Wastewater Treatment Operator Class III key terms

A laboratory measure of the oxygen that microorganisms consume while breaking down organic matter in a water sample over a set period, usually five days at 20 degrees Celsius. It estimates how much biodegradable organic material the wastewater carries.

A test that uses strong chemical oxidants, heat, and acid to measure nearly all oxidizable matter in a sample, including material that microorganisms cannot easily degrade. Results come quickly and are often compared with BOD results.

The concentration of solid particles that remain on a glass-fiber filter after a measured sample is filtered, dried, and weighed. It shows how much settleable and colloidal material is present in the flow.

The two-step biological conversion of ammonia to nitrite and then to nitrate by specialized autotrophic bacteria that need dissolved oxygen and alkalinity. The process consumes oxygen and can lower the pH of the mixed liquor.

The biological conversion of nitrate to nitrogen gas by facultative bacteria working where free oxygen is absent. The gas escapes to the atmosphere, which removes nitrogen from the wastewater before it leaves the plant.

Clumps of microorganisms, organic matter, and inorganic particles that stick together in a biological treatment system and settle out later in the clarifier. Dense, compact floc separates cleanly from the water.

A channel or tank placed after the screens where flow slows enough for heavy abrasive material such as sand, eggshells, and coffee grounds to settle out while lighter organic solids move downstream. Removing grit protects pumps and digesters.

A settling tank that holds flow long enough for settleable solids to drop to the bottom and floatable material to rise to the surface, removing a large share of suspended solids and BOD before biological treatment.

The layer of settled solids that accumulates on the bottom of a clarifier. Its depth shows whether sludge is being removed fast enough to prevent carryover and septic conditions in the tank.

The flow through a clarifier divided by its surface area, usually expressed as gallons per day per square foot. It describes the upward velocity of water and predicts whether particles have time to settle.

The total concentration of solids, both biological and inorganic, suspended in the aeration tank. Operators use it with the food-to-microorganism ratio to control how much biomass is kept in the system.

The pounds of BOD entering the aeration system each day divided by the pounds of microorganisms under aeration. It guides sludge wasting decisions and helps predict whether the sludge will settle well.

The average length of time biological solids stay in the treatment system, also called solids retention time. It is controlled mainly by wasting and affects nitrification, oxygen demand, and sludge production.

A calculated number from a settled sludge test that shows how many milliliters one gram of mixed liquor solids occupies after 30 minutes of settling. Higher values mean poorer settling and greater bulking risk.

Settled biological solids collected in the secondary clarifier and pumped back to the aeration tank to keep the desired biomass concentration in the mixed liquor. Return rates are adjusted to control blanket depth.

Excess biological solids deliberately removed from the treatment system, usually from the return line, to hold the solids retention time and biomass inventory at the levels the operator wants.

Long, thread-like bacteria that form the backbone of healthy activated sludge floc at low numbers but can overgrow to cause poor settling, cloudy effluent, and bulking when conditions favor them.

A fixed-film biological reactor where wastewater trickles over beds of rock or plastic media coated with a slime layer of microorganisms that absorb and break down organic matter as air moves through the voids.

The amount of chlorine remaining in the water after the initial demand is satisfied, present as free available chlorine or as combined chloramines. It is measured to confirm disinfection and protect the receiving stream.

The product of disinfectant residual concentration and the contact time achieved, adjusted for temperature and pH. It is used to judge how much pathogen inactivation a disinfection process is providing.

A group of bacteria used as indicators of fecal contamination and of treatment effectiveness. Their presence in disinfected effluent suggests that pathogens may have survived the disinfection step.

The addition of a reducing chemical such as sulfur dioxide or sodium bisulfite to remove leftover chlorine before discharge, protecting fish and other aquatic life from toxic residual levels in the receiving water.

Treatment that inactivates microorganisms by exposing them to ultraviolet light. Effectiveness depends on lamp output, the clarity of the water, and how evenly the flowing water receives the dose.

A tank where solids settle and compact under their own weight, producing a denser underflow and a clearer overflow so that downstream digestion and dewatering equipment can be smaller and run more efficiently.

Mechanical removal of water from sludge using equipment such as belt presses, centrifuges, or screw presses, raising the solids content from a few percent to a manageable, stackable cake.

The concentrated solid product leaving a dewatering device, expressed as a percentage of total solids. Higher cake solids mean less water is hauled, which lowers transport and disposal costs.

A process in which microorganisms break down organic solids without oxygen, producing methane-rich gas and reducing the mass and pathogen content of the sludge. Digesters are usually heated and mixed.

Infiltration is groundwater entering a sewer through defects; inflow is stormwater entering through direct connections such as roof drains and manholes. Both add clear water that overloads the collection system and the treatment plant.

A release of untreated wastewater from the collection system before it reaches the treatment plant, often caused by blockages, heavy infiltration, or pump failures. It must be reported to regulators.

A facility with pumps and a wet well that raises wastewater to a higher elevation so it can continue flowing by gravity, or that pushes it through a force main toward the plant.

The storage chamber at a pumping station that receives incoming flow and holds it while the pumps cycle. Its volume and pump settings must be sized to avoid septic conditions and excessive pump starts.

A single portion of wastewater collected at one moment and one place, useful for parameters that change quickly or must be tested immediately, such as dissolved oxygen, pH, and chlorine residual.

A sample made by combining portions of wastewater collected at intervals over time, or in proportion to flow, to represent the average quality of a discharge or process stream.

The documented record of who collected, handled, and stored a sample at each step, with dates and signatures, so that results remain defensible for regulatory reporting.

The water's capacity to neutralize acid, contributed mainly by bicarbonate, carbonate, and hydroxide. It buffers pH swings and is consumed as a side effect of nitrification in the aeration tank.

A federal permit issued under the Clean Water Act that sets effluent limits, monitoring requirements, and reporting duties for wastewater discharged to surface waters of the United States.

The periodic report a permittee submits to the regulatory agency showing monitoring results, any violations, and compliance status as required by its discharge permit. Errors or late submittals can bring enforcement action.

An enclosed area with limited entry and exit that is not designed for continuous occupancy and may hold a hazardous atmosphere. Entry requires air testing, ventilation, an attendant, and a completed entry permit.

A procedure for isolating energy sources with locks and tags so that equipment cannot be started while maintenance or repair work is in progress, protecting workers from sudden release of stored energy.