Calculators
Chemical Dosage
Enter a dose and a flow and get the feed rate three ways: pounds per day, gallons per day and a pump setting in mL/min. The math is shown with your numbers underneath.
Your numbers, step by step
- Convert the flow to MGD
- Pounds of active chemical6 mg/L × 4 MGD × 8.34 = 200.2 lb/day
- Scale up for product strength200.2 ÷ 0.125 = 1,601 lb/day of product
- Weight of one gallon of product8.34 × 1.2 = 10.01 lb/gal
- Gallons of product per day1,601 ÷ 10.01 = 160 gal/day
- Pump rate160 gal/day × 3,785 mL/gal ÷ 1,440 min/day = 420.6 mL/min
Why it works
The first step is the same multiplication as the Davidson pie chart: milligrams per liter times million gallons per day times 8.34 gives pounds per day. That answer is pure chemical, the amount the water needs to see.
You are rarely pumping pure chemical. A drum of 12.5% hypochlorite is mostly water, so you have to move eight pounds of product to deliver one pound of chlorine. Dividing by the strength as a decimal makes that correction. The Demand, Strength and Purity page walks through it in more detail.
Pumps are set by volume, not weight, so the last move turns pounds into gallons. Liquid chemicals are heavier than water; specific gravity tells you by how much. A gallon of product weighs 8.34 × SG pounds.
Example: disinfecting plant effluent
Your plant discharges 4.0 MGD and the chlorine contact chamber needs a 6 mg/L dose to hold the residual your permit calls for. You feed 12.5% sodium hypochlorite with a specific gravity of about 1.2.
- 6 × 4.0 × 8.34 = 200.2 lb/day of chlorine
- 200.2 ÷ 0.125 = 1,601 lb/day of hypochlorite solution
- 1,601 ÷ (8.34 × 1.2 = 10.01 lb/gal) = 160 gal/day
- 160 × 3,785 ÷ 1,440 = 420.6 mL/min at the pump
A 160 gal/day feed also tells you a 1,000-gallon tote lasts a little over six days. That is worth knowing before a holiday weekend.
Does the dose look reasonable?
Wastewater doses are set by a target, not picked from a table. Use these broad ranges as a sanity check on what you typed: if your number is far outside them, recheck the units and what the dose is expressed "as" before questioning the plant.
| Job | Chemical | What sets the dose | Sanity range |
|---|---|---|---|
| Effluent disinfection | Sodium hypochlorite | Residual and bacteria limits at the end of the contact tank | About 2–15 mg/L as Cl2; lower for filtered or nitrified effluent |
| Dechlorination | Sodium bisulfite | Chlorine residual leaving the contact tank | About 1.5–2 mg/L per 1 mg/L of Cl2 residual |
| Phosphorus removal | Ferric chloride | Fe:P molar ratio, tuned by jar test and effluent TP | About 2.7–4.5 mg/L as Fe per 1 mg/L of P removed |
| Phosphorus removal | Alum | Al:P molar ratio, tuned by jar test and effluent TP | About 1.3–2.2 mg/L as Al per 1 mg/L of P removed |
| Alkalinity / pH | Caustic soda | pH or alkalinity target, often to support nitrification | About 0.8 mg/L NaOH per 1 mg/L of alkalinity (as CaCO3) added |
For metal-salt doses written as Fe or Al, set the calculator's strength to the metal content, not the product strength: 40% ferric chloride is roughly 13.8% Fe, and 48.5% liquid alum is roughly 4.4% Al. Check your supplier's certificate of analysis for the exact figure.
Before you touch the pump
- Match the flow units. SCADA often reads gpm; the formula wants MGD. Pick the unit above instead of converting in your head.
- Dose is not residual. Chlorine demand eats part of what you add. If the sample tap reads 1.0 mg/L, the dose was higher; size the feed on the dose.
- Know what the dose is "as." A phosphorus-removal dose written as mg/L Fe or mg/L Al is not the same as mg/L of product. Use the strength that matches how the dose is expressed.
- Bleach loses strength. Hypochlorite breaks down with heat and time, so an old tote delivers less than its label. If the residual is drifting down at a steady pump rate, suspect the product.
- Dry feeders stop at pounds. For powder or gas, use the active lb/day and set strength to 100%. Gallons and mL/min don't apply.
- Prove it with a drawdown. Pump dials drift. Time the drop in the calibration column and compare it to the mL/min here.
This is a study and planning aid. Your plant's SOPs, permit and the product's SDS take priority over any number on this page.