Chlorination Disinfection: Chemistry, CT and Dosing

What actually happens when you add chlorine to water?

Whether dosed as chlorine gas (Cl₂) or as sodium hypochlorite (NaOCl), the active species in water is the same. Chlorine gas hydrolyses almost instantly:

The two forms are far from equal: HOCl is roughly 80–100× more germicidal than the hypochlorite ion OCl^-, because the neutral molecule penetrates the cell membrane while the anion is repelled. Since pK_a ≈ 7.5, at pH 6 about 97% of FAC is HOCl, at pH 7.5 it is roughly 50/50, and at pH 8.5 only ~10% remains as HOCl. This is why chlorination is markedly less effective in alkaline water and why pH control is part of any serious design.

How does pH set the fraction of active HOCl?

The active fraction follows directly from the Henderson–Hasselbalch relationship applied to the HOCl/OCl^- equilibrium:

Because the germicidal work is done almost entirely by HOCl, the effective disinfectant concentration is f_HOCl × FAC, not the measured total. A residual of 1.0 mg/L free chlorine at pH 8.0 delivers only ~0.24 mg/L as HOCl — roughly a quarter of the active dose you would get at pH 6.5. Designers therefore either hold pH low through the contact tank or compensate with a higher residual and longer contact time.

Free vs combined chlorine and the breakpoint curve

If ammonia or organic nitrogen is present, chlorine reacts with it to form chloramines — the combined available chlorine:

Chloramines are disinfectants too, but far weaker and slower — monochloramine is roughly 1/100th the biocidal rate of HOCl, needing much larger CT. As chlorine dose increases, the classic breakpoint curve emerges: residual first rises as chloramines form, then falls as continued dosing oxidises those chloramines to nitrogen gas, and finally rises again as true free chlorine accumulates past the breakpoint.

Stoichiometrically the breakpoint sits near a Cl₂:NH₃-N mass ratio of ~7.6:1 (theoretical), typically 8–10:1 in practice because side reactions consume chlorine. Dosing to break-point is how you guarantee a genuine free-chlorine residual rather than a sluggish combined one. For nitrogen-bearing effluents this interacts with upstream ammonia removal, so read it alongside the wider industrial wastewater treatment train.

What is the CT concept and how do you use it?

Disinfection is a kinetic process. The CT concept — disinfectant residual concentration multiplied by contact time — is the standard regulatory and design metric (US EPA Surface Water Treatment Rule). It follows from Chick–Watson first-order kinetics:

When n ≈ 1, achieving a target log-kill needs a fixed CT = C × t value, which is why CT tables are tabulated per organism, temperature and pH. Cryptosporidium and viruses need very different CT from bacteria. The contact tank must also be plug-flow: the regulatory contact time is the T₁₀ (the time at which 10% of a tracer has passed), so baffled tanks with a high baffling factor are specified to stop short-circuiting.

Note how CT rises by orders of magnitude for protozoan cysts — the reason chlorine-resistant organisms drive utilities toward complementary disinfection equipment rather than ever-higher chlorine doses.

Worked example: sizing a chlorine contact tank

Design a contact tank to achieve 4-log (99.99%) virus inactivation on a treated secondary effluent.

Inputs: design flow Q = 200 m³/h; operating pH 7.0 and 10 °C; required free-chlorine CT for 4-log virus ≈ 6 mg·min/L; baffling factor (T₁₀/T) = 0.5 for a moderately baffled tank.

• Choose a residual: target a free-chlorine residual of C = 1.0 mg/L at the tank outlet.
• Contact time from CT: t = CT / C = 6 / 1.0 = 6 min of effective (T₁₀) contact.
• Correct for hydraulics: because only T₁₀ counts, the mean hydraulic residence time must be T = t / 0.5 = 6 / 0.5 = 12 min.
• Tank volume: V = Q × T = 200 m³/h × (12/60) h = 40 m³.
• Chlorine dose: if the effluent exerts a demand of ~2 mg/L before the 1.0 mg/L residual, applied dose ≈ 3 mg/L. Mass rate = 3 g/m³ × 200 m³/h = 600 g/h = ~14.4 kg/day as Cl₂.

So a ~40 m³ baffled contact tank with a ~3 mg/L applied dose meets the duty. Raise the pH to 8.0 and the same CT would demand a higher residual or a larger tank, because f_HOCl collapses — a direct consequence of the equilibrium above. Detailed geometry and baffle layout are then confirmed at the contact-tank detailed design stage.

Disinfection by-products — the trade-off you must design around

Free chlorine reacts with natural organic matter (humic and fulvic acids) to form disinfection by-products (DBPs): chiefly trihalomethanes (THMs) such as chloroform, and haloacetic acids (HAAs), both regulated as suspected carcinogens. UK/EU drinking-water limits cap total THMs at 100 µg/L; the US EPA sets TTHM at 80 µg/L and HAA5 at 60 µg/L.

DBP formation rises with organic (TOC) precursor concentration, chlorine dose, contact time, temperature and pH. The design tension is clear: more CT means better disinfection but more DBPs. Mitigation strategies include removing precursors upstream (coagulation, GAC), minimising excess dose, or switching part of the duty to alternative oxidants. For effluent going to a watercourse, the residual and by-product limits are set by the discharge permit — see UK effluent discharge standards.

Dechlorination and chlorine alternatives

Chlorine residual is toxic to aquatic life at very low levels (often <0.02 mg/L consented), so treated effluent is usually dechlorinated before discharge. The common route is sulphur dioxide or a sulphite/bisulphite salt, which reduces free and combined chlorine rapidly:

Where DBPs, chlorine-resistant protozoa or dechlorination costs dominate, alternatives compete:

• Chlorine dioxide (ClO₂): a strong oxidant that does not hydrolyse, so it works across a wide pH band and forms far fewer THMs — but produces chlorite/chlorate, which are themselves regulated.
• Sodium hypochlorite: chemically identical FAC to chlorine gas but safer to handle; it decays on storage and raises pH, so dose control matters.
• UV disinfection: no chemical residual and highly effective against Cryptosporidium; see our UV disinfection design guide for where it beats chlorine.

In practice many plants combine methods — UV or ClO₂ for the resistant fraction, a small free-chlorine residual for distribution or network protection.

How to specify a chlorination disinfection system

1. Characterise the water. Measure flow, pH, temperature, ammonia/organic nitrogen, TOC and the target pathogen and log-removal. These set demand, breakpoint and DBP risk.
2. Select the target log-inactivation and CT. From CT tables for the governing organism at the design pH and temperature, read the required CT (mg·min/L).
3. Fix pH and residual. Choose an operating pH (lower favours HOCl) and a free-chlorine residual; compute effective HOCl from f_HOCl.
4. Size the contact tank. Compute effective time t = CT/C, divide by the baffling factor to get mean residence time, then V = Q × T. Specify baffles for a high T10/T.
5. Set the dose and dosing plant. Add chlorine demand to the target residual to get applied dose; size gas/hypochlorite dosing and control on residual analysers.
6. Provide dechlorination. Add sulphite/SO2 dosing (~1 mg per mg residual) to meet the discharge chlorine limit, with residual monitoring downstream.