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.

Avg flow Q^[1] (1k–500k)m³/d

Peak factor^[1] (1.5–3)×

NH₃-N^[2] (0–15)mg/L

BOD_eff^[2] (5–30)mg/L

Turbidity^[6] (1–10)NTU

TOC^[7] (5–25)mg/L

Temperature^[3] (10–32)°C

pH^[4] (6–9)—

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.

Mode^[2]—

Target^[3]—

Target residual^[1] (0.5–5)mg/L

Baffling T10/τ^[5] (0.1–1.0)—

Min contact^[1] (15–30)min

NaOCl strength^[8] (5–15)%

Storage^[8] (3–30)d

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.

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