FE Environmental key terms

The mass of dissolved oxygen consumed by microorganisms while decomposing organic matter in a water sample incubated at 20 °C in the dark for five days. Reported as mg/L and used to gauge organic pollution strength and treatment performance.

The oxygen demand from microbial oxidation of carbon-containing organic matter only, with nitrogenous demand suppressed by a nitrification inhibitor. Distinguishes carbon oxidation from ammonia oxidation when interpreting BOD test results.

The total oxygen demand exerted as biodegradable organic matter is fully oxidized, approached asymptotically over long incubation. Related to five-day BOD through the first-order decay constant and used to size aeration and estimate remaining demand.

A first-order rate coefficient, base e or base 10, describing how quickly BOD is exerted over time. Typical municipal values near 0.23 per day (base e) at 20 °C; adjusted for temperature using a theta factor.

Biological oxidation of ammonia to nitrite by Nitrosomonas-type bacteria and then to nitrate by Nitrobacter-type bacteria, consuming about 4.57 g oxygen per g ammonia nitrogen. Causes the nitrogenous second-stage rise in BOD curves.

Reduction of nitrate to nitrogen gas by facultative bacteria under anoxic conditions using nitrate as the electron acceptor. Used for total nitrogen removal and can recover alkalinity and reduce aeration demand.

A reactor compartment with no dissolved oxygen but containing nitrate, where denitrifying bacteria convert nitrate to nitrogen gas. Often precedes or follows aerobic zones in biological nutrient removal configurations.

A suspended-growth biological process in which a mixed microbial culture aerobically consumes organics and forms floc that is settled in a clarifier and partly returned to the aeration basin to maintain biomass.

The concentration of suspended solids, mostly biomass, in the aerated mixed liquor of an activated sludge basin. Used with return rates and clarifier loading to control biomass inventory and effluent quality.

Solids retention time, the average time biomass remains in the treatment system, computed as mass of solids in the system divided by mass wasted plus lost per day. Governs nitrification, settling, and stability.

The mass of organic substrate entering per day divided by the mass of microorganisms in the aeration system. High values favor dispersed growth; low values favor stable floc and nitrification.

A fixed-film biological reactor in which wastewater trickles over media coated with biofilm, and air moves through voids to supply oxygen. Organic loading and hydraulic loading per unit media volume guide design.

A fixed-film process using partially submerged rotating disks whose biofilm alternately contacts wastewater and air, providing oxygen transfer and sloughing control. Staged units improve nitrification and organic removal.

The mass of organic matter, usually BOD or COD, applied per unit reactor volume or media surface per day. Used to size biological units and predict performance and clogging risk.

The volume of water applied per unit area or surface per day, expressed as flow per area. Controls contact time, overflow rates in clarifiers, and percolation performance in soil or filter systems.

The flow divided by the clarifier or basin surface area, expressed as volume per area per day or as a velocity. Sets the settling velocity a particle must exceed to be captured.

The effluent flow divided by the total length of the outlet weir, expressed as volume per length per day. Excessive values can draw solids over the weir and degrade clarifier performance.

The product of disinfectant residual concentration and contact time, used with a required CT to demonstrate inactivation of pathogens. Adjusted by baffling factors and temperature-dependent credit.

The amount of chlorine consumed by oxidizable substances in water before a free residual persists, including ammonia, organics, and reduced metals. Determines dosing needed to achieve a target residual.

The chlorine dose beyond which combined chlorine has been oxidized and additional chlorine appears as free residual. Dosing past the breakpoint improves disinfection and reduces chloramine taste and odor.

A group of gram-negative, lactose-fermenting bacteria used as indicator organisms for fecal contamination and treatment effectiveness. Regulated as total coliform and fecal or E. coli limits in drinking water and effluents.

The cloudiness of water caused by suspended and colloidal particles that scatter light, measured in nephelometric turbidity units. Low turbidity is required for effective disinfection and is a key drinking water limit.

Chemical and hydraulic steps that destabilize colloidal particles and gather them into larger, settleable flocs. Common coagulants include alum, ferric salts, and polymers; pH and mixing energy strongly affect performance.

Gravity separation of suspended particles denser than water in a basin with low horizontal velocity. Design uses overflow rate and detention time; type I settling of discrete particles follows Stokes' law.

An expression for the terminal settling velocity of a spherical particle in a viscous fluid, proportional to the density difference and particle diameter squared, and inversely proportional to viscosity. Basis for sizing grit chambers and clarifiers.

A pretreatment unit that removes dense, abrasive solids such as sand and gravel while keeping lighter organics in suspension. Designed around a target particle size and a controlled velocity or detention time.

A tank in which sludge is aerated so microorganisms consume volatile solids, reducing mass and odor potential. Performance depends on solids retention time, temperature, and oxygen supply.

An enclosed, oxygen-free reactor where bacteria convert sludge solids to methane and carbon dioxide in stages including hydrolysis, acidogenesis, acetogenesis, and methanogenesis. Produces biogas and stabilizes solids.

Liquid that has percolated through solid waste and carries dissolved and suspended contaminants. Collected and treated or recirculated; composition varies widely with waste age and moisture.

A liner-covered drainage layer of permeable material and perforated pipes that intercepts liquid above the base liner and conveys it to a sump for removal. Prevents groundwater contamination and head buildup on the liner.

A landfill barrier made of a low-permeability geomembrane in direct contact with a compacted clay layer, so each material complements the other's weaknesses. Regulated design includes leachate collection above it.

An estimate of the total methane a mass of waste can produce under anaerobic decomposition, often expressed as volume per dry mass. Used with decay rates in landfill gas models for collection and energy design.

Transfer of volatile contaminants from water to air by contacting the two phases in a packed tower or diffused system. Removal depends on Henry's law constant, temperature, and air-to-water ratio.

A partition coefficient relating the concentration of a dissolved volatile substance to its partial pressure in the gas phase at equilibrium. Larger values indicate greater volatility and easier air stripping.

A porous carbon adsorbent used to remove dissolved organics, taste and odor compounds, and some contaminants from water or air. Capacity is described by isotherms; spent carbon is regenerated or replaced.

An empirical relationship expressing adsorbed mass per adsorbent mass as a power function of equilibrium concentration. Used with batch tests to estimate carbon dose and breakthrough behavior.

A multiplier applied to a rate constant to adjust for temperature, typically of the form theta raised to the temperature difference. Common theta values approximate 1.047 for BOD and 1.07 for biological treatment.

An accounting of material entering, leaving, accumulating in, and being generated or consumed within a control volume. The core tool for solving mixing, reactor, and emission problems on the exam.

An ideal reactor assumption in which contents are instantly and uniformly mixed, so effluent concentration equals reactor concentration. Used for aeration basins and chemical feed tanks in mass balances.

An ideal reactor assumption in which fluid moves as ordered slices with no longitudinal mixing, giving a residence time distribution with no spread. Contrasts with complete mix behavior in disinfection and kinetics.