GATE ES key terms

Biochemical oxygen demand measured over five days at 20 °C in the dark, expressing the oxygen consumed by microorganisms degrading biodegradable organic matter in a water sample, reported in mg/L.

Chemical oxygen demand: oxygen equivalent of organic (and some inorganic) matter oxidized by a strong chemical oxidant such as potassium dichromate under acidic, heated conditions, reported in mg/L.

The ratio BOD5/COD, used to judge whether a wastewater stream is amenable to biological treatment; values around 0.4 or higher generally indicate good biodegradability.

Volumetric oxygen mass-transfer coefficient (per hour) describing the rate at which oxygen dissolves into mixed liquor; it determines whether aeration can meet the oxygen demand of the biomass.

A suspended-growth biological process in which recycled biomass oxidizes dissolved organics in an aerated tank, followed by gravity separation in a clarifier to produce a clarified effluent.

Volume in millilitres occupied by one gram of settled activated sludge after 30 minutes of settling; high values indicate poor settling (bulking) and threaten solids washout from the clarifier.

The average period water remains in a treatment unit, computed as reactor volume divided by flow rate; used with solids retention time to size biological reactors.

Two-step microbial oxidation of ammonium to nitrite and then nitrate by autotrophic bacteria, consuming oxygen and alkalinity; it can be followed by denitrification to remove total nitrogen.

Anoxic microbial reduction of nitrate to nitrogen gas using organic carbon as electron donor, the principal pathway for removing total nitrogen from wastewater.

Sequential microbial breakdown of organic solids without oxygen through hydrolysis, acidogenesis, acetogenesis and methanogenesis, producing biogas (methane and carbon dioxide) while stabilizing sludge.

The fraction of a solid sample lost on ignition at about 550 °C, used as a surrogate for organic content in sludge and for measuring the degree of stabilization.

Mass of particles retained on a glass-fibre filter after drying, expressed in mg/L; a key effluent quality parameter and a driver of turbidity and disinfection interference.

Reduction in clarity of water caused by suspended particles scattering light, measured in nephelometric turbidity units (NTU); a rapid proxy for particulate contamination and filter performance.

Optical measurement of light scattered at an angle (commonly 90°) by particles in a sample; the technique underlying turbidimeters used in water treatment and instrument calibration.

Quantitative technique measuring absorbance of light at a chosen wavelength by a coloured species, with concentration obtained via Beer-Lambert law; widely used for nutrients, metals and COD analysis.

Destabilization of colloidal particles by coagulant addition, followed by gentle mixing that grows particles into settleable flocs; the core of conventional surface-water treatment.

Electric potential at the slipping plane of a particle in suspension, indicating the strength of electrostatic repulsion between colloids; near-zero values favour coagulant-driven destabilization.

Dosing chlorine beyond the point where reducing agents and ammonia are fully oxidized, after which free residual chlorine rises steeply, providing reliable disinfection.

Product of disinfectant residual concentration and contact time, used to express the disinfection dose delivered against a target pathogen; higher CT means greater inactivation.

Model relating adsorbed amount per unit mass of adsorbent to equilibrium concentration, assuming a finite number of identical sites forming a monolayer; gives a saturation capacity.

Empirical power-law relation between adsorbed amount and equilibrium concentration, q = Kf·C^(1/n), widely fitted to adsorption data for activated carbon and soils.

Accumulation of dissolved species at the surface of a solid such as activated carbon or soil, driven by surface forces; a key removal mechanism for trace organics and metals.

Analytical steady-state solution of the advection-diffusion equation giving downwind pollutant concentration from a continuous elevated point source under specified stability and wind conditions.

Classification of atmospheric turbulence from A (very unstable) to F (very stable) based on solar radiation, cloud cover and wind speed, used to select dispersion coefficients.

Sum of physical stack height and plume rise due to buoyancy and momentum; the virtual release height used in dispersion calculations for a stack.

Depth of the atmospheric layer through which a pollutant is mixed by turbulence, often capped by a temperature inversion; a key parameter in box and dispersion models.

Atmospheric layer in which temperature increases with height, suppressing vertical mixing and trapping pollutants near the ground, commonly forming on calm clear nights.

A-weighted sound level of a steady noise that would deliver the same acoustic energy as a fluctuating noise over a stated period; the standard descriptor for environmental noise.

Sound pressure expressed in decibels relative to 20 micropascals, using a logarithmic scale so that doubling acoustic energy adds about 3 dB.

Downward movement of soluble constituents from solid waste or soil into percolating water, generating leachate that must be contained and treated at landfills.

Liquid that has percolated through waste and extracted dissolved and suspended contaminants, characterized by high organic strength and often heavy metals; requires collection and treatment.

Coefficient describing the ease with which water flows through a porous medium under a unit hydraulic gradient; governs seepage, landfill liner performance and aquifer yield.

Linear relation stating that specific discharge through a porous medium equals hydraulic conductivity times hydraulic gradient; the foundation of groundwater flow analysis.

A saturated geologic formation capable of storing and transmitting water in usable quantities; characterized by storativity and transmissivity in groundwater assessment.

Direct runoff hydrograph resulting from one unit of effective rainfall uniformly distributed over a catchment in a specified duration, used to derive flood hydrographs for other storms.

Time required for water to travel from the hydraulically most distant point of a catchment to the outlet; sets the duration of design storms in rational-method calculations.

Dimensionless fraction of rainfall that becomes surface runoff, depending on land use, soil and slope; multiplied by rainfall intensity and area in the rational method.

Structured process predicting environmental consequences of a proposed project before approval, comparing alternatives and defining mitigation; the resulting statement supports decision-making.

Maximum population or development intensity an ecosystem or region can sustain indefinitely without degrading its supporting resources and assimilative capacity.

Numerical measure combining species richness and evenness in a community, such as the Shannon-Wiener index; used to compare ecological quality before and after disturbance.

Statistical procedure comparing observed data with a null hypothesis, yielding a p-value; rejection occurs when p is below the chosen significance level, risking type I error.

Range of plausible values for a population parameter constructed from sample data at a stated confidence level; width shrinks as sample size increases.