Glossary

125 terms, A to Z.

A

Activated sludge: A biological treatment process in which a suspended population of microorganisms (mixed liquor) is aerated with wastewater, separated in a clarifier and returned.
Aerobic: A condition with dissolved oxygen present.
Aerobic digestion: Stabilization of sludge by aeration, used mostly at smaller plants.
Alkalinity: The capacity of water to neutralize acid, reported as mg/L CaCO3. Nitrification consumes 7.14 mg/L per mg/L ammonia-N.
Alpha factor: Ratio of oxygen transfer in mixed liquor to transfer in clean water. Often 0.4 to 0.7 for fine bubble diffusers.
Ammonia (NH3-N): Reduced nitrogen in wastewater, toxic to aquatic life and oxidized by nitrifiers. Usually reported as mg/L nitrogen.
Ammonia based aeration control (ABAC): Control strategy that adjusts DO setpoints from an ammonia measurement.
Anaerobic: A condition with no dissolved oxygen and no nitrate.
Anaerobic digestion: Stabilization of sludge without oxygen, producing biogas (methane and carbon dioxide).
Anoxic: A condition with no dissolved oxygen but nitrate present; where denitrification occurs.
AOR: Actual oxygen requirement: oxygen needed by the process under field conditions, lb/d.
Autotroph: An organism that uses inorganic carbon (CO2) for growth, such as nitrifying bacteria.

B

Biochemical oxygen demand (BOD): The oxygen used by microorganisms to oxidize organic matter over 5 days at 20 C (BOD5).
Biosolids: Treated sewage sludge that meets regulatory requirements for beneficial use.
Blanket (sludge blanket): The layer of settled sludge at the bottom of a clarifier.
Blower: Machine that supplies air to diffusers. Types include centrifugal, turbo and positive displacement.
BNR: Biological nutrient removal: biological removal of nitrogen and/or phosphorus.
Bulking: Poor settling and compaction of activated sludge, usually from excess filaments or slime.

C

Carbonaceous BOD (CBOD): BOD with nitrification inhibited, measuring only carbon oxidation.
Centrate: Liquid removed by a centrifuge during dewatering; often high in ammonia.
Chemical oxygen demand (COD): Oxygen equivalent of organic matter oxidized by a strong chemical oxidant.
Chloramine: Compound formed when chlorine reacts with ammonia; a weaker disinfectant than free chlorine.
Chlorine demand: Chlorine consumed by reactions with substances in water before a residual forms.
Chlorine residual: Chlorine remaining after demand is satisfied.
Clarifier: A tank where solids settle by gravity; primary or secondary.
Coagulant: Chemical (alum, ferric, PAC) that destabilizes particles so they clump.
Complete mix: Reactor where contents are uniformly mixed.
Composite sample: Sample made of multiple portions collected over time, often flow proportional.
Contact stabilization: Activated sludge variation with a short contact tank and a separate RAS reaeration tank.
Critically loaded: State point condition where the clarifier is near its thickening limit.

D

Denitrification: Biological conversion of nitrate to nitrogen gas under anoxic conditions.
Detention time: Volume divided by flow; the average time water spends in a tank.
Dewatering: Removing water from sludge to make cake, using centrifuges, belt presses or screw presses.
Diffuser: Device that releases air bubbles into the aeration basin.
Dispersed growth: Bacteria growing as single cells rather than floc; causes turbid effluent.
Dissolved oxygen (DO): Oxygen dissolved in water, mg/L.

E

EBPR: Enhanced biological phosphorus removal using phosphorus accumulating organisms.
Effluent: Water leaving a process or plant.
Endogenous respiration: Microorganisms consuming their own cell material when food is scarce.
EPS: Extracellular polymeric substances: sticky material produced by bacteria that binds floc.
Equalization: Storing flow to even out variations in flow and load.
Extended aeration: Activated sludge with long detention and SRT, low F/M.

F

F/M ratio: Food to microorganism ratio: lb BOD per day per lb MLVSS under aeration.
Facultative bacteria: Bacteria that can use oxygen or other electron acceptors such as nitrate.
Filament: A chain of bacterial cells; excess causes bulking or foam.
Filtrate: Liquid from a belt press or filter during dewatering.
Floc: Clump of bacteria and particles held together by EPS.
Flux (solids flux): Mass of solids moving through a unit area per unit time, lb/d/ft² or kg/m²/h.
FOG: Fats, oils and grease.
Food: Organic matter (BOD) used by microorganisms.
Free-swimming ciliate: Protozoan that swims using cilia and feeds on dispersed bacteria.

G

Grab sample: A single sample taken at one time.
Gram stain: Staining technique that separates bacteria by cell wall type; used in filament ID.
Grit: Heavy inorganic particles such as sand removed at the headworks.

H

Headworks: The preliminary treatment area: screens, grit removal, flow measurement.
Heterotroph: Organism that uses organic carbon for growth.
Hindered settling: Settling of a concentrated suspension as a mass with a clear interface.
Hydraulic loading: Flow per unit area or volume.

I

Influent: Water entering a process or plant.
Internal recycle (IR): Pumping nitrified mixed liquor from the aerobic zone to the anoxic zone.
Inventory: Total pounds of solids under aeration (and sometimes in clarifiers).

L

Limiting flux: The maximum solids flux a clarifier can thicken at a given underflow rate.

M

MCRT: Mean cell residence time: SRT including clarifier solids.
Mixed liquor: The mixture of wastewater and activated sludge in the aeration basin.
MLE: Modified Ludzack-Ettinger: anoxic zone ahead of aerobic zone with internal recycle.
MLSS: Mixed liquor suspended solids, mg/L.
MLVSS: Mixed liquor volatile suspended solids, mg/L; approximates active biomass.
Most open valve control: Blower pressure control that keeps one air valve nearly fully open.

N

Neisser stain: Stain that shows polyphosphate granules and helps identify filaments.
Nematode: Small roundworm seen in older, stable sludge.
Nitrate (NO3-N): Oxidized nitrogen produced by nitrification.
Nitrification: Biological oxidation of ammonia to nitrite and nitrate.
Nitrite (NO2-N): Intermediate in nitrification; normally low.
Nocardioforms: Branched, Gram positive filaments that cause stable brown foam.
NPDES: National Pollutant Discharge Elimination System permit program.

O

ORP: Oxidation reduction potential, mV; indicates aerobic, anoxic or anaerobic conditions.
OUR: Oxygen uptake rate, mg/L/h.
Overflow line: Line on a state point diagram with slope equal to SOR.
Overloaded (clarifier): State point condition where solids arrive faster than the clarifier can thicken.

P

PAO: Phosphorus accumulating organism.
Pin floc: Small, dense floc particles that settle fast but leave a hazy supernatant.
Plug flow: Reactor where flow moves through without much back mixing.
Polymer: Long chain organic chemical used to flocculate solids for thickening and dewatering.
Pounds formula: lb/d = flow (MGD) x concentration (mg/L) x 8.34.
Primary clarifier: Settling tank ahead of biological treatment.
Protozoa: Single celled organisms (ciliates, amoebae, flagellates) that graze on bacteria.

R

RAS: Return activated sludge pumped from the clarifier back to the aeration basin.
RAS chlorination: Controlled chlorine dosing to RAS to reduce filaments.
Rising sludge: Sludge floating to the clarifier surface, usually from denitrification gas.
Rotifer: Multicellular animal with a ciliated crown; indicates mature sludge.

S

SBR: Sequencing batch reactor.
Screenings: Debris removed by bar screens.
Scum: Floating material such as grease and foam skimmed from tanks.
Selector: A small zone at the head of a basin with high F/M that favors floc formers.
Septic: Stale wastewater with no oxygen, often with sulfide and odor.
Settleometer: Container for the 30 minute settling test.
SLR: Solids loading rate on a clarifier, lb/d/ft².
SOR (aeration): Standard oxygen requirement: oxygen needed at standard clean water conditions.
SOR (clarifier): Surface overflow rate, gpd/ft².
SOTE: Standard oxygen transfer efficiency of a diffuser, percent.
SOUR: Specific oxygen uptake rate, mg O2 per g VSS per hour.
SRT: Solids retention time: average time solids stay in the system.
SSV30: Settled sludge volume after 30 minutes, mL/L.
Stalked ciliate: Protozoan attached to floc by a stalk; indicates stable sludge.
State point: The intersection of the overflow line and the MLSS on a solids flux diagram.
Step feed: Feeding influent at several points along an aeration basin.
Straggler floc: Light, fluffy floc floating in the supernatant.
Supernatant: Clear liquid above settled sludge.
SVI: Sludge volume index: mL occupied by 1 g of sludge after 30 minutes of settling.

T

Testate amoeba: Amoeba with a shell; often seen at long SRT.
Thickening: Concentrating sludge before digestion or dewatering.
TKN: Total Kjeldahl nitrogen: organic nitrogen plus ammonia.
Total suspended solids (TSS): Solids retained on a filter, mg/L.

U

Underflow line: Line on a state point diagram with slope equal to the underflow (RAS) rate per area.
Underloaded (clarifier): State point condition with thickening capacity to spare.
UV disinfection: Disinfection by ultraviolet light.

V

VA/ALK ratio: Volatile acids to alkalinity ratio in an anaerobic digester; below 0.1 is normal.
Vesilind equation: Settling model Vs = V0 x e^(-kX) used in state point analysis.
Viscous bulking: Bulking caused by excess EPS slime rather than filaments.
Volatile solids (VS): Portion of solids lost on ignition at 550 C; approximates organic matter.
Volatile solids reduction (VSR): Percent of volatile solids destroyed in digestion.

W

WAS: Waste activated sludge removed from the system to control SRT.
Weir: Edge over which clarified water flows; notched weir plates spread flow evenly.
Weir overflow rate: Flow per foot of weir, gpd/ft.

Z

Zone settling velocity: Settling velocity of the sludge blanket interface in a hindered settling test.