (Under-Chlorination)
                          -> Fuels Ammonia-Oxidizing Bacteria (AOB)
                          -> Triggers Distribution System Nitrification

   4.5:1 to 5.0:1         OPTIMAL MONOCHLORAMINE ZONE
                          -> >95% Monochloramine Residual
                          -> Maximum Residual Stability
                          -> No Noxious Odors or Consumer Complaints

   > 5.5 : 1              Entering Breakpoint Curve (Over-Chlorination)
                          -> Dichloramine and Nitrogen Trichloride Form
                          -> Severe "Locker Room / Bleach" Odors
                          -> Eventual Chloramine Destruction to Breakpoint

Advantages in the Arid Southwest

Chloramines are much weaker oxidants than free chlorine (requiring substantially higher CT values for Giardia and virus inactivation, making them unsuitable as sole primary disinfectants for surface water). However, they offer extraordinary advantages as secondary disinfectants across Arizona:

1. Thermal Stability: In sprawling distribution systems (e.g., Phoenix, Mesa, Glendale, Tucson) where treated water traverses hundreds of miles of pipe and sits in massive storage tanks under summer ground temperatures exceeding 85°F–90°F, free chlorine decays within 24 to 48 hours. Monochloramine remains persistent for days to weeks.
2. DBP Suppression: Switching from free chlorine to chloramines for secondary distribution residual halts the continued formation of TTHMs and HAA5, allowing utilities to maintain compliance with the 0.080 mg/L and 0.060 mg/L MCLs at maximum residence time monitoring locations.
3. Biofilm Control: Because monochloramine is less reactive with pipe surfaces, it penetrates deeper into distribution iron tuberculation and organic biofilms than free chlorine.

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Distribution System Nitrification: Mechanics, Warnings & Control

The primary operational risk in chloraminated distribution systems is nitrification—the biological oxidation of excess free ammonia into nitrite and nitrate by autotrophic bacteria.

Biological Mechanism

When the feed ratio drops below 4.5:1 or when chloramines naturally decompose, unreacted free ammonia ($NH_3\text{-N}$) enters the distribution system. This ammonia serves as an energy source for two groups of nitrifying bacteria:

1. Ammonia-Oxidizing Bacteria (AOB, e.g., Nitrosomonas):

$2 NH_3 + 3 O_2 \xrightarrow{\text{AOB}} 2 NO_2^- + 2 H^+ + 2 H_2O \quad (\text{Ammonia to Nitrite})$

1. Nitrite-Oxidizing Bacteria (NOB, e.g., Nitrobacter):

$2 NO_2^- + O_2 \xrightarrow{\text{NOB}} 2 NO_3^- \quad (\text{Nitrite to Nitrate})$

The Self-Accelerating "Nitrification Spiral"

Nitrification creates an aggressive operational cycle. The generated nitrite ($NO_2^-$) exerts a direct, rapid chemical demand on remaining monochloramine:

$NO_2^- + NH_2Cl + H_2O \rightarrow NO_3^- + NH_4^+ + Cl^- + H^+$

This chemical reaction rapidly destroys the chloramine residual while liberating additional free ammonium ($NH_4^+$) and hydrogen ions ($H^+$). The newly liberated ammonia feeds more AOB bacteria, accelerating the destruction of residual in a compounding loop. As nitrification proceeds, storage tanks experience total loss of disinfectant residual, drop in pH, drop in dissolved oxygen, and massive bacterial regrowth.

Early Warning Diagnostic Indicators

Operators must monitor distribution sample taps and storage tanks weekly for the five cardinal indicators of nitrification:

1. Elevated Free Ammonia: Free $NH_3\text{-N} > 0.10$ mg/L (signals under-chlorination or decomposition).
2. Sudden Drop in Total Chloramine Residual: Loss of $>0.5$ mg/L residual without a change in plant dosing.
3. Nitrite ($NO_2^-$) Spike: Background nitrite in a healthy system is $<0.01$ mg/L. A confirmed nitrite reading $\ge 0.05$ mg/L is the definitive early warning trigger for active nitrification.
4. Depression of Water pH: Nitrification produces hydrogen ions ($H^+$), lowering local water pH by 0.2 to 0.5 units.
5. Surge in Heterotrophic Plate Count (HPC): HPC bacteria spike above 500 CFU/mL due to lack of residual.

Remediation & Control Protocols

• Tank Cycling & Water Age Reduction: Maintain high turnover rates in distribution storage reservoirs, drawing down water levels to $<50%$ capacity daily to keep water age below 5 to 7 days during summer months. Install mechanical mixers or active jet mixing systems to destroy thermal stratification.
• Unidirectional Flushing (UDF): Aggressively flush distribution mains and stagnant dead ends at scouring velocities ($>5.0$ ft/sec) to discharge accumulated sediment and biological slime.
• Free Chlorine Burnout (Burn): The definitive corrective action. The utility temporarily halts ammonia dosing and feeds free chlorine across the entire distribution network for 2 to 4 weeks (typically performed in spring or fall). The free chlorine residual oxidizes accumulated nitrite to nitrate, kills sessile AOB bacteria and biofilm, and re-establishes an inert distribution baseline before the system transitions back to monochloramine.

Chlorine Dioxide ($ClO_2$)

Chlorine dioxide ($ClO_2$) is a synthetic, yellowish-green dissolved gas that operates as a powerful selective oxidant and disinfectant.

Generation & Physical Chemistry

Unlike chlorine gas, chlorine dioxide does not hydrolyze in water to form acids; it exists in solution as a true dissolved neutral gas. Because concentrated chlorine dioxide gas is unstable and violently explosive at partial pressures exceeding 10% in air (76 mmHg), it cannot be compressed or liquefied for transport. It must be generated strictly on-site at the point of application.

• Chlorite-Chlorine Generation: Sodium chlorite solution ($NaClO_2$) reacted with chlorine gas under vacuum:

$2 NaClO_2 + Cl_2\text{ (gas)} \rightarrow 2 ClO_2\text{ (gas)} + 2 NaCl$

• Chlorite-Acid Generation: Sodium chlorite solution reacted with concentrated hydrochloric acid ($HCl$):

$5 NaClO_2 + 4 HCl \rightarrow 4 ClO_2 + 5 NaCl + 2 H_2O$

Performance Characteristics

1. pH Independence: Chlorine dioxide does not dissociate in water. Its oxidation-reduction potential and germicidal efficacy remain completely constant across a broad pH range (pH 4.0 to 10.0), providing massive advantages over free chlorine in high-pH Arizona waters.
2. Zero Halogenated DBPs: Chlorine dioxide reacts with organic matter predominantly by single-electron transfer rather than electrophilic substitution. It does not react with humic substances to form trihalomethanes (TTHMs) or haloacetic acids (HAA5).
3. Taste & Odor Control: Rapidly destroys phenolic compounds, mercaptans, and algae-produced tastes and odors; oxidizes soluble iron ($Fe^{2+}$) and manganese ($Mn^{2+}$) instantaneously.

Disinfection Byproducts & Health Standards

When chlorine dioxide oxidizes organic matter, approximately 50% to 70% is reduced to inorganic chlorite ($ClO_2^-$), with smaller fractions oxidized to chlorate ($ClO_3^-$):

• Chlorite Primary MCL = 1.0 mg/L: Ingested chlorite causes oxidative hemolytic anemia and methemoglobinemia. Utilities must perform daily entry-point monitoring and monthly 3-sample distribution sets.
• Chlorine Dioxide MRDL = 0.8 mg/L: Maximum Residual Disinfectant Level to prevent respiratory and neurological impacts.

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Ozone ($O_3$)

Ozone is an allotrope of oxygen ($O_3$) possessing an extraordinary oxidation-reduction potential (2.07 V), making it the most powerful chemical oxidant and primary disinfectant utilized in municipal water treatment.

Generation & Contactor Architecture

Ozone is an unstable molecule that decomposes rapidly into oxygen ($O_2$) with a half-life in water of only 10 to 30 minutes. Like chlorine dioxide, it cannot be stored or shipped and must be generated continuously on-site.

• Corona Discharge Generation: Dry air or concentrated liquid oxygen (LOX, dew point $<-60^\circ\text{C}$) passes through a high-voltage electrical discharge gap (6,000 to 20,000 volts at 50 to 1,000 Hz). The high-energy electric field cleaves molecular oxygen ($O_2$) into oxygen atoms, which recombine with unreacted $O_2$ to form ozone ($O_3$):

$3 O_2 \xrightarrow{\text{High Voltage Corona}} 2 O_3$

• Contactor Design: Treated water enters deep concrete contactors (18 to 22 feet deep) where ozone gas is introduced through submerged fine-bubble ceramic diffusers or venturi side-stream injection to maximize gas-liquid mass transfer.
• Off-Gas Destruct Units: Because ozone is highly toxic and corrosive to human lung tissue (OSHA permissible exposure limit = 0.1 ppm), exhaust gas from the contactor headspace passes through a catalytic (manganese dioxide) or thermal ozone destruct unit that converts residual $O_3$ back to pure $O_2$ before atmospheric release.

Pathogen Inactivation & DBP Constraints

1. Cryptosporidium Inactivation: Ozone rapidly destroys Cryptosporidium parvum oocysts and Giardia cysts with CT requirements that are roughly 100 times lower than free chlorine.
2. Zero Distribution Residual: Because ozone decomposes in minutes, it leaves zero chemical residual in finished water. Utilities must add a secondary disinfectant (free chlorine or chloramines) prior to distribution.
3. Assimilable Organic Carbon (AOC): Ozone cleaves complex, refractory organic macromolecules into smaller, biodegradable fragments (Assimilable Organic Carbon). If pumped directly into pipes, AOC causes massive bacterial regrowth and slime formation. Consequently, ozone must always be followed by Biological Activated Carbon (BAC) or granular media filtration to bio-filter AOC before disinfection.
4. Bromate Formation ($BrO_3^-$): When source water contains natural dissolved bromide ($Br^-$), ozone oxidizes it to bromate ($BrO_3^-$), a potent human carcinogen:

$Br^- + O_3 \rightarrow OBr^- \xrightarrow{2 O_3} BrO_3^- \quad (\text{Bromate Formation})$

• Bromate Primary MCL = 0.010 mg/L (10 µg/L): Colorado River water delivered via the Central Arizona Project (CAP) contains background bromide concentrations ranging from 50 to 150 µg/L. Ozone facilities treating CAP water must practice aggressive bromate mitigation—such as pH depression (acid addition to pH 6.0–6.5) or ammonia addition—to suppress bromate formation below 10 µg/L.

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Ultraviolet (UV) Irradiation

Ultraviolet disinfection is a physical, non-chemical process that inactivates pathogens by delivering electromagnetic radiation within the UV-C germicidal spectrum.

Photobiological Inactivation Mechanism

Germicidal UV operates at wavelengths between 200 and 300 nm, with peak DNA/RNA absorption occurring at 260 nm. Standard low-pressure mercury lamps emit monochromatic radiation almost exclusively at 253.7 nm (254 nm).

When microorganisms pass through the UV reactor, 254 nm photons penetrate the cell envelope and are absorbed by pyrimidine bases (thymine in DNA, uracil in RNA). The energy breaks hydrogen bonds and induces covalent cross-linking between adjacent thymine molecules, forming cyclobutane pyrimidine dimers (thymine-thymine dimers). This photochemical bond distorts the DNA double helix, preventing the microorganism from unzipping its DNA for replication or transcription. While the pathogen may remain physically intact, it is rendered biologically sterile and completely non-infectious.

Pathogen Sensitivity Profile

• Cryptosporidium and Giardia (Highly Sensitive): Protozoan parasites are exceptionally vulnerable to UV. A minimal UV fluence (dose) of $5\text{ to }12\text{ mJ/cm}^2$ achieves 3-log to 4-log inactivation of both Cryptosporidium and Giardia.
• Enteric Viruses (Highly Resistant): Double-stranded DNA enteric viruses (particularly Adenovirus) possess complex enzymatic repair mechanisms. Achieving 4-log inactivation of Adenovirus requires an immense UV dose exceeding $120\text{ to }186\text{ mJ/cm}^2$.
• Dual Disinfection Design: In modern surface water plants, UV is optimized at low doses (40 mJ/cm²) for primary Cryptosporidium and Giardia inactivation, while chemical chlorination provides virus inactivation and secondary distribution protection.

Key Operational & Maintenance Parameters

• UV Transmittance (UVT): The percentage of 254 nm light transmitted through a 1-centimeter path of water ($%UVT = 100 \times 10^{-A}$). High turbidity, suspended solids, dissolved iron, manganese, and natural organic color absorb UV photons, decreasing UVT and starving pathogens of the required dose. Surface water UV systems typically require UVT $>85%–90%$.
• UV Fluence (Dose): Measured in millijoules per square centimeter ($ ext{mJ/cm}^2$):

$\text{Dose } (\text{mJ/cm}^2) = \text{UV Intensity } (I\text{ in mW/cm}^2) \times \text{Exposure Time } (t\text{ in seconds})$

• Quartz Sleeve Maintenance: Submerged mercury lamps are housed inside high-purity quartz sleeves to isolate electrical components from water. High water hardness, calcium, iron, and heat cause mineral scale to bake onto the quartz sleeve exterior, blinding the lamp. Systems must be equipped with automated mechanical wiper rings (pneumatic or motorized) and undergo periodic chemical cleaning with mild acid solutions (citric or phosphoric acid).
• Zero Distribution Residual: UV produces zero physical or chemical residual in finished water. Under A.A.C. Title 18, Chapter 4, utilities must inject a chemical disinfectant (free chlorine or chloramines) following UV treatment to maintain minimum distribution residuals (0.2 mg/L free chlorine or 0.5 mg/L total chlorine).

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Comparison of Drinking Water Disinfection Technologies