Dechlorination Dose Calculator

Chlorinated effluent must be dechlorinated before discharge. Enter your chlorine residual and flow to size the bisulfite or sulfur dioxide feed.

Flow

Flow unit

Chlorine residual to neutralize (mg/L as Cl₂)Measured residual ahead of the dechlorination point.

Dechlorination chemical

Design safety factorCovers mixing and side demand. 1.1–1.5 typical; default 1.25.

Permit effluent TRC limit (mg/L)For reference only; typical NPDES limits are 0.01–0.05.

Dechlorination Dose

Item	Value

How the calculation works

Formulas
Theoretical ratio = molecular weight of chemical ÷ molecular weight of Cl₂ (70.90)
  NaHSO₃: 104.06 ÷ 70.90 = 1.47 mg/mg Cl₂
  Na₂S₂O₅: 190.11 ÷ (2 × 70.90) = 1.34 mg/mg Cl₂
  SO₂: 64.06 ÷ 70.90 = 0.90 mg/mg Cl₂
Design dose (mg/L) = residual (mg/L) × theoretical ratio × safety factor
Feed rate (lb/day) = 8.34 × flow (MGD) × dose (mg/L)

Chemical	Theoretical mg/mg Cl2	Typical design ratio
Sodium bisulfite (NaHSO3)	1.47	1.5–2.0
Sodium metabisulfite (Na2S2O5)	1.34	1.5
Sulfur dioxide (SO2)	0.90	1.0–1.2

Notes & limitations

Theoretical ratios come from balanced redox stoichiometry (Metcalf & Eddy, Wastewater Engineering; WEF MOP 8). Actual demand varies with mixing, contact time, and side reactions — verify the effluent total residual chlorine (TRC) with DPD testing and trim the feed.

Size the feed system for the peak residual and peak flow combination, not the average — permit compliance is measured at all times.
Sulfur dioxide is a toxic gas; gas feed systems need leak detection, scrubbers, and operator training per OSHA requirements.
Liquid bisulfite products (typically ~38% solution) need a density-based conversion from the dry lb/day shown here — check the product data sheet.
Overfeeding bisulfite depresses dissolved oxygen in the effluent; some permits limit the effluent sulfite residual as well.

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