Pollutant load is concentration multiplied by flow
Concentration and load answer different questions. Concentration in mg/L says how much pollutant rides in each unit of water. Load in pounds per day says how much actually reaches the receiving water. Dilution lowers concentration without changing load, which is why NPDES permits, pretreatment surcharges and TMDL allocations are usually written in mass terms.
The standard American formula is load (lb/day) = flow (MGD) × concentration (mg/L) × 8.34, where 8.34 is the weight of one gallon of water in pounds. A plant discharging 1 MGD at 200 mg/L BOD is releasing 1,668 lb/day, or about 304 short tons a year. Enter your permit limit and the tool also reports the load allowed at that limit and the extra removal you would need.
In that example a 30 mg/L limit allows 250.2 lb/day, so the stream is over and needs 85 % removal. That figure is not a coincidence: the federal secondary treatment standards at 40 CFR 133 set 30 mg/L monthly average for BOD5 and TSS along with a minimum 85 % removal requirement, and 45 mg/L as the weekly average.
Those federal numbers are the floor, not the whole story. Your actual limits come from the permit itself, which may impose stricter water quality based limits, mass caps, or industry specific effluent guidelines. Values here reflect the rules as of September 2026. Confirm applicable limits with your permitting authority and have a licensed environmental engineer review compliance strategy.
Frequently Asked Questions
No. Dilution as a substitute for treatment is prohibited, and where a mass cap applies the load does not change anyway. The pollutant has to be removed in the process.
Use average daily flow with a flow weighted composite concentration. When the swing is large, pull totalizer readings and composite sample results so the daily mass reflects what actually left the outfall.