Wastewater Treatment Plant Operator key terms

A device of parallel bars or rods set in the influent channel that traps rags, plastics, and other large debris so downstream equipment and pumps are not damaged or clogged.

A channel or tank, often with a velocity-control device, that slows flow enough for dense abrasive material such as sand and eggshells to settle while lighter organic solids stay suspended.

A settling tank that holds raw wastewater long enough for settleable solids to form sludge on the bottom and grease to float, reducing the load on downstream biological treatment.

The biological process in which a mixed population of microorganisms, kept suspended by aeration or mixing, consumes dissolved and colloidal organic matter in wastewater and is then separated in a clarifier.

The total concentration of suspended solids, mostly bacteria plus some inert material, in the aeration tank contents, usually measured in milligrams per liter and used to gauge the biomass inventory.

Pounds of biochemical oxygen demand entering the aeration system per day divided by pounds of volatile suspended solids under aeration; it shows how heavily the biomass is loaded and guides wasting decisions.

The average time, in days, that solids remain in the activated sludge system, calculated by dividing the total solids in the system by the solids leaving each day; also called sludge age.

The volume in milliliters occupied by one gram of settled mixed liquor after 30 minutes of settling in a one-liter cylinder; a high value signals poor settling and possible bulking.

The two-step biological oxidation of ammonia to nitrite and then to nitrate by nitrifying bacteria, a process that consumes oxygen and alkalinity and is sensitive to cold water and low dissolved oxygen.

The biological conversion of nitrate to nitrogen gas by bacteria that use nitrate as an oxygen source when dissolved oxygen is low; the gas escapes to the atmosphere, removing nitrogen from the water.

A bed of coarse media over which wastewater is distributed and trickles downward, growing a biological film that absorbs and oxidizes organic matter, with air supplied by natural draft or blowers.

A pond, usually one to two meters deep, whose upper layer stays aerobic from algae and surface aeration while the bottom stays anaerobic; settling and biological treatment occur in the same basin.

A treatment train that pairs a suspended-growth biological process with a microporous or ultrafiltration membrane separating solids from water, replacing the secondary clarifier and producing very low-solids effluent.

A bed of sand, anthracite, or similar material through which secondary effluent passes so remaining fine particles are trapped, lowering suspended solids and turbidity before disinfection or reuse.

A membrane process in which pressure pushes water through a semipermeable membrane that rejects dissolved salts and most other dissolved constituents, producing a purified permeate and a concentrated reject stream.

The amount of chlorine consumed by microorganisms, ammonia, organic matter, and other reducing substances in the water before a measurable free chlorine residual can persist.

The product of the disinfectant residual concentration, in milligrams per liter, and the effective contact time, in minutes; regulators use it to show that required pathogen inactivation has been achieved.

The residual formed when chlorine reacts with ammonia to create chloramines, which persist longer than free chlorine but are much weaker disinfectants and are harder to remove from the water.

The percentage of ultraviolet light at 254 nanometers that passes through a water sample over a fixed path; low transmittance means a higher UV dose is needed to inactivate pathogens.

The addition of a reducing chemical such as sodium bisulfite or sulfur dioxide to effluent after chlorination so the toxic chlorine residual is eliminated before discharge to the receiving water.

A tank where sludge is held quietly so solids settle and concentrate at the bottom, reducing the volume that must be pumped to digesters, dewatering units, or hauled away.

A heated, mixed, oxygen-free process in which bacteria break down organic solids in stages, producing methane-rich biogas while reducing volatile solids, pathogens, and odors in the sludge.

The percentage decrease in volatile organic solids across a digester, computed from feed and withdrawn sludge samples; it is the standard measure of digestion performance and stability.

A dewatering machine that conditions sludge with polymer, then squeezes it between moving porous belts under increasing pressure to release water and form a handleable cake.

Infiltration is groundwater seeping into sewers through cracks and joints; inflow is stormwater or drainage entering directly through manhole covers, roof drains, and illegal connections. Both add clear water the plant must handle.

The storage chamber at a lift station into which incoming sewage flows and where pumps are started and stopped by float switches or level sensors to move wastewater to a higher elevation.

A release of untreated or partially treated sewage from a sanitary sewer system before it reaches the treatment plant, usually caused by blockages, roots, grease, structural failures, or excess flow.

A pipe carrying wastewater under pressure from a pump station to a point where gravity flow can resume; it typically needs air-release valves and careful velocity control to prevent solids deposition.

A laboratory test measuring the oxygen consumed by microorganisms as they break down organic matter in a sample over a set incubation period, commonly five days at 20 degrees Celsius.

The mass of filterable solids in a measured volume of water, expressed in milligrams per liter after drying the filter at a specified temperature; used to track treatment performance and permit compliance.

A sample made by combining several portions collected at intervals or in proportion to flow over a period, giving a result that represents average conditions rather than a single moment.

A written, signed record of who collected, handled, transported, and stored a sample and when, so the resulting data can be defended as legally valid in enforcement or permit reporting.

The comparison of an instrument's reading against a known standard and adjustment of the instrument so its output matches that standard, documented before use to keep measurements trustworthy.

The formation and collapse of vapor bubbles in a pump when local pressure drops below the liquid's vapor pressure, producing noise, vibration, and pitting damage to the impeller and casing.

A space large enough for a worker to enter, with limited entry and exit, and not designed for continuous occupancy; permit-required examples at plants include wet wells, digesters, and manholes.

A colorless, highly toxic, flammable gas with a rotten-egg odor that forms where sewage turns septic; it deadens the sense of smell at higher concentrations and corrodes concrete and metal.

A required procedure in which energy sources are isolated, locked, and tagged before maintenance so equipment cannot be started and stored energy cannot be released while a worker is exposed.

A federal and state permit authorizing a facility to discharge treated wastewater to surface waters, setting effluent limits, monitoring requirements, and reporting obligations the operator must meet.

The periodic report a permittee submits to the regulatory agency listing monitoring results, flow, and compliance status for each effluent limit, signed by an authorized official.