Environmental Engineering 1 Flashcards

In NCEES PE Environmental engineering, the Food-to-Microorganism (F/M) ratio in an activated sludge system is correctly expressed as:

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.

Under the Clean Water Act, a National Pollutant Discharge Elimination System (NPDES) permit is required for:

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.

Under RCRA (Resource Conservation and Recovery Act), the four characteristics that can cause a solid waste to be classified as hazardous are:

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).

In Gaussian plume air dispersion modeling, the downwind distance at which maximum ground-level centerline concentration occurs corresponds to the condition where:

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.

The EPA Stage 2 Disinfectants and Disinfection Byproducts Rule (Stage 2 DBPR) sets the Maximum Contaminant Level (MCL) for total trihalomethanes (TTHMs) at:

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.

In NCEES PE Environmental engineering, the Solids Retention Time (SRT) — also called sludge age or mean cell residence time (MCRT) — is defined as:

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.