Wastewater Terminology, Defined Plainly
The glossary every new operator keeps open in a second tab.
BOD (Biochemical Oxygen Demand)
Dissolved oxygen consumed by microorganisms breaking down organic matter.
MLSS (Mixed Liquor Suspended Solids)
Concentration of solids in the aeration tank's mixture of wastewater and activated sludge.
SRT (Solids Retention Time)
Average time solids remain in the treatment system before being wasted or removed.
Zone Settling Velocity
The rate at which a sludge blanket settles during clarification.
COD (Chemical Oxygen Demand)
A rapid chemical test estimating the total oxidizable material in a sample.
TSS (Total Suspended Solids)
The mass of solid material suspended in a water sample, measured by filtration and drying.
VSS (Volatile Suspended Solids)
The organic (combustible) portion of suspended solids, a proxy for living biomass.
HRT (Hydraulic Retention Time)
The average time water spends in a tank or process unit, driven by flow and volume.
TKN (Total Kjeldahl Nitrogen)
The sum of organic nitrogen and ammonia nitrogen in a sample, a key input for nitrogen removal calculations.
Alkalinity
The water's capacity to resist pH change, commonly expressed as mg/L CaCO3.
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