Dechlorination Chemical Dose
The sulfur-based chemical to neutralize a chlorine residual before an NPDES discharge or a fish-bearing stream: reagent dose (mg/L) = a stoichiometric ratio x the chlorine residual to remove; feed (lb/day) = dose x flow (MGD) x 8.34 / purity. The ratio (mg reagent per mg Cl2) is reagent-specific and editable: ~0.9-1.0 for SO2, 1.34 sodium metabisulfite, 1.46 sodium bisulfite, 1.77 sodium sulfite, ~0.56 sodium thiosulfate. Removing a 2.0 mg/L residual from 5 MGD with sodium bisulfite is a 2.92 mg/L dose and 121.8 lb/day of 100% product; less-pure product needs more. Sulfur dechlor consumes dissolved oxygen and depresses pH, so an over-dose has its own permit cost -- dose to just neutralize, and confirm a near-zero residual downstream of mixing. A dosing estimate; the discharge permit, the reagent assay, and the state primacy agency govern.
Formula and source
reagent_dose_mg_l = stoich_ratio x chlorine_residual_mg_l; feed_lb_day = reagent_dose_mg_l x flow_mgd x 8.34 / (purity_pct/100). Ratio (mg reagent/mg Cl2): SO2 ~0.9-1.0, metabisulfite 1.34, bisulfite 1.46, sulfite 1.77, thiosulfate ~0.56.
Dechlorination stoichiometry with the pounds formula (Standard Methods / wastewater-operations practice), by name; the discharge permit, the reagent assay, and the state primacy agency govern.
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