NCEES National Council of Examiners for Engineering and Surveying Practice Environmental Engineering 1 — Questions and Answers
Question 1: In NCEES PE Environmental engineering, the Food-to-Microorganism (F/M) ratio in an activated sludge system is correctly expressed as:
- Influent BOD load (kg/d) divided by the mass of MLVSS in the aeration tank (kg) (Correct answer)
- Effluent BOD concentration divided by the return activated sludge (RAS) flow rate
- Mixed Liquor Suspended Solids (MLSS) concentration divided by hydraulic retention time (HRT)
- Oxygen uptake rate divided by the sludge volume index (SVI)
Correct answer: Influent BOD load (kg/d) divided by the mass of MLVSS in the aeration tank (kg)
F/M = (Q × S₀) / (V × X), where Q is influent flow, S₀ is influent BOD, V is reactor volume, and X is MLVSS. It quantifies substrate loading per unit biomass and controls treatment efficiency and sludge settleability.
Question 2: Under the Clean Water Act, a National Pollutant Discharge Elimination System (NPDES) permit is required for:
- Atmospheric dry deposition of particulates onto navigable surface waters
- Any point source discharge of pollutants to waters of the United States (Correct answer)
- Nonpoint source agricultural runoff leaving cropland during storm events
- Groundwater infiltration from subsurface septic drain fields
Correct answer: Any point source discharge of pollutants to waters of the United States
NPDES permits are mandated by CWA Section 402 for any discrete conveyance (point source) discharging pollutants to U.S. waters. Nonpoint sources, atmospheric deposition, and groundwater infiltration are generally outside NPDES permitting scope.
Question 3: Under RCRA (Resource Conservation and Recovery Act), the four characteristics that can cause a solid waste to be classified as hazardous are:
- Ignitability, corrosivity, reactivity, and toxicity (Correct answer)
- Volatility, radioactivity, carcinogenicity, and flammability
- Persistence, bioaccumulation potential, acute lethal dose, and pH extremes
- Oxidizing potential, vapor pressure, solubility, and flash point below 100°F
Correct answer: Ignitability, corrosivity, reactivity, and toxicity
RCRA Subpart C (40 CFR Part 261) defines four hazardous waste characteristics using the acronym ICRT: Ignitability (flash point <60°C), Corrosivity (pH ≤2 or ≥12.5), Reactivity (unstable/explosive), and Toxicity (TCLP leachate exceeds regulatory limits for listed metals/organics).
Question 4: In Gaussian plume air dispersion modeling, the downwind distance at which maximum ground-level centerline concentration occurs corresponds to the condition where:
- The horizontal dispersion coefficient σy equals the effective stack height H
- The vertical dispersion coefficient σz equals the effective stack height H divided by √2 (Correct answer)
- Atmospheric stability class transitions from unstable to neutral
- Wind speed at stack height equals the surface friction velocity u*
Correct answer: The vertical dispersion coefficient σz equals the effective stack height H divided by √2
Differentiating the ground-level Gaussian plume equation with respect to downwind distance and setting it to zero yields σz = H/√2 at the point of maximum ground-level concentration. This relationship is used to estimate worst-case receptor impacts in permit modeling.
Question 5: The EPA Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBPR) sets the Maximum Contaminant Level (MCL) for total trihalomethanes (TTHMs) at:
- 40 μg/L evaluated as a locational running annual average (LRAA)
- 80 μg/L evaluated as a locational running annual average (LRAA) (Correct answer)
- 100 μg/L evaluated as a system-wide annual average
- 120 μg/L evaluated as a maximum contaminant level goal (MCLG)
Correct answer: 80 μg/L evaluated as a locational running annual average (LRAA)
Stage 2 DBPR (40 CFR Part 141, Subpart V) retains the 80 μg/L TTHM MCL from Stage 1 but tightens compliance by requiring it be met as a Locational Running Annual Average (LRAA) at every monitoring location, eliminating the system-wide averaging that previously masked high-exposure sites.
Question 6: In NCEES PE Environmental engineering, the Solids Retention Time (SRT) — also called sludge age or mean cell residence time (MCRT) — is defined as:
- The aeration tank volume divided by the influent volumetric flow rate
- The total mass of mixed liquor suspended solids (MLSS) in the system divided by the mass of solids wasted per day (Correct answer)
- The ratio of effluent suspended solids concentration to influent suspended solids concentration
- The time for dissolved oxygen to drop from saturation to 2 mg/L under process loading
Correct answer: The total mass of mixed liquor suspended solids (MLSS) in the system divided by the mass of solids wasted per day
SRT = (V × X) / (Q_w × X_w + Q_e × X_e), representing the average time a microorganism remains in the system. SRT controls biomass selection: nitrifiers require SRT > ~10 days at 20°C, while shorter SRTs favor heterotrophic BOD removal only.
NCEES National Council of Examiners for Engineering and Surveying Practice Environmental Engineering 1
In NCEES PE Environmental engineering, the Food-to-Microorganism (F/M) ratio in an activated sludge system is correctly expressed as: