A licensed engineer prepares the noncompliance report required by an industrial facility's NPDES permit. The permit limits total suspended solids to a monthly average mass loading of 250 lb/day. For the reporting month the average discharge is 1.8 MGD at a total suspended solids concentration of 22 mg/L. The percentage by which the monthly average loading exceeds the permitted mass limit is most nearly:
- (A)24%
- (B)32%
- (C)76%
- (D)132%
Show worked solution
Answer: (B)
Loading of 330 lb/day against a 250 lb/day limit places the month 32 percent above the permitted mass discharge.
Mass loading, not concentration, is the permitted quantity, so flow and concentration combine through the standard 8.34 lb per million gallons per mg/L factor.
Exceedance in a noncompliance report is measured against the permit limit, which is the fixed regulatory reference, rather than against the observed loading.
Restating the limit as an equivalent permitted concentration of 16.7 mg/L against the measured 22 mg/L reproduces the same margin.
FE Reference Handbook — Environmental Engineering: Mass Loading Rate (Flow-Concentration Product) and Water/Wastewater Unit Conversions
Why the other choices appear
- (A)Divides the 80.3 lb/day excess by the measured loading instead of the permit limit: 80.3/330.3 = 24.3%.
- (C)Reports the limit as a fraction of the measured loading, 250/330.3 = 75.7%, mistaking that ratio for the exceedance.
- (D)Reports the full ratio of loading to limit, 330.3/250 = 132%, without subtracting the 100% baseline.