Design Manual › Disinfection › Chlorine
FREE ENGINEERING TOOL · DISINFECTION
Chlorine Disinfection
Disinfection is the one unit process where the design is a product, not a rate:
concentration × time. This tool builds the dose from the water's actual chlorine demand, converts the
target log-inactivation into a CT requirement corrected for temperature and pH, and sizes the contact
tank from the baffled contact time — because a perfectly sized tank with a poor baffling factor
will still fail.
1Effluent quality & duty
Chlorine demand is set by what is in the water, not by the target. Ammonia dominates:
oxidising it to the breakpoint costs 7.6 kg of chlorine per kg of ammonia-nitrogen.
2Disinfection target & chemistry
Free chlorine is a far stronger disinfectant than chloramine — but getting to free
chlorine means paying the full breakpoint dose. Chloramine costs less chlorine and forms fewer
byproducts, and buys it back with a much longer contact time.
Results — dose & contact
Chlorine dose
—mg/L
Cl₂ consumption
—kg/d
Contact volume
—m³
| Item | Value | Basis |
|---|
3CT — the actual design criterion
CT is corrected outward from the 20 °C, pH 7 reference in both directions: cold water
needs far more, and high pH does too, because the hypochlorite ion is a weaker disinfectant than
hypochlorous acid.
| Item | Value | Basis |
|---|
4Chemical supply & dechlorination
If the receiving water is sensitive, the residual has to come back off before discharge.
Dechlorination is a small step that is often forgotten until the permit arrives.
| Item | Value | Basis |
|---|
Operating envelope & risks
| Item | Value | Basis |
|---|
5Instrument schedule
An honest note: residual chlorine is not in our online catalogue. ORP is — and used
properly, with periodic DPD checks, it is a workable continuous proxy.
| Point | Instrument | Why here |
|---|
Quote the disinfection instruments at factory price
ORP, pH, turbidity and flow — all mappable to our catalogue and supplied at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the device).
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Sources & parameter basis
| [1] | METCALF & EDDY / EPA — wastewater disinfection: dose 5–20 mg/L for secondary effluent, residual 0.5–2 mg/L (up to 5 for chloramine), contact 20–30 min at peak flow. |
| [2] | Breakpoint chlorination: Cl₂:NH₃-N ≈ 7.6:1 by weight to reach the breakpoint and leave free residual; monochloramine formation sits around Cl₂:N ≈ 3–5:1. Other reducing demand taken as ≈0.3·BOD + 1 mg/L. |
| [3] | CT values at 20 °C, pH 7 (mg·min/L) — free chlorine: coliform 2-log ≈1.5, virus 3-log ≈5, Giardia 3-log ≈53. Chloramine: ≈50, ≈750, ≈1100 respectively. Temperature: CT ∝ 1.07(20−T) (matches EPA tables from 5 to 25 °C). |
| [4] | pH: CT scales as ≈1.25(pH−7), reflecting the HOCl/OCl⁻ split (HOCl ≈76 % at pH 7, ≈24 % at pH 8). |
| [5] | Baffling factor T10/τ: unbaffled 0.1, poor 0.3, average 0.5, superior 0.7, plug flow 1.0. Design on T10, not on the nominal HRT. |
| [6] | Turbidity shields pathogens and consumes chlorine; keep effluent below about 2 NTU, ideally under 1 NTU, before disinfection. |
| [7] | Disinfection by-products: THM formation rises with dose and TOC. Watch it above roughly 10 mg/L dose with TOC above 5 mg/L. |
| [8] | Sodium hypochlorite: 5–15 % available chlorine; it decays with age, heat and light — size storage for turnover, not for calendar weeks. Dechlorination: NaHSO₃ ≈1.46 mg per mg Cl₂, SO₂ ≈0.9 mg per mg Cl₂. |
Full derivation and worked example: Chlorine-Dosing-Calculation-Sheet.md. Switch
the mode from chloramine to free chlorine and watch what happens: the chlorine bill roughly doubles, and
the contact tank shrinks. That trade is the whole decision.
What is the breakpoint and how much chloramine dose is needed?
The calculator reports the breakpoint chlorine demand and the chloramine dose separately, because chloramines need far less chlorine than free chlorine once the breakpoint is passed.
How is the CT value corrected for temperature and pH?
CT demand rises with temperature and with pH, so the calculator applies both corrections to the CT value instead of treating the reference temperature as universal.
How is the baffled contact tank volume calculated?
Contact tank volume follows from the flow, the required contact time from the corrected CT, and the baffle layout - the calculator sizes the tank and the contact time together.
What about dechlorination and DBP risk?
The calculator also returns the dechlorination duty and flags turbidity shielding and disinfection by-product risk, so residual chlorine and DBP are checked rather than ignored.
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