🧪 Disinfection & Chlorination: CT Values and DBPs

The Last Line of Defense

Disinfection is the treatment step that inactivates the disease-causing microorganisms — bacteria, viruses, and protozoa — that survive earlier processes. It is arguably the most important public-health intervention in water supply: chlorination of drinking water in the early twentieth century all but eliminated waterborne typhoid and cholera in developed nations. Understanding disinfection means understanding chlorine chemistry, the dose-time relationship, and the unwelcome byproducts that come with it.

Chlorine Chemistry

When chlorine gas or hypochlorite is added to water, it forms hypochlorous acid (HOCl) and the hypochlorite ion (OCl⁻):

Cl₂ + H₂O → HOCl + HCl;   HOCl ⇌ H⁺ + OCl⁻

Together these are called free available chlorine. The balance between them depends strongly on pH: HOCl is far more germicidal than OCl⁻ — perhaps 80–100 times — yet it dominates only at lower pH (below about 7.5). As pH rises, more chlorine shifts to the weaker OCl⁻, so disinfection becomes less effective. This is why pH control matters as much as dose.

Breakpoint Chlorination

Real water often contains ammonia, and chlorine reacts with it before it can act as a free disinfectant. As chlorine dose increases, the chemistry passes through distinct stages:

1. Chlorine reacts with reducing agents and is consumed (little residual forms).
2. Chlorine combines with ammonia to form chloramines (combined chlorine); the residual rises.
3. Additional chlorine oxidizes and destroys those chloramines; the residual falls to a minimum — the breakpoint.
4. Beyond the breakpoint, further chlorine remains as free chlorine, the desired disinfecting residual.

Operating past the breakpoint guarantees a free-chlorine residual and removes ammonia and many taste-and-odor compounds.

The CT Concept

Disinfection is not instantaneous; it depends on both how much disinfectant is present and how long the microbes are exposed. This is captured by the CT value:

CT = C × T

where C is the disinfectant residual (mg/L) and T is the contact time (minutes). Regulators publish required CT values for inactivating specific pathogens to a target log-reduction, indexed by temperature and pH (colder water and higher pH need more CT). A plant proves compliance by demonstrating it achieves the required CT in its contact basins and pipelines before the water reaches the first customer. The contact time is usually based on the T₁₀ — the time for 10% of the water to pass through — to account for short-circuiting.

Chloramines

Some utilities deliberately use chloramines (formed by adding ammonia with chlorine) as the residual disinfectant in the distribution system. Chloramines are weaker and slower than free chlorine, so they are poor primary disinfectants, but they are far more stable — they persist longer in long pipe networks and, importantly, form fewer regulated disinfection byproducts. Many systems now disinfect primarily with chlorine, ozone, or UV and then add chloramines purely to carry a lasting residual.

Disinfection Byproducts

Chlorine's effectiveness comes at a cost. When it reacts with natural organic matter (NOM) in the water, it produces disinfection byproducts (DBPs), some of which are suspected carcinogens. The two regulated groups are:

Controlling DBPs forces a balancing act: too little disinfection risks pathogens; too much (with high organics) creates DBPs. The best strategy is to remove the organic precursors first — through enhanced coagulation, granular activated carbon, or membranes — so less DBP forms when chlorine is finally added. Switching to chloramines for the residual and optimizing the point of chlorination also help.

UV and Ozone Alternatives

Two chemical-free or low-byproduct disinfectants have become mainstream:

• Ultraviolet (UV) light: inactivates microbes by damaging their DNA. UV is exceptionally effective against chlorine-resistant Cryptosporidium and Giardia, works in seconds, and creates no chlorinated DBPs — but provides no residual.
• Ozone (O₃): an extremely powerful oxidant that destroys pathogens and oxidizes taste, odor, and color compounds rapidly. It too leaves no residual and can form bromate (its own byproduct) in bromide-bearing water.

Because neither leaves a residual, both are used as primary disinfectants, followed by a small chlorine or chloramine dose to protect the water as it travels through the distribution system. The modern trend is a multi-barrier disinfection approach — combining UV or ozone with chlorine — to inactivate the full range of pathogens while keeping byproducts low.