Glossary of Key Terms
Glossary of Key Terms
| Term | Definition |
| Acclimation | The adaptive response of microorganisms becoming more tolerant to an inhibitory concentration of a toxic substance, either through neutralization or selective growth. |
| Activated Sludge | A biologically active mass, continuously mixed with biodegradable matter in an aeration basin, which is subsequently separated by sedimentation and partially recirculated. |
| Aerobic | A process that proceeds in the presence of free oxygen. |
| Anaerobic | A process that proceeds in the absence of free oxygen. |
| Aufban & Abban | A hypothesis describing microbial growth as the result of competition between two processes: Aufban (assimilation or building up) and Abban (endogenous metabolism or breaking down). |
| Biochemical Oxygen Demand (BOD) | A measure of the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample. It is used as a measure of wastewater characteristics. |
| Biological Solids Retention Time (θx) | Also known as Sludge Age or Mean Cell Retention Time. It is the ratio of the total active microbial mass in the treatment system to the total quantity of active microbial mass withdrawn daily. |
| Biomass | A collection of microorganisms (such as bacteria, yeast, molds, protozoa) in a gelatinous mass, often forming a biological floc with good settling characteristics. |
| Chemical Oxygen Demand (COD) | A measure of the amount of oxygen required to chemically oxidize the organic and inorganic compounds in water. It is used to characterize wastewater. |
| Completely Mixed Reactor | An idealized reactor type in which the contents are well stirred and uniform in composition throughout. Influent is dispersed rapidly through the mass. |
| Denitrification | The third stage of biological nitrogen removal, where nitrites and nitrates are converted to atmospheric nitrogen under anaerobic conditions, often with a supplemental carbon source like methanol. |
| Endogenous Respiration | The internal metabolic process in microorganisms that results in the auto-digestion or self-destruction of cellular material to provide energy for maintenance, occurring when the external food supply is limited. |
| Fixed-Film System | A biological treatment system (e.g., trickling filter, RBC) where the biomass remains stationary, attached to a solid supporting media (like rocks or plastic), while wastewater flows around it. |
| Haldane Equation | A kinetic equation that describes inhibitory substrate reactions, showing that the specific growth rate of microorganisms decreases at very high substrate concentrations due to toxic effects. |
| Hydraulic Retention Time (t̄) | Also known as Detention Time. It is the average length of time a soluble compound remains in a reactor, calculated as the ratio of the reactor volume to the volumetric feed rate (V/Q). |
| Microbial Decay Rate (b) | The rate of auto-oxidation or decay of microbial cells, representing the loss of cell mass due to endogenous respiration. |
| Mixed Liquor | The combination of wastewater and activated sludge within an aeration basin. |
| Monod Equation | A widely accepted kinetic equation that describes the relationship between the specific growth rate of microorganisms (μ) and the substrate concentration (S), where growth rate increases with substrate concentration up to a maximum. |
| Nitrification | The second stage of biological nitrogen removal, an aerobic process where ammonia is oxidized first to nitrites (by Nitrosomonas) and then to nitrates (by Nitrobacter). |
| Plug Flow Reactor | An idealized reactor type in which fluid flows in an orderly progression without any element of the flowing fluid overtaking another. |
| Process Loading Factor (U) | Also known as the Food to Microorganisms (F/M) Ratio. It is the ratio between the mass of substrate utilized per day and the mass of active microorganisms in the reactor. |
| Rotating Biological Contactor (RBC) | A fixed-film treatment process where large, rotating plastic disks are half-submerged in wastewater, allowing biomass to grow on the surface and absorb oxygen from the air and substrate from the water. |
| Sloughing | The process in a trickling filter where an old, thick microbial film detaches from the filter medium, often due to anaerobic activity at the film-media interface, allowing a new film to grow. |
| Substrate | The degradable substances, usually organic, present in wastewater that serve as food for microorganisms during biological treatment. |
| Suspended-Contact System | A biological treatment system (e.g., activated sludge) where both the biomass and the substrate are kept in suspension or motion within the reactor. |
| Toxicity | The property of a substance to deter or inhibit the metabolic processes of cells, which is a function of the substance’s nature, concentration, time of exposure, and environmental conditions. |
| Trickling Filter | A fixed-film biological treatment process where wastewater is applied intermittently to a fixed bed of media (like stones or plastic), allowing a microbial slime to grow and treat the wastewater as it trickles through. |
| Volatile Suspended Solids (VSS) | The volatile fraction of suspended solids in wastewater. In a biological reactor, VSS is often assumed to represent the mass of active microorganisms present per unit volume. |
| Yield Coefficient (Y) | The growth yield coefficient, representing the ratio of cell mass synthesized (ΔX) to the amount of substrate removed (ΔS) in a given time. |