5.3 Chloramines, Chlorine Dioxide, Ozone & UV

5.3 Chloramines, Chlorine Dioxide, Ozone & UV

Every disinfectant is a trade. Gain oxidizing power and you usually gain disinfection byproducts; gain persistence and you usually lose inactivation strength. South Carolina's facility grouping makes this concrete: a plant that adds chloramine moves into Group II, while a plant using chlorine dioxide, ozone, or UV lands in Group III, V, or VI depending on source water — so the disinfectant you select literally determines the operator class the plant requires.

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1. Relative Strength and Persistence

$$\textbf{Oxidizing strength: } \text{Ozone} > \text{Chlorine Dioxide} > \text{Free Chlorine} > \text{Chloramine}$$ $$\textbf{Residual persistence: } \text{Chloramine} > \text{Free Chlorine} > \text{Chlorine Dioxide} > \text{Ozone (none)}$$

That inversion is the whole story. The strongest disinfectant persists the least; the most persistent disinfects the least. Most modern plants therefore run a two-stage strategy: a strong primary disinfectant for inactivation at the plant, and a persistent secondary disinfectant to hold a residual through the distribution system.

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2. Chloramines

Chloramines form when free chlorine reacts with ammonia. The species depends on the chlorine-to-ammonia-nitrogen weight ratio and the pH:

Advantages: long-lasting residual deep into the distribution system; substantially lower trihalomethane and haloacetic acid formation; better control of biofilm regrowth over long detention times.

Disadvantages and operational risks:

• Weak disinfectant — provides very little practical CT credit; must not be relied on as the primary disinfectant.
• Nitrification. Excess free ammonia feeds ammonia-oxidizing bacteria in the distribution system. The signature is falling chloramine residual, rising nitrite and nitrate, falling dissolved oxygen and pH, and rising heterotrophic plate counts — most often in summer, in dead ends, and in oversized storage tanks. Control with tank turnover, flushing, residual maintenance, and periodic free chlorine burnouts.
• Toxic to dialysis patients and to fish, requiring notification of hospitals, dialysis centers, and aquarium owners before conversion.

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3. Chlorine Dioxide (ClO₂)

Generated on site from sodium chlorite, because it is explosive under pressure and cannot be shipped as a compressed gas.

• Effective across a wide pH range — unlike free chlorine, its performance does not collapse as pH rises.
• Does not form trihalomethanes or haloacetic acids — a major advantage on high-TOC source water.
• Excellent for taste and odor and for iron and manganese oxidation.
• Regulated byproducts: chlorite carries an MCL of 1.0 mg/L, and the chlorine dioxide MRDL is 0.8 mg/L. Chlorite must be monitored daily at the entry point.
• Can cause odor complaints described as kerosene or cat-urine-like, arising from reactions with new carpet and paint indoors.

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4. Ozone (O₃)

Generated on site by passing dry air or oxygen through a high-voltage corona discharge.

• The strongest common disinfectant and highly effective against Cryptosporidium, which resists chlorine almost entirely.
• Excellent for taste and odor, color, iron and manganese.
• Leaves no residual whatsoever — a secondary disinfectant is mandatory.
• Forms bromate when the source water contains bromide. Bromate has an MCL of 0.010 mg/L, and controlling it may require pH depression or ammonia addition.
• Breaks large organics into smaller, more biodegradable fragments (AOC/BDOC), which feed regrowth downstream. Ozone is therefore usually paired with biologically active filtration to consume that material before it reaches the distribution system.
• Ozone is toxic to breathe; ambient ozone monitors and off-gas destruct units are required.

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5. Ultraviolet (UV) Light

UV inactivates organisms by damaging their nucleic acid so they cannot reproduce. Dose is expressed in millijoules per square centimeter (mJ/cm²).

The exam point: UV is superb against protozoa at low dose — exactly the organisms chlorine struggles with — but achieving virus credit requires an order-of-magnitude higher dose. Plants therefore commonly use UV for Crypto/Giardia credit and chlorine for virus credit.

Operational considerations:

• No residual — a secondary disinfectant is mandatory.
• UV transmittance (UVT) governs performance; turbidity, iron, and organics absorb UV and shield organisms.
• Sleeve fouling and lamp aging reduce delivered dose; sleeves need mechanical or chemical cleaning, and lamps are replaced on an hours-based schedule.
• No disinfection byproducts are formed, which is a significant advantage on high-TOC water.

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6. Selection Summary