Environmental PE

The PE exam often provides EPA default exposure parameters. For cancer risk, AT = 70 years × 365.25 days = 25,550 days regardless of actual exposure duration. For non-cancer effects, AT = ED × 365.25. Always check which type of assessment is required before selecting AT values.

Standard BOD₅ test assumes k ≈ 0.23 day⁻¹ at 20°C. Remember that BOD₅ is typically 65-70% of ultimate BOD for domestic wastewater. Temperature corrections use k₍T₎ = k₂₀ × θ^(T-20) where θ = 1.047.

Surface overflow rate is a key design parameter - memorize typical ranges! Primary clarifiers: 600-800 gpd/ft², Secondary: 300-800 gpd/ft². Always check if the problem gives diameter (calculate area first) or surface area directly. Watch units carefully - flow is often given in MGD but answer needed in gpd.

This is the fundamental CSTR equation - memorize it! Often given as part of treatment train calculations. Watch units carefully: detention time θ = V/Q must match k units (usually hours). Common on activated sludge and disinfection problems.

Darcy's law appears frequently on PE Environmental exams for groundwater flow calculations and contaminant transport problems. Remember: Q is total volumetric flow, while v is actual seepage velocity through pores (always less than Darcy velocity). Know typical K values for different materials and be ready to calculate travel times for contaminant plumes.