Water - Wastewater
A B C D E F G H I J K L M N O P Q R S T U V W
Anthropogenic Relating to the
science of man.
Amino acids Compounds, containing within their
structure at least one amino group (NHy) and one carboxylic group (-COOH)).
Sub-units of amino acids make up proteins.
Amplitude The magnitude of the maximum
displacement of an oscillating sound wave.
Anaerobic digestion Digestion of organic matter
by anaerobic microbial action, resulting in the production of methane
Anaerobiosis The presence of life in an anaerobic
Anion A negatively charged ion.
Anion exchange capacity The ability to exchange
positively charged particles of two or more compounds, measured in
milliequivalents per 100 grams.
Aquaclude Rocks and soils which transmit water
with difficulty, e.g. clay, shale and unfractured granite.
Aquifer Rocks and soils which transmit water with
ease through their pores and fractures, e.g. limestone, sandstone and
Aromatic compounds Compounds which contain a six-membered
ring of carbon atoms, known as a benzene ring, e.g. vitamin B.
Atmospheric boundary layer A region near the
earth in which the relative velocity increases from zero with elevation,
due to the motion of the atmosphere relative to the earth’s surface.
Atmospheric inversion A term which describes the
rising of warm air above cold air when two air masses of different
temperatures, humidity and pressure meet.
Attached growth Fixed microbial growth on the
media surface in a trickling filter.
Attentuate and disperse landfill sites The
traditional type of landfill site from which the leachate produced seeps
through soil assures and pores into the underlying saturated zone, where
it is diluted.
Attrition A gradual abrasion.
Auto-ignition temperature The temperature to
which a reactive mixture must be raised so that, under certain
conditions of pressure and after a specific period of time, that mixture
will spontaneously ignite.
Autotrophic A term applied to organisms which
produce their own organic constituents from inorganic compounds
utilizing energy from sunlight or oxidation processes.
Available water content The water available in
the soil for plant use, i.e. the difference between the permanent
wilting point and the field capacity.
Avogadro number The number of atoms of carbon in
exactly twelve grams of the carbon-12 isotope, i.e. 6.023x1023.
Bag filter A row of fabric bags
through which a gas stream is passed for the removal of particulate
Baling A compaction process in which the volume
of waste requiring to be stored or transported is reduced.
Bandspreading The spreading of fertilizers in
thick bands, 300 mm apart.
Baseflow Water which enters streams from
persistent, slowly varying sources and maintains streamflow between
Basidia The reproduction cell of the fungal
group, Basidiomycetes. which contains the mushroom puffballs and
Bearing capacity A measure of the load per unit
area that a material can withstand before failure.
Benthic Of the bottom.
Bentonite A natural clay whose particles form a
skin of very low permeability on an excavated soil face The skin
supports the soil hydrostatic pressure, thereby promoting stability.
Benzene ring The basic structure of benzene: six
carbon atoms arranged in a ring, each with a hydrogen atom attached.
Binary fission A form of reproduction of
micro-organisms in which the cell mass is passed on as two new
individuals to the succeeding generation and the biomass is retained
within the population.
Biochemical oxygen demand (BOD)
A measure of the
amount of oxygen used by bacteria in the
Biocide A chemical toxic or lethal to living
Biodegradable Capable of decomposition by living
Biodiversity The infinite range of living
organisms found within an ecosystem.
Biome A major regional ecological community,
characterized by distinct life forms and principal plant or animal
Biosolids The semi-solid end product of
Biosphere The part of the earth and the
atmosphere in which life can occur.
Biotope The smallest geographical unit of the
biosphere or of a habitat, characterized by its biota, that can be
defined by convenient boundaries.
Bluff body A body which is of angular, rather
than aerodynamic, shape.
Bound water A thin film of water held by adhesion
to the surface of soil particles.
Buoyancy The upward force that acts on a body
which is totally immersed in a fluid and is equal to the weight of the
fluid displaced by the body.
Bubbling bed Expansion and fluidization of the
sand of a fluid-bed incinerator caused by high rate passage of air.
Budding A type of asexual reproduction in which
new cells are formed as outgrowths of a parent cell.
Buffer A solution which undergoes only a slight
change in pH when H+ or OH- ions are added to it.
Buffer stripping The cultivation of narrow strips
of land across the slope of the land rather than parallel to it, with
the aim of reducing soil erosion.
Bulking agent A low density material, usually
domestic refuse, straw or woodchips which is mixed with compost to
permit air circulation while the compost is digesting.
Buttress zone Zone of protection.
Can velocity The velocity of the
gas in the passages between the filter units in the filter house of a
Capilliarity The rise of water in tubes of small
bore due to the adhesion between the water molecules and the surface of
the vessel wall.
Capilliary suction time (CST) A
laboratory-determinable parameter defining the dewaterability rate of a
Capilliary water Water held in soil micropores by
weak capilliary forces.
Capsid A protein coat surrounding the nucleic
acid of a simple virus.
Capsule A layer of well-organized materials lying
outside and adhering to the bacterial cell wall.
Carotenes See carotenoids.
Carotenoids A group of plant pigments of an
orange, yellow or red color which assist in photosynthesis, absorb light
in the violet – blue range but whose presence is usually masked by
chlorophyll. They contain the groups carotenes and xanthophylls.
Catalyst A substance which alters the rate of a
chemical reaction but which is not used up and is unchanged chemically
at the end of the reaction.
Catchment A natural drainage basin which channels
rainfall into a single outflow.
Cation A positively charged ion.
Cation exchange capacity The ability to exchange
negatively charged particles of two or more compounds, measured in
milliequivalents per 100 grams.
Cavity zone A region within which there is little
mixing of air.
Cellular storm A rainfall event consisting of a
number of discreet rainfall-bearing cells (clouds).
Cell wall The outer supporting layer of a plant
cell made by the protoplast and consisting largely of cellulose.
Chemical oxygen demand (COD) A quick chemical
test to measure the oxygen equivalent of the organic matter content of
wastewater that is susceptible to oxidation by a strong chemical.
Chemisorption Adsorption involving very strong
Chemotrophic A term applied to organisms which
produce their own organic constituents from inorganic compounds
utilizing the energy obtained from the oxidation of hydrogen sulphide.
Chloracne A widespread acneform eruption due to
exposure to compounds such as dibenzofurans, dibenzodioxins and
Chloramine A compound composed of chlorine and
Chlorination A disinfection technique used in
water treatment, involving the addition of Cl2 gas, chlorine
dioxide, sodium hypochlorite or calcium hypochlorite.
Chlorofluorocarbons (CFCs) Compounds containing
chlorine, fluorine or bromine, used as aerosol propellants,
refrigerants, foaming agents and solvents and which, on decomposition by
sunlight, produce oxides of chlorine responsible for the removal of
ozone from the stratosphere.
Chlorophyll A photosynthetic plant pigment which
absorbs red and blue light but reflects green light. The chlorophyll
molecule has a square head, magnesium at the center and a long tail.
Chlorophyll a The most important of the pigments
in chlorophyll, found in all photosynthetic plants except bacteria.
Chlorophyll b One of the constituent pigments of
chlorophyll, found in higher plants and green algae.
Chloroplast A chlorophyll-containing, cytoplasmic
body of plant cells where photosynthesis occurs.
Chromatophore A plastic containing colored
Cilia Whiplike structures of 5 – 20 µm length
which allows bacterial mobility by beating with a swimming action.
Circulating bed Recovery of solids from the gas
phase of a fluidized bed combustion reactor, followed by reinjection
into the sand bed.
Closed loop recycling The remanufacture of a new
product from a retired product of the same type.
Coagulation The water/wastewater treatment
process of destabilizing colloidal particles to facilitate particle
growth during flocculation by either double-layer compression, charge
neturalization, interparticle bridging or precipitate enmeshment.
Coarse fish Fish, e.g. mullet, which are able to
tolerate low oxygen levels.
Coepod species A phylum containing the Crustacea,
i.e. small freshwater and marine animals, of which some plankton is
Conductivity A measure of the ability of a
solution to conduct an electrical current and is proportional to the
concentration of ions in the solution.
Coliforms Non-pathogenic bacteria present in the
intestines of warm-blooded animals, water and wastewater, whose numbers
Colloids Very small particles in suspension, e.g.
Combustion A high temperature process involving
the decomposition of organics in an excess of air.
Completely mixed reactor An aeration tank in
which, on entering, the influent wastewater is dispersed immediately
throughout the reactor volume.
Composting The biological stabilization of wastes
of biological origin under controlled conditions.
Compound A substance, the molecules of which
consist of two or more different kinds of atoms.
Compression settling Particles are present in
such a high concentration that they touch each other and settling can
occur only by compression of the particle mass.
Condensation point That level above the earth’s
surface to which a parcel of unsaturated air must ascend before becoming
Constructed wetland A biological wastewater
treatment system which utilizes plants for the degradation of organic
Contact stabilization A wastewater treatment
plant in which there are two tanks, one for the adsorption of organic
matter onto the suspended solids and another for oxidation of the
Containment landfill sites The modern landfill
site, in which the leachate generated is contained by bottom liners,
collected and treated.
Contaminated site A landfill into which hazardous
polluting waste has been dumped.
Contour ploughing Ploughing across the slope of
the land rather than with it, to prevent soil erosion.
Convection Transport of heat by vertical movement
of a heated body.
Convective precipitation When a parcel of air which is less dense
than the air surrounding it, rises, it
cools and loses moisture which falls to the earth as
Coriolis force A transverse force, caused by the movement of the
earth about the sun, which causes a build-up in the level of water to
the right of a tidal current in the northern hemisphere and to the left
in the southern hemisphere.
Corona The upper portion of a body part.
Criteria pollutant Emissions to the urban air traditionally seen
as polluting, e.g. carbon monoxide (CO2), sulfur dioxide (SO4).
Cryogenic Producing very low temperatures.
Cyanide A highly poisonous salt of hydrocyanic acid, used
frequently in the extraction of gold and silver.
Cyclone separator A means of purifying an air stream by using
both gravitational and centrifugal forces.
Cytotoxic Damaging to cell structure and cell division
.Daphnid species The phylum containing the
Branchiopodia, i.e. marine solitary, benthic animals with a shell of two
Denitrification The chemical reduction of nitrate and nitrite to
gaseous forms: nitric oxide, nitrous oxide and dinitrogen: NO3-
NO à N2 Oà
Deoxyribose nucleic acid (DNA) A large organic molecule found in
the cell nucleus, containing a phosphate group, five-carbon sugars (deoxyribose)
and four different nitrogenous bases in a repetitive structure.
Detritivores Organisms which feed on fragmented particulate
Dewatering of sludge A mechanical unit operation which increases
the dry solids concentration of the sludge from 3.9 percent after
digestion to 25 – 30 percent thereby ensuring that the sludge
effectively behaves as a solid for handling purposes.
Diffusion The process by which gases and liquids spread
themselves throughout any space into which they are put.
Dilute-phase bed The stage in fluidized bed combustion at which
the bubbling of the reactor bed becomes so great that the boundary
between the bed and the gas above it becomes indistinct.
Dimiclic A term to describe a lake whose thermocline is disrupted
due to two periods of free circulation or overturn per year in the lake.
Dioxin Tetrachlorodibenzoparadioxin (TCDD), a highly toxic and
environmentally persistent product of the manufacture of the pesticide
Direct contact condenser The vapor stream is in direct contact
with the drying medium, hot air or gas. The drying medium (hot air or
flue gas) leaves the drier with the water vapor coming from the sludge.
The drying temperature is 80 – 150 ºC.
Directivity index The difference between the measured sound power
level and the value based on the assumption of uniform radiation in all
Discrete settling Particles settle as independent units, without
interaction of flocs.
Disinfection The removal or inactivation of pathogenic organisms.
Dissolved oxygen A measure of the amount of oxygen dissolved in
water, expressed as either:
(i) mg/1 – which is the absolute amount of oxygen dissolved in the
(ii) as percentage saturation of the water with O2 (% sat)
Dissolved solids The total colloidal and suspended solids in a
liquid. Any particle passing a 1.2 µm filter is defined as dissolved.
Dominant group The highest ranking group in a social order of
dominance sustained by aggressive or other behavioural patterns.
Downflow column e.g. Sand filtration where water flows through
the filter by gravity. Also used in anaerobic digestion, where the
wastewater enters at the upper levels and flows down through a packed
medium. Opposite to upflow column.
Downwash The drawdown of a plume after emission due to a low
pressure area downwind of the stack.
Dry absorption A method of controlling acids in flue gas
emissions, by injection of dry calcium
hydroxide into the gases leaving the furnace of an incinerator.
Dry weather flow The combination of wastewater and dry weather
infiltration flowing in a sanitary
sewer during times of low precipitation.
Ecology That branch of science dealing with
living organisms and their surroundings.
Ecosystem A community of interdependent organisms together with
the environment which they inhabit
and with which they interact, e.g. a pond.
Ecotron A controlled, in-house, ecological experiment to recreate
a particular ecosystem.
ECU The EU unit of monetary currency.
Effluent The outflow from a sewage treatment plant.
Electrical double layer A name given to the combination of the
Stern layer and the diffused layer of
both negatively and positively charged ions which surround it.
Electron Negatively charged particle contained within an atom,
the weight of which is about two
thousand times less than that of the hydrogen atom.
Electrostatic precipitation A means of purifying an air stream by
attraction and adhesion of ionized
particles to an electrode.
Element A substance, the molecules of which have all the same
Endotoxin An environmental toxin which attacks the endocrine
glands, i.e. kidney, liver, etc.
Environmental impact assessment (EIA) A review to which all
commencing projects must be
subjected with regard to their impact on the environment.
Enzyme A substance produced by living cells which acts like a
catalyst in promoting reactions within
Epilimnion The zone in a stratified lake just below the
near-surface water in which temperature
decreases rapidly with depth.
Epilithic Relating to organisms growing on rocks or on other
hard, inorganic substances.
Equilibrium concentration The concentration of the dissociated
ions when the rates of both backward
and forward reactions are equal.
Equilization basin A holding tank within which variations in
sewage inflow rate and liquid nutrient
concentrations are averaged.
Equivalence The number of protons donated in an acid-base
reaction or the total change in valence in
an oxidation-reduction reaction.
Eucaryotic cell A cell whose nucleus is enclosed by a membrane,
e.g. algae, higher plants and animals.
Euphotic zone The surface zone of large lakes through which
sufficient light penetrates for photo-
synthesis to occur.
European Communities (EC) A precursor to the EU, created by the
merger of the European Coal and
Steel Community, the European Economic Community and the European
European Economic Community (EEC) An organization established in
1957 under the Treaty of
Rome to co-ordinate the activities of its member countries in the
coal and steel industry, the
establishment of a common market and the pooling of atomic energy
European Union (EU) A supranational organization which replaced
the EEC in 1993, with the
objective of peace and prosperity for its members by achieving
complete economic and political union.
Eurytopic A term describing an organism which is tolerant of a
wide range of habitats.
Eurotrophic A term describing freshwater bodies which are rich in
plant nutrients and therefore highly
Eutrophication An increase in the concentration of nutrients in
an aquatic ecosystem, causing:
(i) the increased productivity of autotrophic green plants,
leading to the blocking out of sunlight
(ii) elevated temperatures within the water body
(iii) depletion of the world’s oxygen resources
(iv) increased agal growth
(v) reduction in the level of and variety of fish and animal
Evaporation The changing of liquid water from rivers, lakes, bare
soil and vegetative surfaces into
Evapotranspiration A collective term for all the processes by
which water in the liquid or solid phase
at or near the earth’s land surfaces becomes atmospheric water
Exothermic reaction A chemical reaction during which heat is
Extended aeration Involves an aeration period of more than 24
hours and a high rate of return sludge to
allow cell decay during the endogenous respiration phase of the
Facultative aerobes/anaerobes Having the ability
to live either with or without oxygen.
Fickian diffusion Molecular diffusion, governed by Fick’s law,
which says that the rate of flow of
molecules across a unit area of a certain plane is directly
proportional to the concentration gradient.
Field capacity The amount of water which can be held in the soil
against the force of gravity, i.e. Water
which will not drain freely out of the soil.
Filtration A process whereby suspended and colloidal matter is
removed from water and wastewater by
passage through a granular medium.
Five-day biochemical oxygen demand (BOD5) A measure of
the amount of oxygen used by bacteria to
degrade organic matter in a sample of wastewater over a 5 day period
at 20 ºC, expressed in mg 1-1.
Fixed bed A bed of dry carbon which recovers volatile organic
carbons from an air stream.
Flagellae Whiplike structures of 100 – 200 µm length which
allow bacterial mobility by undulating in
planar or helical waves.
Flash point The lowest temperature at which a flammable vapor/air
mixture exists at the surface of a
Flocculation The water treatment process in which particle
collisions are induced in order to encourage
the growth of larger particles.
Flotation A process by which suspended matter is lifted to the
surface of a liquid to facilitate its
removal. Frequently done by the bubbling of air through the liquid.
Flow duration curve A means of summarizing temporal variability
by averaging precipitation over a
selected time period.
Flowing well When the groundwater is flowing in a confined
aquifer, it is under hydrostatic pressure.
Should a standpipe be inserted into the aquifer, the water will rise
in the standpipe.
Flue gases Gas by-products of the incineration process whose
temperature is a measure of incinerator
efficiency and whose constituents may be polluting.
Flue gas scrubber Equipment used for the removal of suspended
particulates and acid gases from flue
Fluidized bed combustion An incineration technique in which waste
is destroyed by combustion on a
Fluoridation The addition of fluoride to drinking water within
the limits 0.7 – 1.2 mg/1-1 to help
prevent the occurrence of tooth decay.
Foaming agent Anti-foaming chemicals added to wastewater in the
aeration tank to disperse the
contaminating foam caused by the action of the surface aerators and
the presence of detergents in
Food/micro-organism ratio (F/M) A measure of the organic loading
rate of a wastewater treatment
system, i.e. the ratio between the daily BOD load and the quantity of
activated sludge in the system
Fugitive emissions Emissions from non-point sources, e.g.
loading/unloading, transferring, trans-
porting, storing and processing of materials.
Fumigating A term describing a plume from an emission stack which
is trapped by a stable inversion
above the stack mouth, thereby hitting the ground level very close to
Functional group A group of atoms on which the characteristic
properties of a particular homologous series depends, e.g. the alkanes,
alcohols and esters.
Furans Compounds causing chloracne, liver damage and liver
cancer. Strictly C4H4O, but more
commonly one of a range of polychlorinated dibenzofurans that are
produced as contaminants from the incomplete incineration of chlorinated
Gamete A mature cell, involved in
Gas chromatography A process whereby compounds
become separated by being physically carried by a gas over a liquid of a
high molecular weight.
Gas flaring The burning of recovered landfill gas
from a stack under controlled conditions to help eliminate the discharge
of harmful constituents to the atmosphere.
Gasification A high temperature process involving
the decomposition of organics in the absence of oxygen. Some of the
energy stored as chemical energy from the organic material will be
released as burnable gas.
Genotype The genes which an organism possesses or
the genetic make-up of an organism.
Groundborne vibrations Vibrations caused by the
reaction of tyres of heavy vehicles with irregularities in the road
Groundwater Water under a pressure greater than
atmospheric pressure which is present in the saturated zone of the soil.
Haematins A group of colored
plant pigments, including the red pigment, haematochrome.
Haloform A basic organic unit of the halogen
Halogen The reactive members of Group 7 of the
Periodic Table, including chlorine, bromine, fluorine and iodine.
Hardness in water The sum of the calcium and
magnesium ion concentrations. A hard water will leave a scale on the
inside of kettles and will form a scum rather than a lather with soap.
Hazardous waste A substance which exhibits
ignitability, reactivity, corrosivity, and/or toxicity.
Heat of adsorption Adsorption is the process of
retaining a gas molecule by either physical or chemical means onto an
adsorbent (a solid, e.g. activated carbon). The heat change taking place
during this process (loss of heat of gas, increase in temperature of
adsorbent) is the heat of adsorption.
Heat of condensation The quantity of heat
required to bring about a phase change from a gas to a liquid.
Heat of solution The heat change which takes
place when one mole of a substance is dissolved in excess solvent.
Heavy metal Inorganic species of large atomic
weight. Usually chromium (Cr3+), lead (Pb2+),
mercury (Hg2+ ), zinc (Zn2+), cadmium (Cd2+)
and barium (Ba2+).
Herbivores Animals which feed on plant material
only, e.g. rabbits.
Heterotrophic A term applied to organisms which
need ready-made food materials from which to produce their own
constituents and to obtain all their energy.
High rate aeration An increased rate of aeration
of MLSS in an activated sludge system requiring less activated sludge
and shorter aeration periods.
Homogenous Consisting of only one phase.
Humus The vegetative upper layers of the soil.
Hydraulic conductivity See permeability.
Hydrograph A graph of stream discharge versus
Hydrolysis The breakdown of high molecular
compounds to low molecular compounds.
Hydrophilic Displaying an affinity for water.
Hydrophobic Displaying an aversion for water.
Hydrothermal vent An opening in the earth through
which heated or superheated water is ejected.
Hydraulic jump An area of turbulence and of loss
of energy associated with the transmission from shooting to tranquil
Hydrological cycle The endless recirculatory
transport process of the earth’s water resources, linking the
atmosphere, the land and the oceans.
Hyetograph A graph of water input to a catchment
Hypha A tubular filament which is the basic unit
structure of most fungi and some bacteria.
Hypolimnion The lower layer of water in stratified lakes which
retains the winter temperature.
Ion Atoms or groups of atoms which have either
lost or gained electrons and so have become either
positively or negatively charged.
Ion exchange Ion exchange can be illustrated by the following
reaction: Ca2+ + Na2 Z à
CaZ + 2Na+.
Incineration Chemical oxidation at high temperatures where
organic material is converted into heat
energy, flue gas and slag.
Inclusion body Organic or inorganic bodies containing glycogen,
protein or lipids, present in the
cytoplasm of a bacterium.
Indirect contact condenser A condenser in which there is no
direct contact between the heating medium and the vapor stream, but a
partition divides the two. The water vapor is removed separately from
the heating medium. The drying temperature is 100 – 250 º C.
Invertebrate Animal without cranium and spinal column.
Irrigation requirement The difference in volume between effective
precipitation and evapotranspiration.
Isohyet A line on a map connecting areas of equal precipitation.
lsocyanate Derivatives of nitrogen-substituted carbamic acids,
containing carbon, oxygen and an
organic amine group.
Isotropic A substance whose physical properties are the same in
Jute Fiber from the bark of some plants, used
mostly for sacking.
Karst Landforms of chemically weathered
limestone, characterized by underground channels and
caverns, swallow holes and open joints.
Ketone An organic compound containing three carbon atoms, one of
which is double-bonded to an
oxygen atom, the other two each attached to three hydrogen atoms e.g.
Landfill A repository in the ground for unwanted
Landfill gas This is produced principally from the anaerobic
decomposition of biodegradable organic
waste and includes ammonia, carbon dioxide, carbon monoxide,
hydrogen, hydrogen sulfide,
methane, nitrogen and oxygen.
Landfill liner Used to limit the movement of leachate and
landfill gases from the landfill site. Can be
made of natural clay material or composite geomembrane and clay
Lapse rate The rate of temperature change with height for a
parcel of dry air rising adiabatically.
Latent heat of evaporation The quantity of heat required to bring
about a phase change from a liquid to
Leachate Liquid, composed of external rainfall, groundwater, etc.
which has percolated through solid
waste and has extracted both biological and chemical, dissolved or
Legumes Legumes are specific plants, e.g. clover, soybeans and
lupins, which carry nodules on their
roots and, together with bacteria of the genus Rhizobium are
responsible for the biological fixation
of nitrogen in the soil.
Life cycle assessment (LCA) The assessment of the steps in a
product life cycle, including: raw
materials acquisition, bulk material processing, materials
production, manufacture, assembly, use,
retirement and disposal.
Ligand Molecules of a complexing agent in a complex ion, i.e. an
aggregate formed when a metal
ion bonds to several other ions or molecules which cluster around it.
In the reaction
AgC1 + 2NH3à Ag(NH3)2+
+ Cl-, NH3 is the ligand.
Light compensation point The depth in a sea or lake below which,
because of low light intensities,
plants use up more organic matter in respiration than they make
Liquid injection incineration A method of incineration of liquid
waste by high-rate injection into a
Lithotrophic A term describing organisms which use inorganic
compounds as electron donors in their
Littoral zone The shore of a lake to a depth of about 10 meters.
Lofting A term describing a plume from an emission stack which
remains aloft due to a stable inversion
below the mouth of the stack.
Lower explosive limit (LEL) The concentration at
which a gas forms an explosive mixture with air.
Lysis The rupture of cells.
Magnetic separation A process
which utilizes the magnetic properties of ferrous metals to extract them
from the waste stream.
Masking agent A substance which will remove an
offending odor from an air stream by decomposition or conversion to an
Materials recovery facility (MRF) Depots where
reusable waste material is recovered.
Mean cell residence time (MCRT) The average time
a single microbe will remain in an activated sludge system and is
calculated by: Total mass of cells/Rate of cell wastage
Meiosis A type of cell nuclear division in which
the daughter nuclei receive only half the original number of chromosomes
in the parent nucleus.
Membrane process The removal of dissolved solids
from water by passage through a membrane of minute pore diameter (3*10-10
Mesophilic temperatures Those temperatures in the
range 10 – 45 ºC.
Mesotrophic A term to describe waters having
intermediate levels of the minerals required by green plants.
Methanogenesis Intermediate compounds are
converted to the final products of methane and carbon dioxide.
Methanogenic bacteria Obligate anaerobes and
methanobacteria (e.g. methanosarcina, methanobacilli) which produce
methane gas from the decomposition of acids and alcohols:
CH4 + CO2
CO2 + H2O + NH3à
Micro-organisms Neither plant nor animal, these
are small, simple organisms which are either unicellular or
multicellular, consisting of protozoa, algae, fungi, ricettsiae, viruses
Mineralization The process by which organic N is
reconverted to mineral form by a wide variety of heterotrophic organisms
– bacteria, fungi and actinomycetes.
Mixed liquor suspended solids (MLSS) The
microbial suspension in the aeration tank containing living and dead
micro-organisms and inert biodegradable matter, the operating
concentration of which may vary in the range 1500 to 4000 mg/1
Mole A mole of any substance is that amount of it
which contains the Avogadro Constant number of particles. A mole of any
substance is equal to its molecular mass or atomic mass expressed in
Molecular diffusion The drifting of molecules
under random kinetic motion from a low concentration region to a high
Monomielic A term to describe a lake having a
single period of free circulation or overturn per year.
Morphology The study of the form of animals and
Mouse system Software for the hydrodynamic and
hydrochemical design of a wastewater collection system from the Danish
Mutagenic Causing alteration of the genetic
material of an organism, leading to inherited differences.
Mycelium A mat of branching hyphae found
particularly in actinomycetes.
Negative project A commencing
project for which an Environmental Impact Assessment will not be
Negative sorting Manual sorting of waste to
remove the unwanted fractions. Is recommended only for dry waste.
Neutrality An ion or ion group which has an equal
number of electrons and protons, i.e. neither a positive nor negative
Niche The ecological role of a species in a community.
Nitrification The conversion of the ammonium ion, NH4+,
into the nitrite ion, NO3+. It occurs in two
(i) 2NH4+ + 3O2 = 2NO2-
+ 2H2O + 4H+ by the bacteria genus Nitrosomonas
(ii) 2NO2 + O2 = 2NO3-
by the bacteria genus Nitrobacter.
Non-point source pollution Pollution from diffuse and not easily
identifiable sources, e.g. a field.
Normality A concentration unit which is defined as: number of
equivalents of solute/number of liters of solution.
Nucleic acid See Deoxyribose nucleic acid and
Nucleoid An irregularly shaped region in the
procaryotic cell containing the genetic material.
Nucleus The well-defined region surrounded by the
cell wall which contains the chromosomes, i.e. the materials of
inheritance of the cell.
Nutrient removal Tertiary treatment introduced to
remove some of the trace compounds and elements contained in most
domestic wastewaters, e.g. inorganic ammonia, nitrates, phosphates and
sulfates, which are little affected by conventional treatment processes.
Octave band The interval between
a given frequency and twice that frequency within the audible frequency
Odor threshold The minimum level or value of an
odor necessary to elicit a public response.
Off-specification stock Stock which is produced
by errors in processing, leading frequently to customer dissatisfaction.
Olfactometer An instrument used to measure
relative odour levels.
Oligotrophic A term describing freshwater bodies
which are poor in plant nutrients and are therefore unproductive.
Open loop recycling The manufacture of a new and
simpler product from a retired, downcycled, more complex product.
Organochlorine A highly persistent and
carcinogenic compound containing one or more chlorine atoms found in
pesticides such as DDT.
Organoleptie parameters These are properties
which can be detected by the human senses – eyes, nose and mouth.
Organotrophic A term describing organisms which
use organic compounds as electron donors in their energy-producing
Overflow rate The rate at which water is drawn
off from the surface of primary and secondary clarification tanks. It is
an important tank design parameter and is derived from the analysis of
Overland flow The lateral movement of water over
the ground surface due to gravitational forces.
Oxidizing waste A waste which loses electrons in
an oxidation-reduction reaction, thereby becoming reduced itself.
Oxidation A process in which there is loss of
electrons from an element or ion.
Oxidation ditch A ring-shaped channel, 1 – 1.5
m deep, around which wastewater circulates at 0.3-0.6 m/s-1,
is aerated by mechanical rotors and undergoes biological treatment by
the resident microbes.
Oxidation number The charge which an atom of an
element has, or appears to have, in a compound, e.g. chlorine has an
oxidation number of – 1.
Oxygen sag curve The longitudinal profile of
oxygen concentration in a river.
Ozone A triatomic gas, particularly prevalent in
the stratosphere, formed by the reaction:
O + O2 + energy à
O3 + energy.
Ozone-depleting gases Gases (e.g. oxides of nitrates, chlorine
nitrate, halocarbons and water vapor)
which cause destruction of the ozone layer, thereby allowing
increased amounts of sunlight to reach the earth.
PAH Polycyclic aromatic
Parasite An organism which lives on or in another
living organism of a different species (the host), from which it obtains
food and protection, e.g. tapeworms, greenflies.
Parr Fry, i.e. young fish.
Partial pressure The pressure a single gas within
a contained mixture of gases would exert if it were the only gas in the
Peat bed filter Odorous compounds are removed
from an air stream by passage through a bed of uncompacted peat from the
upper layer of a bog.
Percentage exceedance The inverse of the return
period, i.e. a frequency (e.g. 90 percent) of occurrence of a flood
event or rainfall whose associated depth is exceeded that percent (90
percent) of the time.
Periplasmic space A space between the plasma
membrane and the outer membrane of a biological cell, sometimes filled
with a loose network of peptidoglycan.
Permanent wilting point The water content of the
soil beyond which plants cannot exert sufficient suction to extract
Permeability The rate at which a fluid flows
through a porous medium under the hydraulic head operating within the
medium. Usually, the greater the porosity, the greater the permeability.
Pesticide A material used for the mitigation,
control or elimination of plants or animals detrimental to human health
pH A measure of the acidity or basicity of a
solution i.e. the negative of the logarithm of the hydrogen ion
Phagocytose Ingestion of solid particulate matter
by a cell.
Phenol An organic compound with a hydroxyl (OH)
group bonded directly to a benzene ring.
Phenotic compound Compounds containing a
phenol group, i.e. those containing hydrogen, six carbon atoms joined by
alternating single and double bonds, and a hydroxyl group attached to
the first carbon atom.
Photo-oxidation Oxidation initiated by sunlight.
Photolytic process A process in which radiant
energy causes chemical decomposition.
Phototrophic An orientation response to light.
Physico-chemical parameters Instrumental methods
of analysis such as turbidimetry, colorimetry, polarography, adsorption
spectrometry, spectroscopy and nuclear radiation.
Physiology The science of functioning of living
Phytoplankton Plankton consisting of
photosynthesising plants, such as algae.
Plasma arc destruction A method of incineration
in which very hot plasma, heated by the conversion of electrical to
thermal energy, pyrolyses and atomizes waste.
Plasma membrane A membrane of 5 – 10 nm
containing proteins and lipids, surrounding the cytoplasm of all cells.
Plastid A membrane-bounded body found in the
cytoplasm of most plant cells. See chloroplast.
Point-source pollution Pollution from sources
which are easily identified, e.g. slurry tank.
Pollination The transfer of pollen, usually by
insects or wind, from the anther of a stamen (male part of the flower)
to the stigma of a carpel (female part of the flower).
Polyampholite A type of polymer.
Polyaromatic compounds (PAHs) Long chain
compounds, very persistent in nature, containing the hydroxyl group in a
Polychlorinated biphenyls (PCBs)
A generic term
covering a family of chlorinated isomers of biphenyl found in sewage
outfalls and industrial and municipal solid wastes.
Polyelectrolytes Long-chain molecules used in the conditioning of
sludge which, by neutralizing surface charges, cause bridging across
fine particles or floes to form larger particles.
Polymer Giant molecules built up from thousands
of smaller molecules, combined together to form a repetitive structure.
Porosity The proportion of void spaces in the soil. The porosity
of fine soils, e.g. clay, is low, whereas that of coarser gravelly soils
Positive project A commencing project for which an Environmental
Impact Assessment is considered essential.
Precipitation The depth of rainfall plus the water equivalent of
snow, sleet and hail falling during a given measurement period.
Precipitation reaction A physical or chemical reaction which
results in the precipitation of one of the products formed.
Predator An organism which lives by killing and consuming other
living things, e.g. sparrows, rabbits.
Primary consumer Organisms which feed directly on the primary
producers. These include herbivores, detritus feeders, scavengers and
decomposers (animals which feed on dead plant remains).
Primary pollutant Air pollutants which are emitted from an
identifiable source, e.g. carbon monoxide from the car engine.
Primary producer Organisms which are capable of using solar
energy to make food by the process of photosynthesis, e.g. plants.
Primary succession An ecological succession commencing in a
habitat or on a substrate that has never previously been inhabited.
Procaryotic cell A cell which lacks a distinct nucleus, e.g.
Profundal zone The zone of a lake lying below that depth at which
the light compensation point occurs
Project thresholds Pre-determined levels relating to project
size, production or site required which, jf exceeded by a commencing
project, define the necessity for an Environmental Impact Assessment.
Protista The microbial kingdom to which unicellular or cell
groups of eucaryotic organisms which lack true tissues, e.g. protozoa,
Protozoa Aquatic, free-living and parasitic organisms, these are
the most basic of all animals, with only one single cell and measuring
no more than 5 – 1000 µm in size.
Proteins Substances containing the elements carbon, hydrogen,
oxygen, nitrogen and occasionally sulfur, whose main function is cell
growth and repair.
Proton Positively charged particle contained within the nucleus
of an atom.
Protoplasm The living matter of a cell, comprising both the
nucleus and the cytoplasm.
Pseudopodium A temporary protrusion of cytoplasm from the surface
of a cell which serves for both cell motion and ingestion.
Psychrophilic temperatures Those in the range 0 – 10 ºC.
Pyrolysis A high temperature process involving the decomposition
of organics without oxygen, air at steam. Burnable gas is released as a
Quench tank A tank containing water to cool ashes
and unburned materials which fall from the grates into a residue hopper
Radiation The emission of rays and particles
characteristic of radioactive substances.
Radical An element or atom, or group of these, normally forming
part of a compound and remaining unaltered during that compound’s
ordinary chemical changes.
Radioactive cloud An artificially generated atmospheric cloud
containing radioactive compounds.
Rapid gravity filter A filter used in water treatment which
removes suspended solids from water by passing it through a sand bed,
where the solids collect as a surface mat and in the sand interstices.
The water should previously have been treated by coagulation,
flocculation and sedimentation.
Recharge The process of renewing underground water by
Redox potential The oxidizing or reducing power of a reactant.
Reduced groundwater Groundwater which contains no oxygen.
Reduction A process in which an atom or ion gains electrons.
Regeneration rate The rate of reproduction of
bacteria, the method of which is usually by binary fission.
Regolith All loose earth material above the
underlying soil rock.
Retention time The length of time a wastewater
remains in a clarification tank, an important design parameter in the
optimization of settling of suspended solids.
Return activated sludge Settled activated sludge
from the clarifier which is returned to the aeration tank to ensure an
active population of microbes will be mixed with the incoming
Return period The long-term average of the
intervals between successive exceedances of a flood magnitude.
Reverse osmosis A membrane process in which
solutions of two different concentrations are separated by a
semi-permeable membrane. An applied pressure gradient greater than the
osmotic pressure ensures flow from the more concentrated to the less
Reynold’s number The ratio of inertial to
viscous forces in a fluid, the value of which will determine whether the
Quid flow is turbulent or viscous.
Ribonucleic acid (RNA) A large organic molecule
found in the cell cytoplasm containing a phosphate, five-carbon sugars
(ribose) and four nitrogenous bases in a repetitive structure.
Ribosomes Tiny bodies containing RNA responsible
for protein synthesis and found in the bacterial cell cytoplasm.
Riffle area An area of the river bed covered by
grains of too large a size and weight to be carried by the water and so
Root zone The soil layer from which plant roots
can extract water during transpiration.
Rotary screen See
Rotary kiln incineration A process by which waste
enters an inclined, rotating kiln, is mixed with air and combusted.
Rotating biological contactor A form of
biological treatment in which fixed media is grown on circular discs
mounted on a horizontal axle. These discs are partially submerged in
wastewater while the axle rotates, allowing bio-oxidation of the
wastewater, using oxygen from the air.
Roughing filter A high-rate trickling filter of
depth 1 – 2m, hydraulic loading 10-40m3/m2/d and
organic loading 0.32 – 1.0 kgBOD/m3/d through which
wastewater may be passed prior to an activated sludge treatment.
Salmonoids Belonging to the
salmon family, Salmonidae.
Saprophytic An organism which obtains food by
absorbing dissolved organic materials resulting from organic breakdown
Schmutzdeck The surface mat of suspended
particles which forms on the surface of a slow sand filter.
Scoping The second stage of an Environmental
Impact Assessment which decides the key issues for review within the EIA.
(i) The final sorting stage necessary
for high-quality compost, during which uncomposted particles such as
wood, glass or plastic are removed by passing through a fine mesh. (ii)
The first stage of an Environmental Impact Assessment (EIA) in which the
projects to be subjected to an EIA are chosen.
Scrubbing A process by which suspended particles
and acid gases are removed from a flue gas stream, the former by
absorption onto liquid droplets and the latter by diffusion into the
Scum well A box used to store the scum which
forms on the surface of a wastewater in a clarification tank. Scum is
usually drawn off by a horizontal, slotted pipe that can be rotated by a
lever or a screw.
Secondary consumers Organisms which feed on the
herbivores or other primary consumers, e.g. foxes, lions.
Secondary pollutant Air pollutants which are
formed in the atmosphere by chemical reactions, e.g. ozone.
Secondary production The assimilation of organic
matter by a primary consumer.
Secondary succession An ecological succession that takes place in
an area where a natural community
existed and was removed.
Selective catalytic reduction (SCR) A
pre-combustion method of decomposition of NO, in an ajar stream to
nitrogen and water by injection of ammonia into the catalytic bed of a
Selective non-catalytic reduction
post-combustion method of decomposition of NOx in an air
stream to nitrogen and water by injection of ammonia downstream of a
Semi-dry absorption process A method of
controlling acids in flue gas emissions, by injection of a calcium
hydroxide and water solution into the gases leaving the furnace of an
Sensible heat That portion of the heat radiated
by the sun which is required to heat the earth.
Sensitive area A water body which may
intermittently suffer eutrophication.
Sequencing batch reactor A time-stepped batch
process for the biological treatment of liquid hazardous waste.
Sere A term used to describe a succession of
communities, each following one after the other and finally reaching a
Settling chamber The purification of an air
stream by reducing the velocity of the gas so that the particles drop
out by gravity.
Settling tank A rectangular or circular tank in
which particle velocities within the liquid are sufficiently reduced to
allow the suspended material to be removed from the liquid by gravity
Settling velocity This is the velocity at which a
particle will fall to the bottom of a settling tank and is equal to the
surface overflow rate for a rectangular tank.
Severance The physical or psychological division
of an existing community or property due to traffic development.
Sewage Wastewater and other refuse such as faeces,
carried away in sewers.
Sewerage System of pipes and treatment plants
which collect and dispose of sewage in a town.
Sheath A hollow, tubelike structure found in most
bacteria surrounding a chain of cells.
Sisal Strong, durable white fiber of agave used
in the making of ropes.
Slag The fused bottom ash produced by the
incineration process containing incombustibles, the ash fraction of
combustibles and any undestroyed pollutants.
Sloughing A term which describes the falling off
of the slime layer of micro-organisms on the media of a trickling filter
due to the development of anaerobic conditions and lack of food caused
by an increase in slime thickness.
Slow sand filter A filter which removes suspended
solids from raw water by passing it through a sand bed, where the solids
collect as a surface mat and in the sand interstices. Filtration rates
are in the order of 2 – 51/m2/min.
Sludge The accumulation of solids resulting from
chemical coagulation, flocculation and sedimentation after water or
Sludge bulking A phenomenon caused when a large
number of filamentous micro-organisms present in the mixed liquor
interferes with the compaction of the floe and produces a sludge with a
poor settling rate.
Sludge conditioning Addition of chemicals,
polyelectrolytes or heat treatment to improve the rate of dewatering.
Sludge dewatering The mechanical unit operation
used to reduce the moisture content of sludge to 70 – 75 percent and
thus ensure that the remaining sludge residue effectively behaves as a
solid for handling purposes.
Sludge stabilization The process of destroying or
Sludge volume index (SVI) A measure of the
ability of sludge to settle, coalesce and compact on settlement.
Smog Dense, smoky fog, the formation of which is
promoted by reactions between unsaturated hydrocarbons and oxides of
nitrogen in the presence of sunlight and under stable meteorological
Soilbed A large tract of land, the microbes
within which remove odorous compounds from an air stream
Soil horizons The soil layers seen in a vertical
soil profile, characteristic of soil-forming processes over time.
Soil profile A vertical cut through the soil
revealing a sequence of horizons.
Soil suction Water pressure within a soil which
is less than atmospheric pressure.
Solid waste All the wastes arising from human and
animal activities which are normally solid and are discarded as useless
Solubility product The equilibrium constant for a
reaction involving a precipitate and its constituent ions, e.g. for
magnesium sulfate MgSO4 = Mg+2 + SO4-2,
the solubility product = [Mg+2][SO4-2]
Solute A substance dissolved in a fluid.
Solution The conversion of a solid or gas into
liquid form by mixing with a solvent.
Solvent A liquid capable of or used for
Sound exposure level (SEL) Used to express the
energy of isolated noise events, the SEL is that constant level in
decibels lasting for one second which has the same amount of acoustic
energy as a transient noise.
Sound intensity The average sound power per unit
area normal to the direction of propagation of a sound wave.
Sound power The rate, measured in watts, at which
energy is transmitted by oscillating sound waves.
Spates A river in flood.
Specific flux A measure of rate of flow per unit
Specific resistance to filtration
laboratory-determinable wastewater sludge parameter.
Spectrophotometry An instrumental method of
measuring the intensity of light in various parts of the spectrum.
Spore A unicellular or multicellular microscopic
body involved in plant, bacteria and protozoan reproduction.
Stable inversion On moving downwards through the
atmosphere, a cool parcel of air becomes heated and less dense than the
surrounding air, thereby being pushed back up. It finds itself in a
stable position – wanting neither to move up nor down.
Stabilization pond A quiescent, diked pond in
which wastewater undergoes biological treatment under microbial action.
Stenotopic A term describing an organism which is
tolerant of a narrow range of habit.
Step feed aeration An aeration system in which a
portion of the sewage load is added at each of several inlets, thus
spreading out the oxygen demand over the length of the tank so that
oxygen utilization is more efficient.
Stern layer The innermost ion layer tightly
attached to the surface of a colloidal particle.
Stratosphere The temperature-constant region of
the atmosphere above the troposphere which contains oxygen and ozone.
Supernatant The partially purified water, high in
suspended solids and ammoniacal nitrogen, which is released during the
digestion process and whose quality and amount is dependent on the type
and settling quality of the waste and on the digester system efficiency.
Surface tension The minimization of the surface
of a free body of liquid due to the unbalanced attractions exerted by
the liquid and the air on the liquid surface molecules.
Surge channel A channel or basin designed to take
Suspended growth The free-moving, aerobic,
microbial culture used in the biological treatment of wastewater by the
activated sludge process.
Suspended solids Solids in suspension in a water
or wastewater which can be removed by filtration.
Suspension A substance consisting of particles
suspended in a medium.
Sustainable development Projects undertaken with
care to preserve and manage resources, use genetic engineering with
responsibility, search for technical alternatives to existing energy
sources and control land, water and air pollution.
Synoptic storm A storm covering several hundred
miles, associated with frontal activity and/or intense low pressure
Synthetic organics Man-made, organic compounds,
some of which are carcinogenic, including surfactants pesticides,
cleaning solvents and trihalomethanes.
Tapered aeration An aeration
system which equalizes the quantity of air supplied to the demand for
air exerted by the micro-organisms as the liquor flows through the
Temperate climate A climate not exhibiting
extremes of either heat or cold, e.g. the Irish climate.
Tempering A process which brings metals to the
proper hardness and elasticity by heating after quenching.
Teratogenic Causing developmental malformations.
Tertiary consumer Organisms which feed on
secondary consumers, e.g. man.
Thermal drying An operation which involves
reducing the water content of sludge by vaporization of water to air,
resulting in a granular dried product of 92 – 95 per cent dry solids
Thermal plume Heated effluent from an outfall,
usually less dense than the receiving water, causing increased growth
rates and species changes due to local warming.
Thermocline A horizontal temperature
discontinuity layer in a lake in which the temperature falls by at least
1 ºC per meter depth.
Thermophilic temperatures Those in the range 45
– 75 ºC.
Thickening of sludge A process which facilitates
disposal of sludge by increasing the solids content to approximately 4
Thiocyanates Pseudohalide ions, formed from the
oxidation of a CN- group, containing an SCN- group.
Threshold of hearing 10 –12 W/m –2
, i.e. the lowest sound intensity to which the human ear can
Toxin A specific poison of biological organic
Transfer station A location to accomplish
transfer of solid wastes from collection and other small vehicles to
larger transport equipment, with the aim of economizing on waste
Transmissivity A measure of the rate of flow of
water through a water-bearing rock.
Trapping A term describing a plume from an
emission stack which is trapped by a stable inversion above the stack
mouth, but due to mixing below the mouth level, hits the ground downwind
rather than beside the stack.
Trickling flter A biological reactor in which
micro-organisms, growing as a slime on the surface of fixed media,
oxidize the colloidal and dissolved organic matter in wastewater using
atmospheric oxygen which diffuses into the thin film of liquid as the
wastewater is trickled over the slimed surfaces at regular intervals.
Transpiration The loss of water vapor from the
surface of the plant due to evaporation.
Trommel A rotary screen used to separate out the
various size fractions of municipal solid waste.
Trophic levels One of the hierarchical strata of
the food web characterized by organisms which are the same number of
steps removed from the primary producers.
Tropopause The interface between the troposphere
Troposphere The layer of atmosphere extending
from the earth’s surface to the stratosphere.
Tubificid worm (Potamothrix hummonensis)
A benthic worm, tolerant of low oxygen conditions, belonging to the
Tundra A vast, level, treeless region with an
arctic climate and vegetation.
Turbidity The clarity of water, i.e. a measure of
the accumulation of collidal particles, determined by light transmission
through the water.
Turbulent mixing When a flow of liquid or air
becomes large, the streamlines become irregular and parcels of the
flowing substance begin to move in a highly irregular path while
maintaining a net downstream velocity.
technique used in water treatment to separate out bacteria larger than 1
- 100 µm.
Upflow column Where the water/wastewater flows
upward under pressure through a column or tank, instead of downward by
gravity. When used in water filtration, it is akin to the backwashing
process in rapid gravity filters. Also used in aerobic digestion of
industrial wastewaters where the column is packed with aggregate or
Vadose zone The entire zone of
negative water pressures above the water table, the lowest portion of
which is permanently saturated by capilliary rise.
Valency The number of electrons which an atom of
an element must either lose or gain to achieve a noble gas structure.
Van der Waals forces The forces which exist
between the molecules in a crystal.
Vibrating screen Used to remove undersized
components of municipal solid waste.
Virion A mature virus.
Volatile acid A fatty acid with, at most, six
carbon atoms which are water soluble.
Volatile solid Solids, frequently organic, which
volatilize at a temperature of 550 ºC.
Vortex shedding Turbulent eddies which are shed
from the downstream corners of buildings in a wind/water environment.
Waste minimization The general
trend in developed countries to reduce the quantities of waste material produced.
Watershed Line between the headstreams of river
systems, dividing one catchment from another.
Water table The level of water within the soil at
which the pore water pressure is equal to the atmospheric pressure.
Wavelength The horizontal distance between two
successive wave crests or between two Wave troughs or between any two
corresponding points on the wave surface.
Waveperiod The time taken for two successive wave
crests or two wave troughs or any two corresponding points on successive
waves to pass a fixed point in space.
Wet oxidation A method whereby waste, either
dissolved in water or emulsified, is oxidized at very high temperatures
Windrow A form of composting in which pretreated
refuse is laid out in heaps with a triangular cross section of 2 – 3 m
width at the base and a height of 2 m and turned at regular intervals.
Zeta potential A measure of the
charge on a colloidal particle.
Zone settling Particles are so close together
that interparticle forces hinder the settling of neighbouring particles,
causing all the particles to remain in a fixed position relative to each
other and to settle at a constant velocity.