Water Quality
Core concepts
Water-quality work centers on the Clean Water Act: NPDES permits, effluent limitations, water-quality standards (designated uses, criteria, antidegradation), TMDLs, stormwater permits, and pretreatment. Wastewater treatment questions test primary/secondary/tertiary processes, activated sludge operating parameters (MLSS, F/M, SRT/MCRT, HRT, RAS/WAS), clarifier hydraulics, disinfection CT, and residuals. Receiving-water effects include BOD/DO sag, nitrification demand, nutrients/eutrophication, turbidity, and toxicity. Know the difference between CBOD and NBOD, BOD5 and ultimate BOD, and permit limits expressed as daily maximum, monthly average, or loading. Mass balances, removal efficiency, and loading conversions are frequent. Also review mixing zones, whole-effluent toxicity, and biosolids handling. Stormwater often requires SWPPP and best management practices.
Worked example
A plant discharges 1.0 MGD with BOD5 = 200 mg/L. Using 8.34 lb/gal, load = concentration (mg/L) × flow (MGD) × 8.34 = 200 × 1.0 × 8.34 = 1,668 lb/day. If the permit limit is 30 mg/L monthly average, the allowable load is 30 × 1.0 × 8.34 = 250 lb/day; required removal is (200−30)/200 = 85%. Trap: do not multiply by 8.34 twice or mix MGD with liters. If flow changes, loads scale linearly.
Rule of thumb
For dilute aqueous streams, 1 mg/L × 1 MGD ≈ 8.34 lb/day. Check units: mg/L is effectively lb per million lb water.