Environmental Engineering: Mass Balances, Treatment Trains, Air Quality, and Waste Regulation
What this domain tests
The Environmental domain is one of the largest knowledge areas on the FE Environmental exam. It tests applied calculation, not statute recitation: mass balances and unit conversions, drinking-water and wastewater treatment sizing and performance, air-pollutant concentrations and control efficiency, solid and hazardous waste classification, and the federal programs (CWA, SDWA, CAA, RCRA, CERCLA) that set the numbers you design to.
Loading arithmetic and mass balances
Almost every item reduces to mass flow. In water, lb/day = concentration (mg/L) × flow (MGD) × 8.34, because 8.34 lb is the mass of one gallon of water, so mg/L across one million gallons gives pounds. In air, convert between ppm and mg/m³ through molar volume: at 25 °C and 1 atm, mg/m³ = ppm × MW / 24.45 (use 22.414 at 0 °C). Control efficiency is always (mass in − mass out) / mass in.
Water and wastewater treatment
Drinking water is a multibarrier train - coagulation, flocculation, sedimentation, filtration, disinfection - designed to meet turbidity and pathogen log-inactivation targets. Wastewater proceeds preliminary → primary (settling) → secondary (biological) → tertiary. Activated-sludge design rests on solids retention time, food-to-microorganism ratio, MLSS, and return-sludge rate; nutrient limits add nitrification/denitrification and phosphorus removal.
Air quality and waste
The six criteria pollutants are CO, Pb, NO₂, ozone, PM, and SO₂. NAAQS set primary (health-based) and secondary (welfare-based) levels; nonattainment triggers New Source Review with emission offsets. Under RCRA a waste is hazardous if it is listed or exhibits ignitability, corrosivity, reactivity, or toxicity (TCLP). CERCLA governs release reporting and cleanup liability at contaminated sites.
Worked example
A plant treats 2.5 MGD; influent BOD₅ is 220 mg/L and effluent is 30 mg/L. Find the mass removed and the removal efficiency.
- Influent load = 220 × 2.5 × 8.34 = 4,587 lb/day
- Effluent load = 30 × 2.5 × 8.34 = 626 lb/day
- Removed = 4,587 − 626 = 3,961 lb/day
- Efficiency = 3,961 / 4,587 = 86.4%, matching the shortcut 1 − (30/220).
Traps that cost marks
- Applying 8.34 to a flow given in gallons per day instead of MGD.
- Reporting percent removed when the item asks percent remaining.
- Mixing mg/L and µg/L across drinking-water MCLs and air standards.
- Assuming the secondary NAAQS is always the stricter number; for several pollutants it is not.
- Confusing RCRA (cradle-to-grave management of active waste) with CERCLA (abandoned-site response).