Water Glossary

A

Absolute filter rating

A stated particle-retention rating defined by a specified test method and efficiency. It is not an unconditional guarantee that every larger particle will be retained under all operating conditions.

Absorption

Uptake of a substance into the bulk of another material, such as a gas dissolving in a liquid. It differs from adsorption, which occurs at a surface.

Acid aerosol

Very small liquid or solid particles that are acidic and are small enough to become airborne.

Acidic water

Water with a pH below the neutral value at its temperature; at 25 °C, this means a pH below approximately 7.

Acidity

The capacity of water to neutralise a strong base to a specified titration endpoint, often reported as mg/L as CaCO₃. Acidity is not the same as pH or the total number of hydrogen atoms.

Acid neutralising capacity (ANC)

The capacity of a water sample to neutralise strong acid, usually expressed as mg/L as CaCO₃. In USGS analytical usage, ANC is measured on whole water, whereas alkalinity is measured on a filtered sample.

Acid rain

Rain made more acidic by atmospheric pollutants, particularly sulfur and nitrogen oxides that form acids in the atmosphere.

Activated alumina

A porous aluminium-oxide adsorbent used for selected contaminants, including fluoride and some arsenic species. Capacity depends strongly on pH, competing ions and pretreatment.

Activated carbon (activated coal)

A porous carbon-based adsorbent used to reduce selected organic contaminants, taste and odour compounds, and disinfectants. Performance depends on carbon type, contaminant, contact time and water chemistry; it does not remove every dissolved substance.

Activated sludge

A biological wastewater treatment process using suspended microbial flocs. Aeration supports biological oxidation; solids are separated from treated water and a portion is returned to maintain the biomass.

Active groups

Functional groups attached to an ion-exchange resin that provide exchange sites. Their associated counter-ions can be exchanged with ions in the water.

Adsorption

Accumulation of dissolved substances at the surface of a solid, such as activated carbon. Adsorption differs from absorption into the bulk of a material.

Further reading: adsorption.

Advanced oxidation process (AOP)

Treatment that generates highly reactive species, commonly hydroxyl radicals, to transform susceptible contaminants. Examples include UV/hydrogen peroxide and ozone/hydrogen peroxide. Performance and by-products depend on water chemistry; oxidation does not necessarily mean complete mineralisation.

Further reading: advanced oxidation.

Advanced wastewater treatment

Additional treatment to meet objectives beyond conventional secondary treatment, such as nutrient removal, trace-contaminant reduction, filtration or preparation for reuse.

Advanced water treatment

Treatment beyond a conventional process train to meet a particular water-quality objective, such as micropollutant control or high-purity production. There is no universal 85% removal threshold.

Aerated lagoon

A wastewater pond supplied with mechanical or diffused aeration to support biological treatment. Treatment performance depends on loading, mixing, temperature and retention time.

Aeration

Contacting water with air or another gas to transfer gases between phases. Uses include adding oxygen, stripping certain dissolved gases and supporting biological oxidation.

Further reading: aeration.

Aeration tank

A tank where aeration transfers oxygen to water or wastewater, often while mixing the contents and supporting biological treatment.

Aerobic

A process that takes place in the presence of oxygen, such as the digestion of organic matter by bacteria in an oxidation pond.

Aerosol

Very small liquid or solid particles dispersed in air.

Affinity

The relative tendency of an ion-exchange resin or adsorbent to retain a particular species under specified conditions. Competing substances and concentration affect selectivity.

Agglomeration

A process of bringing smaller particles together to form a larger mass.

Aggressive water

Water with a tendency to attack a particular material, such as metals or cement. This depends on pH, alkalinity, salts, dissolved gases, temperature and material; softness alone does not determine corrosivity.

Algae

A diverse group of mainly photosynthetic organisms, ranging from microscopic cells to seaweeds. Duckweed is a flowering plant, not an alga. Cyanobacteria, historically called blue-green algae, are bacteria.

Algal blooms

Rapid increases in algal or cyanobacterial abundance. Some blooms produce toxins or taste and odour compounds, and decomposition can deplete oxygen; not every bloom is toxic.

Aliquot

A measured portion of a sample taken for analysis. One or more aliquots make up a sample.

Alkalinity

The acid-neutralising capacity of water, commonly reported as mg/L as CaCO₃. Bicarbonate, carbonate and hydroxide often contribute. Alkalinity buffers pH changes but does not force water to remain at pH 7.

Alluvium

Sediments deposited by erosion processes, usually by streams.

Ammonia nitrogen

Ammonia and ammonium reported as the mass of nitrogen, commonly mg/L as N. This differs from reporting mg/L as NH₃ or NH₄⁺; the un-ionised ammonia fraction depends on pH and temperature.

Anaerobic

Without free dissolved oxygen. In biological nutrient-removal terminology, an anaerobic zone also lacks nitrate and nitrite as usable electron acceptors; this differs from an anoxic zone.

Anammox

Anaerobic ammonium oxidation: a biological process converting ammonium and nitrite mainly into nitrogen gas. It requires suitable operating conditions and differs from conventional nitrification–denitrification.

Anion

A negatively charged ion, such as chloride (Cl⁻), sulfate (SO₄²⁻) or nitrate (NO₃⁻).

Anion-exchange resin

A resin exchanging negatively charged ions, such as chloride, sulfate or nitrate. Hydroxide-form resins are used in demineralisation; selective resins have application-specific performance.

Anode

The electrode at which oxidation occurs. Metal dissolution is one possible anodic reaction, but some anodes instead support reactions such as oxygen evolution.

Anoxic

In biological treatment, conditions with little or no dissolved oxygen but with nitrate or nitrite available as electron acceptors. Anoxic zones commonly support denitrification.

Antiscalant

A chemical dosed to inhibit or delay mineral-scale formation. Selection and dose depend on water chemistry, recovery and compatibility; antiscalants do not remove dissolved salts.

Aquatic

Growing in water, living in water, or frequenting water.

Aqueous

Describing a solution or process in which water is the solvent or principal liquid medium.

Aqueous solubility

The amount of a substance that can dissolve in water at equilibrium under specified conditions, including temperature, pH and, where relevant, pressure.

Aquifer

A body of permeable rock or unconsolidated sediment that stores and transmits groundwater in useful quantities.

Aromatics

A type of hydrocarbon that contains a ring structure, such as benzene and toluene. They can be found for instance in gasoline.

Assimilation

In biological treatment, incorporation of nutrients or other substances into biomass. In receiving-water discussions, the term can also refer to a water body's processing of added material.

Assimilative capacity

The load a receiving environment can accept while meeting specified environmental-quality objectives. It depends on local conditions and does not imply unlimited or risk-free disposal.

Atom

The smallest unit of matter that is unique to a particular element. They are the ultimate building blocks for all matter.

Atomic number

A specific number that differs for each element, equal to the number of protons in the nucleus of each of its atoms.

Attenuation

A reduction in concentration, mass, mobility or effect through processes such as dilution, sorption, transformation or degradation. Reduced concentration does not always mean destruction.

Attrition

The action of one particle rubbing against the other in a filter media or ion exchange bed that can in time cause breakdown of the particles.

Available chlorine

The oxidising capacity of a chlorine-containing chemical, conventionally expressed as an equivalent amount of chlorine (Cl₂). It differs from the chlorine residual measured after dosing water.

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B

Backflow

Unwanted reversal of flow that can carry contaminated water into a potable-water system. It can result from backpressure or backsiphonage.

Backpressure

Pressure downstream of a connection that exceeds the supply pressure and can cause backflow.

Backsiphonage

Backflow caused by reduced or negative pressure in the water supply, drawing water backwards through a connected outlet.

Backwashing

Reversing the flow of water back through the filter media to remove entrapped solids.

Backwash recovery

Treatment and reuse of water generated during filter backwashing. Its contribution to overall recovery depends on the system boundary and any additional waste streams.

Bacteria

Single-celled prokaryotic microorganisms. Most reproduce by binary fission; bacterial spores are survival structures, not the usual mechanism of reproduction.

Bacterial water contamination

The introduction of unwanted bacteria into a water body.

Base

A substance that accepts protons or contributes hydroxide ions in aqueous solution. A basic aqueous solution has a pH above neutrality at its temperature, not a universal threshold of 7.5.

Bed Load

Sediment particles resting on or near the channel bottom that are pushed or rolled along by the flow of water.

Bed volume (BV)

The bulk volume occupied by a packed bed, including voids. Treated throughput may be expressed as multiples of BV, while flow may be expressed as BV per hour.

Benthic zone

The lower region of a body of water including the bottom.

Bicarbonate

The hydrogen carbonate ion, HCO₃⁻, and salts containing it. Bicarbonate commonly contributes to water alkalinity and reacts with acid to form carbon dioxide and water.

Binder

Chemicals that hold short fibres together in a cartridge filter.

Bioaccumulation

Net accumulation of a substance in an organism when uptake from water, food or other sources exceeds elimination. It differs from biomagnification along a food chain.

Biochemical oxygen demand (BOD)

Dissolved oxygen consumed by biological activity in a water sample under specified test conditions, reported as mg/L O₂. The test conditions determine whether nitrogenous as well as carbonaceous demand is included.

Further reading: BOD and waste water treatment.

Biocide

An agent used to kill or control biological organisms. Effectiveness, material compatibility and environmental impacts depend on the agent, dose, contact time and application.

Biodegradable pollutants

Pollutants that are capable of decomposing under natural conditions.

Biofilm

A community of microorganisms attached to a surface and embedded in an extracellular matrix. Biofilms may be useful in biological treatment or cause fouling in equipment.

Biofouling

Accumulation and activity of microorganisms and their extracellular material on equipment or treatment media, impairing flow, transfer or product quality.

Biological contaminants

Organisms or biological materials that can impair water quality, including pathogens and certain toxins. Viruses are biological agents but are not cellular organisms.

Biologically activated carbon (BAC)

Activated carbon supporting a microbial biofilm, combining adsorption with biological degradation of biodegradable substances. Adsorption capacity and biological activity change during operation.

Biological nutrient removal (BNR)

Biological treatment designed to reduce nitrogen and/or phosphorus, using suitable combinations of aerobic, anoxic and anaerobic conditions and solids removal.

Biological oxidation

Decomposition of complex organic materials by microorganisms through oxidation.

Biomonitoring

The use of living organisms to test the suitability of effluents for discharge into receiving waters and to test the quality of such waters downstream from the discharge.

Bioremediation

Use of organisms or their activity to transform, remove or immobilise contaminants. Treatment may be incomplete and does not necessarily convert all contaminants into carbon dioxide and water.

Biota

All living organisms in a region or ecosystem.

Biotransformation

Conversion of a substance into other compounds by organisms; including biodegradation.

Blackwater

Wastewater containing toilet discharges, including faeces and urine. Definitions for collection and reuse systems may also include other high-strength domestic streams.

Blending

Mixing water streams to achieve a desired composition or flow. The resulting water still needs to meet the applicable quality objectives; dilution is not contaminant destruction.

Blinding

A build-up of particles in a filter medium, that prevents fluids from flowing through.

Blind spots

Any place on a filter medium where fluids cannot flow through.

Blowdown

Withdrawal of water from a boiler, cooling system or other circulating process to limit the accumulation of dissolved or suspended substances.

BOD₅ (BOD5)

Biochemical oxygen demand measured over five days, commonly at 20 °C. Report the test method and whether nitrification was inhibited; BOD₅ and carbonaceous BOD₅ are not necessarily identical.

Boiler feed water

Water supplied to a boiler, often made up of treated make-up water and returned condensate. Required quality depends on boiler pressure, design and operating requirements.

Boiling point

The temperature at which a liquid's vapour pressure equals the surrounding pressure. Pure water boils at approximately 100 °C at standard atmospheric pressure; the value changes with pressure and composition.

Bottled water

Water packaged for drinking in sealed containers. Permitted sources, treatment, composition and labelling depend on the applicable regulations.

Brackish water

Water with salinity between freshwater and seawater. Numerical boundaries vary by classification; specify the salinity or dissolved-solids concentration for engineering work.

Brackish-water reverse osmosis (BWRO)

RO applied to brackish feed water. Membrane selection, pressure, pretreatment and recovery depend on salinity and the full feed-water composition.

Breakpoint chlorination

Chlorine addition sufficient to satisfy demand and pass through the destruction of combined chlorine, after which further dosing produces a free chlorine residual. Reaching breakpoint alone does not establish adequate disinfection.

Breakthrough

The appearance or increase of a target contaminant in a treatment unit's effluent as retention or removal becomes insufficient. In adsorption and ion exchange, it commonly signals exhaustion of usable capacity.

Brine

Water containing a high concentration of dissolved salts. Brine does not necessarily contain heavy metals or organic contaminants; composition depends on its source.

Bromate

An inorganic disinfection by-product that can form when ozone reacts with bromide-containing water. Formation depends on water chemistry and operating conditions.

Buffer

A substance that reacts with hydrogen or hydroxyl ions in a solution, in order to prevent a change in pH.

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C

Cake

Solid dewatered residue on a filter media after filtration.

Calcium hypochlorite

A chlorine-releasing chemical used for disinfection and oxidation. Its available-chlorine content, storage conditions and dosing requirements depend on the product.

Candle filter

A filter using elongated cylindrical elements. Depending on design, it may use porous elements directly or support a precoat/filter cake.

Capillary action

Movement of a liquid in narrow pores or tubes caused by surface tension and interactions with the solid surface. It can raise water above the surrounding free-water level.

Capillary membranes

Small tubular or hollow-fibre membrane elements, commonly used in microfiltration and ultrafiltration. Their dimensions and separation performance depend on the design.

Capillary zone

The region above the water table in which capillary forces draw groundwater upwards into pores. It is also called the capillary fringe.

Carbonate hardness

The portion of total hardness equivalent to carbonate and bicarbonate alkalinity. When both are expressed as CaCO₃, it is commonly taken as the smaller of total hardness and total alkalinity.

Carbonates

Salts or other compounds containing the carbonate ion, CO₃²⁻. Dissolved carbonate participates in alkalinity and calcium-carbonate scale formation.

Carcinogen

An agent capable of causing cancer. It need not be dissolved in water; risk depends on the substance, route and extent of exposure.

Cartridge filter

A replaceable filter element fitted inside a housing. Retention rating, efficiency, pressure limits and compatibility depend on the cartridge; a micron rating alone does not establish microbial safety.

Further reading: cartridge filters.

Catalyst

A substance that increases a reaction rate without being consumed overall. Catalysts participate in reaction steps and may become deactivated during use.

Catch basin

A sedimentation area designed to remove pollutants from runoff before being discharged into a stream or pond.

Cathode

The electrode at which reduction occurs. Possible reactions include metal deposition and hydrogen evolution, depending on the system.

Cation

A positively charged ion, such as sodium (Na⁺), calcium (Ca²⁺) or magnesium (Mg²⁺).

Cation-exchange resin

A resin exchanging positively charged ions. Sodium-form resins are commonly used for softening; hydrogen-form resins can be used in demineralisation.

Centrifugation

A separation process, which uses the action of centrifugal force to promote accelerated settling of particles in a solid-liquid mixture.

More info on centrifugation

Check valve

A valve that allows water to stream in one direction and will then close to prevent development of a back-flow.

Chelating agents

Substances that bind a metal ion through two or more coordinating sites to form a complex. Chelation can alter metal solubility and affect treatment.

Chemically enhanced backwash (CEB)

A membrane backwash assisted by a compatible cleaning chemical. It differs from a longer or more intensive cleaning-in-place procedure; methods are membrane-specific.

Chemical oxygen demand (COD)

The oxygen-equivalent amount of material oxidised by a specified chemical test, reported as mg/L O₂. COD includes many organic and some inorganic reducing substances; it does not measure every pollutant or guarantee complete oxidation.

Chemical pollution

Introduction of chemical contaminants into a water body.

Chemical precipitation

Conversion of dissolved species into a solid phase through changes in chemical conditions, often followed by settling or filtration. It differs from atmospheric precipitation.

Chemical weathering

Dissolving of rock by exposure to rainwater, surface water, oxygen, and other gases in the atmosphere, and compounds secreted by organisms.

Chloramines

Compounds formed when chlorine reacts with ammonia or related nitrogen compounds. Monochloramine is used as a secondary disinfectant; chloramination can also form by-products and does not provide identical pathogen control to free chlorine.

Chlorinated hydrocarbons

Hydrocarbons that contain chlorine. These include a class of persistent insecticides that accumulate in the aquatic food chain. Among them are DDT, aldrin, dieldrin, heptachlor, chlordane, lindane, endrin, Mirex, hexachloride, and toxaphene.

Chlorinated solvent

An organic solvent containing chlorine, such as trichloroethene or tetrachloroethene. Environmental behaviour and health risks differ between substances.

Chlorination

A water purification process in which chlorine is added to water for disinfection, for the control of present microorganisms. It is also used in the oxidation of compound impurities in water.

Chlorine-contact chamber

A chamber providing controlled contact between water and a chlorine disinfectant. Effective contact time depends on flow and hydraulics, including short-circuiting.

Chlorine residual

Chlorine remaining after the water's chlorine demand has been partly or fully satisfied. State whether the result is free, combined or total chlorine and report the measurement basis.

Clarifier

A settling unit separating suspended solids from water or wastewater. Collected sludge and surface scum are removed; clarification does not by itself remove all dissolved contaminants.

Clarity

The clearness of a liquid.

Cleaning-in-place (CIP)

Cleaning equipment by circulating suitable solutions without routine disassembly. Chemicals, sequence, concentration, temperature and exposure time must be compatible with the equipment and foulant.

Coagulant

A chemical used to destabilise colloids or form precipitates that capture particles. Common examples include aluminium and iron salts; selection and dosing depend on water chemistry.

Coagulation

Destabilisation of particles and colloids, commonly by dosing metal salts or other coagulants. Mechanisms include charge neutralisation and entrapment in precipitates; flocculation subsequently promotes aggregation.

Further reading: coagulation.

Coalescence

Liquid particles in suspension that unite to create particles of a greater volume.

Coastal zone

Lands and waters near the coast, whose uses and ecology are affected by the sea.

Coliform bacteria

A group used as indicators in water-quality monitoring. Total coliforms can occur in the environment and are not all evidence of faecal contamination; E. coli is a more specific indicator of faecal contamination.

Coliform index

A reported measure of coliform occurrence or concentration under a specified test. Interpretation requires the method, sample volume and indicator group; it does not by itself prove water is safe.

Collector sewers

Pipes to collect and carry wastewater from individual sources to an interceptor sewer that will carry it to a treatment facility.

Colloids

Very small dispersed particles, commonly described as roughly 1 nm to 1 µm, with boundaries varying by convention. They can remain suspended and may need coagulation or suitable membranes for removal.

Colony-forming units (CFU)

A culture-based estimate of viable organisms capable of forming colonies under specified conditions. One colony may originate from one cell or a cluster; CFU is not a count of all microorganisms.

Combined sewer

A sewer system that carries both sewage and rain water runoff.

Composite sample

A sample formed by combining individual samples over a defined period or set of locations. Collection may be time-proportional or flow-proportional, depending on the objective.

Compounds

Two or more different elements held together in fixed proportions by attractive forces called chemical bonds.

Concentrate

The stream enriched in retained solutes or particles during a separation process. In RO and NF it is the feed-side outlet, also called reject or retentate; it is not material simply trapped in a filter.

Concentration

The quantity of a substance relative to a specified amount of mixture, for example mg/L, mol/L or mass fraction. State the units and reporting basis.

Concentration factor

The ratio of a substance's concentration in a stream to a stated reference, such as concentrate to feed. It is not automatically equal to the theoretical value inferred from water recovery.

Concentration polarisation

Accumulation of retained solutes near a membrane surface, creating a local concentration higher than the bulk feed. It can increase osmotic resistance and scaling or fouling risk.

Concentration process

The process of increasing the number of particles per unit volume of a solution, usually by evaporating the liquid.

Condensate

Water obtained by condensation of water vapour.

Condensation

The change of state from a gas to a liquid.

Conductivity

A measure of water's ability to conduct electrical current, commonly reported in µS/cm or mS/cm. It depends on ionic composition and temperature; conversion to TDS is an estimate, not a universal fixed factor.

Further reading: TDS and conductivity.

Conduit

A natural or artificial channel through which fluids may be transported.

Consumptive water use

Water removed from available supplies without return to a water resources system; water used in manufacturing, agriculture, and food preparation.

Contact time

The period during which water contacts a treatment agent or medium. Effective hydraulic contact time can differ from tank volume divided by flow because of mixing and short-circuiting.

Contaminant

Any foreign component in a substance, for example in water.

Conventional sewer systems

Systems that were traditionally used to collect municipal wastewater in gravity sewers and convey it to a central primary or secondary treatment plant, before discharge on receiving surface waters.

Conveyance loss

Water loss in pipes and channels by leakage or evaporation.

Cooling tower

Large tower used to transfer the heat in cooling water from a power or industrial plant to the atmosphere either by direct evaporation or by convection and conduction.

Corrosion control

Measures to reduce material deterioration and release of metals, using appropriate chemistry, materials and operating conditions. Scale indices alone do not establish corrosion performance.

Corrosivity

Ability of water to dissolve or break down certain substances, particularly metals.

Cross-flow filtration

Filtration in which feed flows mainly along the membrane surface while permeate passes through it. The tangential flow helps limit accumulation but does not eliminate fouling.

Cryptosporidium

A protozoan parasite whose oocysts can cause gastrointestinal illness. Oocysts are highly resistant to routine chlorination; control may use suitable filtration and validated UV or other effective barriers.

CT value

For chemical disinfection, disinfectant residual concentration multiplied by effective contact time, usually mg·min/L. The required value depends on disinfectant, target organism, temperature, pH and the applicable method.

Cultural eutrophication

Human-caused nutrient enrichment of water, which can promote excessive algal or plant growth and subsequent oxygen depletion.

Current

The portion of a stream or body of water, which is moving much faster than the rest of the water. The progress of the water is principally concentrated in the current.

Cyanobacteria

Photosynthetic bacteria sometimes called blue-green algae. Some species can produce toxins; treatment must distinguish intact cells from dissolved toxins.

Cycle

The length of time a filter can be used before it needs cleaning, usually including cleaning time.

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D

Dead-end filtration

Filtration in which the main feed flow passes towards the filter surface and retained material accumulates. Periodic backwashing, cleaning or replacement is needed.

Deaeration

Removal of dissolved gases from water, often specifically oxygen and carbon dioxide from boiler feed water. Thermal, vacuum and membrane methods have different operating requirements.

Dealkalinisation

Any process that serves to reduce the alkalinity of water.

Decant

To draw off the upper layer of liquid after the heaviest material (a solid or another liquid) has settled.

Decarbonation

The process of removing carbon dioxide from water, using contact towers or air scrubbers.

Dechlorination

Reduction or removal of chlorine residual, for example using activated carbon or a suitable reducing agent. It can protect sensitive membranes but also removes a barrier against microbial growth.

Decomposition

Breakdown of material through biological, chemical or physical processes. In wastewater treatment it often refers to microbial conversion of organic matter.

Defluoridation

Reduction of fluoride concentration in water, using suitable processes such as adsorption or membrane treatment. The required level depends on the intended use and applicable requirements.

Defoaming agents

Chemicals used to suppress or break foam in treatment processes. Selection must consider downstream membranes, oxygen transfer and product-water requirements.

Degasification

Removal of dissolved gases, for example by vacuum, stripping, heating or membrane contactors. Effectiveness depends on the gas and operating conditions.

Deionisation (DI)

Reduction of dissolved ionic species, commonly using cation- and anion-exchange media. Deionised water is not automatically free of particles, organic substances or microorganisms.

Demineralisation

Reduction of dissolved mineral ions, using processes such as ion exchange, RO or electrodialysis. The term does not imply absolute removal of every contaminant.

Demiwater

A common term for demineralised water. Its required conductivity and other quality attributes must be specified for the application.

Denitrification

Biological reduction of nitrate or nitrite, usually to nitrogen gas under anoxic conditions. It differs from nitrification, which oxidises ammonia.

Density

Mass per unit volume, commonly expressed in kg/m³. Water density varies with temperature, pressure and dissolved substances.

Deoxygenation

Reduction of dissolved oxygen by methods such as thermal deaeration, vacuum or membrane degassing, or chemical oxygen scavengers. The required residual depends on the application.

Depression storage

The storage of water in low areas, such as ponds, and wetlands.

Depth filtration

Treatment process in which the entire filter bed is used to trap insoluble and suspended particles in its voids as water flows through it.

Desalination

The removal of salt from seawater or brackish water to produce drinking water, using various techniques.

Desorption

The opposite of adsorption; the release of matter from the adsorption medium, usually to recover material.

Detection limit

The lowest quantity or concentration distinguishable from background under a specified analytical method. It differs from a quantification limit and must be considered when interpreting non-detects.

Detention time

The time water remains in a vessel. Nominal hydraulic detention time is working volume divided by flow; actual residence times vary with mixing, dead zones and short-circuiting.

Detergent

A water-soluble cleansing agent, other than soap.

Dewater

The separation of water from sludge, to produce a solid cake.

Differential pressure (ΔP)

The pressure difference between two measurement points, commonly across a filter or membrane stage. A rise can indicate increased flow resistance; it is distinct from transmembrane pressure.

Diffuser

A device distributing a gas into water as bubbles, for example in aeration or ozone contacting. Gas-transfer performance depends on the diffuser and process conditions.

Diffusion

Net molecular transport driven by a concentration or chemical-potential gradient, tending to distribute substances more evenly.

Digester

A reactor where microorganisms stabilise sludge or other organic material. Digesters may be aerobic or anaerobic; anaerobic digestion can produce biogas.

Dilution water

Water used to dilute a sample or solution. In BOD testing it is prepared with specified nutrients, buffering and oxygen conditions, and may require seeding.

Direct run-off

Water that flows from the ground surface directly into streams, rivers, and lakes.

Discharge

The flow leaving a source or treatment system; in hydrology, the volume passing a cross-section per unit time. An effluent discharge can also carry a contaminant load.

Disinfectant

An agent used to inactivate microorganisms, such as chlorine or ozone. Disinfection performance depends on the organism, dose, contact time and water quality.

Disinfection

Inactivation of microorganisms to achieve a specified risk reduction. It is not the same as sterilisation and does not necessarily remove chemical contaminants.

Further reading: disinfection.

Disinfection by-products (DBPs)

Compounds formed when disinfectants react with water constituents. Examples include trihalomethanes and haloacetic acids; treatment balances microbial control and by-product management.

Dissolve

The process during which solid particles mix molecule by molecule with a liquid and appear to become part of the liquid.

Dissolved air flotation (DAF)

Separation using fine air bubbles released from pressurised, air-saturated water. Bubbles attach to particles or flocs, carrying them to a surface layer for removal.

Dissolved organic carbon (DOC)

The organic carbon measured in the filtrate after a specified filtration procedure, usually reported as mg/L as C. DOC is a fraction of total organic carbon.

Dissolved oxygen (DO)

Molecular oxygen dissolved in water, commonly measured in mg/L or as percentage saturation. Temperature, salinity and pressure affect saturation.

Dissolved solids

Material remaining in the filtrate after a specified filtration step, measured by the relevant analytical procedure. Ordinary particle filtration does not remove dissolved salts.

Distillation

Evaporating water and condensing its vapour to separate it from many non-volatile impurities. Volatile contaminants may carry over, so product quality depends on design and operation.

Dosing

Controlled addition of a treatment chemical. Distinguish the concentration of the supplied solution, the applied dose and the residual concentration remaining after reaction.

Double-pass reverse osmosis

A system in which permeate from a first RO pass feeds a second RO pass to improve product quality. A pass is different from a stage on the concentrate side.

Dredging

Cleaning, deepening, or widening of a waterway, using a machine (dredge) that removes materials by means of a scoop or a suction device.

Drought

Term applied to periods of less than average precipitation over a certain period of time.

Duplicates

Two separate samples with separate containers taken at the same time and at the same place.

Dystrophic lakes

Lakes typically rich in humic organic matter, often brown-coloured and acidic. Their ecology varies; low fish abundance is not a defining requirement.

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E

Effluent

Water or wastewater leaving a treatment unit or facility. Effluent is not automatically drinking water or suitable for discharge or reuse without meeting the relevant quality requirements.

Ejector

A device that uses a high-velocity motive fluid to create suction, draw in another fluid or gas, and mix or transport it.

Electrical charge

A property arising from an imbalance of positive and negative charge. An ion's charge reflects the difference between its proton and electron counts; losing electrons makes a cation, gaining electrons makes an anion.

Electrical conductivity (EC)

See Conductivity and Specific conductance. State the measurement temperature or reference temperature and use units such as µS/cm.

Electrodeionisation (EDI)

A process combining ion-exchange resin, ion-selective membranes and direct current to remove ions, typically from RO permeate. Electrical regeneration reduces the need for routine acid/alkali resin regeneration; concentrate and cleaning requirements still apply.

Electrodialysis (ED)

Separation in which an electric field drives dissolved ions through selective ion-exchange membranes into concentrate compartments, producing a lower-salinity stream.

Electrodialysis reversal (EDR)

Electrodialysis with periodic reversal of electrical polarity and corresponding flow functions to help control deposits. It does not eliminate pretreatment or cleaning requirements.

Electrolysis

Use of electrical energy to drive electrochemical reactions. Oxidation occurs at the anode and reduction at the cathode; the products depend on the electrolyte, electrodes and conditions.

Electrolyte

Substance that dissociates into ions when it dissolves in water.

Electrons

Negatively charged subatomic particles occupying quantum states around atomic nuclei. Electron transfer is central to oxidation–reduction reactions.

Elements

The distinctive building blocks of matter that make up every material substance.

Elutriation

Freeing sludge of its mother liquor by washing it with water.

Empty bed contact time (EBCT)

Packed-bed volume divided by volumetric flow rate, commonly expressed in minutes. It is a design parameter for adsorption contactors, not the actual time spent in the bed's water-filled voids.

Emulsifier

A chemical that helps suspending one liquid in another.

Emulsion

A dispersion of droplets of one liquid in another liquid with which it is not fully miscible, such as oil in water.

End-of-pipe treatment

Treatment applied after waste or emissions have been generated, before their release. It differs from preventing waste at its source.

Endotoxin

A lipopolysaccharide-related component of Gram-negative bacteria that can trigger biological responses, including fever. Killing bacteria does not necessarily remove their endotoxins.

Energy recovery device (ERD)

Equipment recovering pressure energy from a concentrate or other high-pressure stream, commonly used to reduce seawater-RO pumping energy.

Enrichment

When the addition of nutrients, such as nitrogen and phosphorus, from sewage effluent or agricultural runoff to surface water, greatly increases algal growth.

Erosion

The wearing away of the land surface by wind, water, ice or other geological agents. Erosion occurs naturally from weather or runoff but is often intensified by human land use practices.

Escherichia coli (E. coli)

A bacterial species found in the intestines of humans and other warm-blooded animals. Its detection in water is an indicator of faecal contamination; some strains can cause disease.

Estuary

Region of interaction between rivers and near-shore ocean waters, where tidal action and river flow mix fresh and salt water. Therefore estuaries mainly consist of brackish water.

Eutrophic

Referring to water that is rich in nutrients such as nitrogen and phosphorous.

Eutrophication

Nutrient enrichment that can increase algal and plant production. Subsequent decomposition may reduce dissolved oxygen; severity depends on the water body and environmental conditions.

Evaporation

The process of the passage of water from liquid to vapour.

Evaporation ponds

Basins designed to concentrate liquids or manage wastewater by evaporation. They can accumulate salts and other contaminants and require appropriate containment and residuals management.

Evapotranspiration

The loss of water from the soil through vaporizing, both by direct evaporation and by transpiration from plants.

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F

Facultative bacteria

Bacteria that can live under aerobic or anaerobic conditions.

Feed water

Water entering a specified treatment unit. Feed-water quality must be assessed at that unit's inlet, after any upstream treatment or mixing.

Fermentation

Microbial conversion of organic compounds without using oxygen as an external electron acceptor, producing substances such as organic acids, alcohols and gases. Methane production is a distinct stage in anaerobic digestion.

Filter medium

The permeable material that separates solids from liquids passing through it.

Filtrate

A liquid that has passed through the filter medium.

Filtration

Separation using a medium or membrane that selectively retains particles or other constituents. What passes through depends on the filter and the substances present.

First-draw sample

Water collected immediately after an outlet is opened following a specified stagnation period. It can assess plumbing-related contamination, but the protocol and sampling objective must be stated.

Fission

Reproduction of microorganisms by means of cell division.

Floc

A flocculent mass that is formed in the accumulation of suspended particles. It can occur naturally, but is usually induced in order to be able to remove certain particles from wastewater.

Flocculant

A substance, often a polymer, used to promote particle aggregation or strengthen flocs. It is distinct from the physical mixing process called flocculation.

Flocculation

Gentle mixing that brings destabilised particles together into larger aggregates called flocs. A polymer may assist the process, but adding a separate flocculant is not always required.

Further reading: flocculation.

Floodplain

The flat or nearly flat land along a river or stream that is covered by water during a flood.

Flotation

Separation that brings dispersed material to the liquid surface, commonly using gas bubbles attached to particles or droplets. It can remove particles denser than water when bubbles provide sufficient buoyancy.

Flow

The discharge rate of a resource, expressed in volume during a certain period of time.

Flow augmentation

The addition of water to meet flow needs.

Flux

Flow through a membrane per unit membrane area, commonly expressed in L/m²/h (LMH). It applies to several membrane processes, not only RO.

Food-to-microorganism ratio (F/M)

The applied organic load divided by the mass of biomass in a biological reactor over a stated time. State the load basis, such as BOD₅ or COD, and the biomass basis.

Fouling

Accumulation of material on or within a membrane, filter, resin or other surface that impairs performance. Foulants may include particles, organics, minerals and biological growth.

Fragmentation

Breakup of a larger object or particle into smaller pieces. The fragments do not necessarily attach to another surface.

Free chlorine

The sum of hypochlorous acid and hypochlorite ion, conventionally reported as mg/L as Cl₂. Their relative proportions depend strongly on pH.

Freezing

Transition from liquid to solid. Pure water freezes near 0 °C at standard atmospheric pressure; dissolved substances and pressure can change this value.

Freshwater

Water with relatively low salinity. A common USGS classification uses less than 1000 mg/L dissolved solids; boundaries depend on context, and freshwater is not necessarily potable.

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G

Gallon

A volume unit still used in water engineering. One US liquid gallon is approximately 3.785 L; one imperial gallon is approximately 4.546 L. Specify which is intended.

Giardia

A protozoan parasite that can cause gastrointestinal illness. Its cysts require appropriate filtration or validated disinfection; resistance and required treatment differ from those of Cryptosporidium.

Granular activated carbon (GAC)

Activated carbon in granular form, used in fixed beds for adsorption and sometimes biological treatment. Performance depends on the target substance, carbon properties, competing contaminants and contact time.

Greywater (gray water)

Wastewater from selected domestic sources other than toilets, commonly showers, baths and laundry. Whether kitchen wastewater is included depends on the local definition and reuse scheme.

Groundwater

Water in the saturated pores and fractures of soil or rock below the water table. It moves in response to hydraulic-head gradients.

Groundwater discharge

Groundwater flowing into springs, rivers, lakes, wetlands, coastal waters or other outlets. Discharge is not inherently contaminated.

Groundwater hydrology

The branch of hydrology that deals with the occurrence, movements, replenishment and depletion, properties and methods of investigation and utilisation of groundwater.

Gully

A deeply eroded channel created by the concentrated flow of water.

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H

Half-life

The time for a quantity or concentration to decrease to half its initial value under specified conditions. It can refer to radioactive decay or a particular transformation process.

Hard water

Water containing appreciable dissolved calcium and magnesium, usually expressed as total hardness in mg/L as CaCO₃. Hardness can promote scale and increase soap consumption; it is not a measure of all dissolved ions.

Further reading: hard water.

Head loss

Loss of hydraulic head as water flows through pipes, fittings, filters or other equipment. It may be expressed as a liquid-column height or equivalent pressure difference.

Heat exchanger

Equipment that transfers heat between fluids or between a fluid and another medium, usually without mixing the streams.

Heavy metals

An imprecise collective term without one universally accepted definition. For water assessment, identify the specific elements, chemical forms and concentrations rather than assuming all metals share the same toxicity.

Further reading: heavy metals.

Heavy water

Water enriched in the hydrogen isotope deuterium, commonly represented as D₂O. It is chemically distinct from ordinary H₂O.

Henry's law

At a fixed temperature and dilute equilibrium conditions, the dissolved concentration of a gas is related to its partial pressure above the liquid. State the convention and units used for the Henry constant.

Holding Pond

A pond or reservoir, usually made of earth, built to store polluted runoff.

Hollow-fibre membrane

A membrane formed as a narrow tube, assembled in bundles. Feed and permeate may flow inside or outside the fibres depending on the module design.

Homeowner water system

A water system that supplies piped water to a single residence.

Humidification

The addition of water vapour to air.

Hydraulic conductivity

A property describing how readily a porous medium transmits a particular fluid under a hydraulic gradient. It depends on both the medium and fluid properties and is commonly expressed in m/s.

Hydraulic gradient

Change in hydraulic head per unit distance along a flow path. It helps determine groundwater-flow direction and rate.

Hydraulic retention time (HRT)

Nominal reactor liquid volume divided by the flow through it. HRT describes water residence, whereas solids retention time describes biomass residence.

Hydrocarbon

Organic compounds that are built of carbon and hydrogen atoms and are often used in petroleum industries.

Hydroelectric power water use

The use of water in the generation of electricity at plants where the turbine generators are driven by falling water.

Hydrogen peroxide

An oxidising chemical, H₂O₂, used in selected oxidation, disinfection and cleaning applications. It can support advanced oxidation when combined with UV, ozone or suitable catalysts.

Hydrogen sulfide (H₂S)

A toxic gas that can form under reducing conditions, for example during sulfate reduction. It has a rotten-egg odour at low concentrations, but smell is not a reliable safety warning.

Hydrogeology

The study of groundwater occurrence, movement and interaction with geological materials, including its chemistry.

Hydrograph

A graph of a water-related variable, such as river discharge, water level or groundwater head, against time.

Hydrologic cycle

The natural cycle of water passing through the environment, including evaporation, condensation, retention and run-off.

Hydrolysis

A chemical reaction in which water participates in breaking or changing chemical bonds. It occurs in both organic and inorganic chemistry.

Hydrophilic

Having an affinity for water.

Hydrophobic

Having an aversion for water.

Hydrosphere

Region that includes all the earth's liquid water, frozen water, floating ice, frozen upper layer of soil, and the small amounts of water vapour in the atmosphere.

Hypochlorite

The anion OCl⁻, present in disinfectants such as sodium hypochlorite and calcium hypochlorite. Its balance with hypochlorous acid depends strongly on pH.

Hypoxic waters

Waters with low dissolved oxygen. A value below about 2 mg/L is often used to describe hypoxia, but biological effects and thresholds vary among organisms and conditions.

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I

Ice

The solid form of water.

Imhoff cone

A clear, cone-shaped container used to measure the volume of settleable solids in a specific volume of water.

Immiscibility

The inability of liquids to mix into a single homogeneous phase in all proportions under specified conditions, as with oil and water.

Impermeable

Not easily penetrated by water.

Impurities

Unwanted constituents relative to the intended water use, including dissolved chemicals, particles and microorganisms. Clear water can still contain impurities.

Indicator

Any biological entity or process, or community whose characteristics show the presence of specific environmental conditions or pollutants.

Indicator organisms

Organisms monitored as evidence of contamination or treatment performance. Their presence or absence does not directly measure every pathogen.

Indicator tests

Tests for a specific contaminant, group of contaminants, or constituent which signals the presence of something else.

Indirect discharge

Introduction of pollutants from a non-domestic source into a publicly owned wastewater treatment system. Indirect dischargers can be commercial or industrial facilities whose wastes enter local sewers.

Infiltration

Penetration of water into a medium, for instance the soil.

Influent

The stream of water that enters any system or treatment unit.

Inhibitor

chemical that interferes with a chemical reaction, such as precipitation.

Injection

The introduction of a chemical or medium into the process water to alter its chemistry or filter specific compounds.

Inorganic chemicals

A broad class generally distinguished from organic carbon compounds. Some carbon-containing species, including carbonates and carbon dioxide, are classed as inorganic.

Integrity testing

Testing for defects or leaks that can compromise a membrane or filter barrier. The method, sensitivity and relationship to removal claims must be specified.

Ion

An atom or group of atoms carrying a net electrical charge. Cations are positive and anions negative; ions can occur in solution and in other states of matter.

Ion-exchange capacity

The quantity of ions a resin can exchange, expressed on an equivalent-charge basis. Total capacity and usable operating capacity differ and depend on regeneration and service conditions.

Ion exchange (IX)

Reversible exchange of ions between water and a solid exchanger while maintaining electrical balance. Applications include softening, demineralisation and selective contaminant removal.

Further reading: ion exchange.

Irrigation

Applying water or wastewater to land areas to supply the water and nutrient needs of plants.

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J

Jar test

A laboratory test procedure with differing chemical doses, mix speeds, and settling times, to estimate the minimum or ideal coagulant dose required to achieve water quality goals.

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K

Kinetic energy

Energy associated with motion. In fluid systems it contributes to the total mechanical energy of flowing water.

Kinetic rate coefficient

A parameter in a rate equation describing the speed of a reaction or process under specified conditions. Its units depend on the model.

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L

Laboratory water

Water purified to a quality appropriate for a laboratory application. Relevant attributes may include conductivity, organic carbon, particles and microbial contamination; no single grade suits every method.

Lagoon

A pond used to store or treat water or wastewater. Treatment lagoons may be aerobic, facultative or anaerobic, depending on design and operation.

Lake

An inland body of standing water occupying a basin. Lakes may be fresh, brackish or saline.

Laminar flow

Flow in which fluid moves in orderly layers with limited cross-stream mixing. The transition to turbulence depends on geometry and operating conditions.

Land Application

Discharge of wastewater onto the ground for treatment or reuse.

Langelier saturation index (LSI)

An index of calcium-carbonate saturation, calculated as measured pH minus the calculated saturation pH. Positive values indicate supersaturation and negative values undersaturation; LSI is not a complete predictor of metal corrosion.

Large water system

A size category whose definition depends on the jurisdiction and programme. US EPA commonly uses more than 50,000 people served for a large public water system; people served is not the same as customer accounts.

Leachate

Water that contains solute substances, so that it contains certain substances in solution after percolation through a filter or soil.

Leaching

The process by which soluble constituents are dissolved and filtered through the soil by a percolating fluid.

Leakage

Unwanted passage of a fluid or substance. In ion exchange, leakage means target ions appearing in the treated effluent despite operation before full exhaustion.

Light absorption

Uptake of light energy by a material. In water analysis, absorbance is measured at a specified wavelength and path length and can indicate particular constituents.

Lime

In water treatment, quicklime (CaO) or hydrated lime (Ca(OH)₂), used for pH adjustment, softening or other treatment. Calcium-carbonate scale is not the same material as lime.

Limnology

The study of the physical, chemical, hydrological, and biological aspects of fresh water.

Liquid

A state of matter, neither gas nor solid, that flows and takes the shape of its container.

LMH

Litres per square metre per hour (L/m²/h), a common unit of membrane flux. Compare flux values only with their operating conditions and membrane duty stated.

Log reduction value (LRV)

Base-10 logarithm of the ratio of inlet to outlet concentration. A 1-log reduction is 90%, 2-log is 99%, and 3-log is 99.9%; detection limits constrain what can be demonstrated.

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M

Make-up water

Water added to replace losses from a circulating or operating system, such as a cooling tower or boiler circuit.

Maximum contaminant level (MCL)

An enforceable contaminant limit under a specified drinking-water regulatory programme. In US federal usage, an MCL applies to public water systems and differs from a non-enforceable health goal (MCLG).

Mechanical aeration

Use of mechanical energy to inject air into water to cause a waste stream to absorb oxygen.

Mechanical flotation

A term used in the mineral industry to describe the use of dispersed air to produce bubbles that measure 0.2 to 2 mm in diameter.

Media

Materials used for filtration, adsorption, ion exchange or biological growth support. The removal mechanism depends on the material and process.

Medium-size water system

A programme-specific category. US EPA commonly uses 3301–50,000 people served for a medium public water system; other authorities may use different boundaries.

Melting

The change of a solid into a liquid.

Membrane

A selective barrier through which some components pass more readily than others. Separation can involve size, charge, diffusion and other mechanisms, depending on the membrane.

Membrane bioreactor (MBR)

Biological wastewater treatment combined with membrane separation of solids, commonly MF or UF. The membrane retains biomass but does not by itself remove all dissolved salts or nutrients.

Membrane stage

A group of membrane pressure vessels operating in parallel at one step of an array. Concentrate may feed the next stage; a second pass instead treats permeate.

Mesotrophic

Reservoirs and lakes which contain moderate quantities of nutrients and are moderately productive in terms of aquatic animal and plant life.

Metabolise

Conversion of food, for instance soluble organic matter, to cellular matter and gaseous by-products through a biological process.

Microbial growth

The multiplication of microorganisms such as bacteria, algae, diatoms, plankton, and fungi.

Microfiltration (MF)

A pressure-driven membrane process mainly retaining suspended particles and selected microorganisms. It normally passes dissolved salts; retention depends on the membrane and its integrity.

Microfiltration system (MFS)

A membrane system for removing particles and selected microorganisms. It generally does not remove dissolved salts, and its microbial performance depends on the membrane and integrity.

Further reading: MFS.

Micron (micrometre, µm)

A unit of length equal to one millionth of a metre, or 0.001 mm. A filter's micron rating must be interpreted with its efficiency and test method.

Microorganisms

Microscopic organisms, including bacteria, some fungi, algae and protozoa. Viruses are often discussed alongside them in water treatment but are acellular biological agents.

Mineral water

Water containing dissolved mineral constituents. The regulated term natural mineral water can have specific source and labelling requirements; mineral content alone does not establish drinking-water safety.

Miscibility

The ability of two liquids to mix.

Mist

Small liquid droplets suspended in a gas, produced for example by spraying, splashing or condensation. Size boundaries between mist, fog and spray depend on the convention.

Mixed-bed ion exchange

Cation- and anion-exchange resins used together in one bed, commonly for polishing demineralised water. Exhausted media require appropriate regeneration or replacement.

Mixed liquor suspended solids (MLSS)

Suspended-solids concentration in the mixed contents of a biological reactor, including biomass and inert solids. Usually reported in mg/L.

Mixture

Various elements, compounds or both, that are mixed.

Molecular weight cut-off (MWCO)

A membrane retention rating based on a specified reference solute and test, commonly expressed in daltons. It is not a sharp universal boundary for every molecule.

Molecules

Combinations of two or more atoms of the same or different elements held together by chemical bonds.

Moving-bed biofilm reactor (MBBR)

A biological reactor using mobile carriers that support biofilm. Carrier retention and downstream solids separation are part of the process design.

Municipal discharge

An outflow from a municipal system, such as treated wastewater entering receiving water. The term is not restricted to coastal drainage basins.

Municipal sewage

Liquid wastes, originating from a community. They may have been composed of domestic wastewaters or industrial discharges.

Municipal Sludge

Semi liquid residue that remains from the treatment of municipal water and wastewater.

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N

Nanofiltration (NF)

A pressure-driven membrane process often used for hardness, colour and selected organic-contaminant reduction. Retention of monovalent salts varies considerably; NF is not interchangeable with RO.

Natural organic matter (NOM)

A diverse mixture of naturally occurring organic substances in water. NOM can affect colour, biological growth, treatment demand, adsorption and disinfection by-product formation.

Net driving pressure (NDP)

For RO/NF, the hydraulic pressure difference available for water transport after allowing for the osmotic-pressure difference and the relevant pressure losses. State how average pressures are calculated.

Neutralisation

Reaction of acidic and basic constituents, commonly used to adjust water pH. The endpoint and target pH depend on the chemistry and treatment objective; equivalence need not occur at pH 7.

Neutrons

Uncharged building blocks of an atom that play a part in radio-activity. They can be found in the nucleus.

Nitrification

Biological oxidation of ammonia to nitrite and then nitrate, consuming oxygen and alkalinity. It changes the nitrogen form but does not by itself remove total nitrogen from water.

Nominal filter rating

A stated particle-retention rating that represents a specified or typical removal efficiency under defined conditions. It is not interchangeable with an absolute rating without comparing test methods and efficiencies.

Non-detect (ND)

A result below the specified analytical detection or reporting threshold. It does not establish that a substance is completely absent.

Non-point sources

Diffuse water pollution sources without a specific point of origin. The pollutants are generally carried off the land by storm water. Common non-point sources are agriculture and atmospheric disposal.

Non-potable water

Water not designated or demonstrated suitable for drinking. It may still be suitable for a specified industrial, irrigation or other use after appropriate assessment.

Normalised membrane performance

Membrane flow, salt passage or pressure behaviour adjusted to reference conditions to distinguish operating-condition changes from deterioration. Use a consistent calculation method.

NTU and FNU

Nephelometric turbidity units and formazin nephelometric units. They reflect specified optical methods and are not automatically interchangeable or equivalent to mg/L suspended solids.

Nucleus

The positively charged central part of an atom, containing protons and, except for ordinary hydrogen, neutrons.

Nuisance Contaminant

Constituents in water, which are not normally harmful to health but may cause offensive taste, odor, color, corrosion, foaming, or staining.

Nutrient

Any substance that promotes growth with living organisms. The term is generally applied to nitrogen and phosphorus in wastewater, but is also applied to other essential and trace elements.

Nutrient Pollution

Contamination of water resources by excessive inputs of nutrients. In surface waters, excess algal production is a major concern.

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O

Oligotrophic lakes

Lakes with low nutrient availability and relatively low biological production. They are often clear, but depth and oxygen conditions vary.

Organic loading rate (OLR)

Organic load applied per unit reactor volume or treatment area per time, commonly based on COD or BOD. State the basis and units.

Organic matter

A diverse mixture of carbon-based substances, often originating from organisms or their decomposition. Organic matter is not limited to dead plant and animal fragments.

Osmosis

Movement of water through a selectively permeable membrane towards the side with lower water chemical potential, commonly the more concentrated solution, until balanced by an opposing pressure.

Osmotic pressure

The pressure required to stop net osmosis across a selectively permeable membrane at specified conditions. In RO, the effective hydraulic driving force must overcome the osmotic-pressure difference.

Outfall

A point or structure through which water or wastewater is discharged to a receiving environment. Outfalls may carry treated effluent or other flows.

Overflow rate

Flow divided by the surface area of a settling unit, usually expressed in m³/m²/h or m/h. Also called surface loading rate; it is a design and operating parameter.

Oxidation

Loss of electrons or an increase in oxidation state. It can transform contaminants without necessarily removing them from water.

Oxidation pond

A man-made body of water in which waste is consumed by bacteria.

Oxidation–reduction potential (ORP)

An electrode-measured potential indicating the oxidising or reducing character of a solution, commonly in mV relative to a specified reference electrode. ORP is not a direct measurement of disinfectant concentration.

Oxygen depletion

The reduction of the dissolved oxygen level in a water body.

Ozone

An unstable oxidizing agent, that consists of three oxygen atoms and can be found in the ozone layer in the atmosphere. It is produced by electrical discharge through oxygen or by specifically designed UV-lamps.

Ozone generator

A device that generates ozone by passing a voltage through a chamber that contains oxygen. It is often used as a disinfection system.

More info on ozone and ozone generators

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P

Parameter

A variable, measurable property whose value is a determinant of the characteristics of a system such as water. Temperature, pressure, and density are examples of parameters.

Partial pressure

For an ideal gas mixture, the pressure attributable to one component, equal to its mole fraction multiplied by total pressure. It helps describe gas–liquid equilibrium.

Particle size

A specified dimension or equivalent diameter of a particle. Results depend on the measurement method and particle shape.

Particulate loading

The mass of particulates per unit volume of water.

Parts per billion (ppb)

A ratio of one part in 10⁹; specify whether it is a mass, volume or other basis. For dilute water near 1 kg/L density, 1 ppb by mass is approximately 1 µg/L.

Parts per million (ppm)

A ratio of one part in 10⁶. For dilute water near 1 kg/L density, 1 ppm by mass is approximately 1 mg/L; the equivalence is not exact for concentrated or denser solutions.

Pasteurisation

Controlled heating for a specified time to reduce target microorganisms. It does not imply complete sterilisation or removal of chemical contaminants.

Pathogens

Disease-producing microorganisms.

Percent saturation

The measured dissolved concentration divided by the equilibrium saturation concentration under specified conditions, multiplied by 100. Dissolved-oxygen saturation depends on temperature, salinity and pressure.

Percolating water

Water that passes through rocks or soil under the force of gravity.

Periodic table

Arrangement of chemical elements by increasing atomic number and recurring properties. Mendeleev's early table used atomic weights; atomic number defines the modern order.

Permeability

A measure of how readily a medium transmits a fluid. In membrane work, the definition and units must specify the driving force and any temperature normalisation.

Permeate

The stream that passes through a membrane. It is generally lower in retained constituents than the feed, but it is not necessarily sterile or suitable for every use.

Persistence

Refers to the length of time a compound stays in the environment, once introduced.

Persistent organic pollutants (POPs)

Organic substances resistant to environmental degradation; some also bioaccumulate and travel long distances. The regulatory category has a defined list under the relevant agreements.

PFAS

Per- and polyfluoroalkyl substances: a large class of fluorinated chemicals. Selected adsorbents, ion exchange and high-pressure membranes can remove some PFAS from water; separation generally transfers them to media or concentrate rather than destroying them.

pH

The negative base-10 logarithm of hydrogen-ion activity. Neutral water has pH approximately 7 at 25 °C, but neutrality changes with temperature. The familiar 0–14 range is not an absolute limit.

Phase

A state of matter. This can be solid, liquid or gaseous.

Photosynthesis

The process of conversion of water and carbon dioxide to carbohydrates. It takes place in the presence of chlorophyll and is activated by sunlight. During the process oxygen is released. Only plants and a limited number of microorganisms can perform photosynthesis.

Physical and chemical treatment

Processes generally used in wastewater treatment facilities. Physical processes are for instance filtration. Chemical treatment can be coagulation, chlorination, or ozon treatment.

Physical weathering

Breaking down of rock into bits and pieces by exposure to temperature and changes and the physical action of moving ice and water, growing roots, and human activities such as farming and construction.

Phytoplankton

Microscopic photosynthetic organisms suspended in water, including algae and cyanobacteria.

Pilot testing

Testing a treatment process at a smaller scale using representative conditions and feed water before full-scale implementation. A pilot can reveal operating constraints not apparent from laboratory tests.

Point-of-entry treatment (POE)

Treatment at or near the inlet to a building or facility, serving most or all of its incoming water.

Point-of-use treatment (POU)

Treatment at a particular outlet or point where water is used, such as a dedicated drinking-water tap.

Point source

A stationary location from which pollutants are discharged. It is a single identifiable source of pollution, such as a pipeline or a factory.

Polar substance

A substance whose molecules have an uneven charge distribution, producing partial positive and negative regions. A polar molecule such as water can have no net electrical charge.

Polishing

A final treatment step to improve already treated water, such as reducing trace ions, organic carbon, particles or microorganisms. The target quality must be specified.

Pollutant

A substance or other input, such as heat, that impairs water quality or its intended use. An adverse effect need not involve immediate death of organisms.

Polyaluminium chloride (PACl)

An aluminium-based coagulant used in clarification and filtration. PACl is different from powdered activated carbon, which is also sometimes abbreviated PAC.

Pore

An opening in a membrane or medium that allows water to pass through.

Potable water

Water that is safe for drinking and cooking.

Potentiation

The ability of one chemical to increase the effect of another chemical.

Potentiometric surface

The surface to which water in an aquifer can rise by hydrostatic pressure.

Powdered activated carbon (PAC)

Fine activated carbon dosed into water for adsorption, then removed with solids. PAC and GAC use related adsorption mechanisms but different contacting and separation arrangements.

Precipitate

An insoluble reaction product in an aqueous chemical reaction.

Pressure sewer

A sewer conveying wastewater under positive pressure, typically using pumps. Its route need not continuously rise in elevation.

Pretreatment

Treatment before a downstream process to condition feed water or reduce troublesome constituents. In industrial discharge programmes, the term also refers to treatment before entry to a municipal sewer.

Primary wastewater treatment

The removal of suspended, floating and precipitated solids from untreated wastewater.

Click here for an overview of the wastewater treatment process

Process water

Water that serves in any level of the manufacturing process of certain products.

Product water

Water produced by a treatment process. Its suitability depends on the intended use and verified quality; the term does not automatically mean potable water.

Protons

Positively charged building blocks of an atom that are centered in the nucleus.

Protozoa

A diverse group of single-celled eukaryotic organisms. Some graze on bacteria in treatment systems; others, including Giardia and Cryptosporidium, can be pathogens.

Public water system

A legally defined drinking-water supply category. Under the US Safe Drinking Water Act, it generally serves at least 15 connections or at least 25 people for at least 60 days per year; it need not be publicly owned.

Putrefaction

Biological decomposition of organic matter; associated with anaerobic conditions.

Pyrogen

A substance capable of producing fever. Bacterial endotoxins are an important example, but not all pyrogens are endotoxins; relevant controls depend on the application.

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Q

Qualitative water assessment

Description of water characteristics or observations, such as appearance or odour. Sensory observations cannot establish chemical or microbiological safety.

Quantification limit

The lowest concentration that can be quantified with the stated accuracy and precision under a specified analytical method. It is generally higher than the detection limit.

Quantitative water assessment

Use of analyses of water properties and concentrations of compounds and contaminants in order to define water quality.

Quicksilver water

A historical name for a mercury-nitrate solution used in metalworking. It is a hazardous mercury-containing chemical, not a type of purified or potable water.

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R

Radioactive

Having the property of releasing radiation.

Raw sewage

Untreated wastewater and its contents.

Raw water

Intake water before any treatment or use.

Reaeration

Transfer of oxygen from air back into water, through natural surface exchange or engineered aeration.

Recarbonation

Addition of carbon dioxide to water, commonly after lime softening, to adjust pH and calcium-carbonate stability. Also called recarbonisation.

Receiving waters

A river, lake, ocean, stream or other watercourse into which wastewater or treated effluent is discharged.

Recharge Area

An area where rainwater soaks through the ground to reach an aquifer.

Recirculation

Return of part of a process stream to an earlier point or repeated circulation in a loop. It does not necessarily include treatment or constitute water reuse.

Recovery

The fraction of feed water converted into product water across a defined process boundary. For a membrane unit, recovery = permeate flow ÷ feed flow × 100%; overall recovery should account for cleaning and other losses.

Redox

Shortened term for reduction/ oxidation reactions. Redox reactions are a series of reactions of substances in which electron transfer takes place. The substance that gains electrons is called oxidising agent.

Reduction

Gain of electrons or a decrease in oxidation state, paired with oxidation of another species.

Regeneration

Restoration of an exchanger or treatment medium's usable capacity. Ion-exchange resins are commonly regenerated with an appropriate acid, alkali or salt solution; requirements depend on resin type and service.

Rejection

A membrane's reduction of a specified solute, commonly calculated as (1 − permeate concentration ÷ feed concentration) × 100%. State the sampling basis and conditions; rejection differs from water recovery.

Remineralisation

Adjustment of mineral content and stability after treatment such as desalination, commonly through dissolution of minerals, chemical dosing or blending. Final quality must suit the intended use.

Reserve Capacity

Extra treatment capacity built into wastewater treatment plants and sewers to be able to catch up with future flow increases due to population growth.

Reservoir

A natural or artificial holding area used to store water.

Residue

The dry solids remaining after the evaporation of a sample of water or sludge.

Resistivity

The reciprocal of electrical conductivity, commonly expressed in MΩ·cm for high-purity water. Pure water is approximately 18.2 MΩ·cm at 25 °C; this does not prove the absence of organics or microorganisms.

Resolution

The breaking of an emulsion into its individual components.

Retentate

The stream or material retained on the feed side of a membrane. In cross-flow operation it is commonly called concentrate; its composition depends on the separation.

Return activated sludge (RAS)

Settled biological solids returned from a secondary clarifier to the biological reactor to maintain biomass concentration.

Reverse osmosis (RO)

A pressure-driven membrane process that produces lower-solute permeate and a more concentrated feed-side stream. Applied pressure overcomes the osmotic-pressure difference. Removal varies by substance, membrane and conditions; RO does not guarantee zero contamination.

Further reading: reversed osmosis.

Run-Off

The part of precipitation water that runs off the land into streams or other surfacewater.

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S

Safe water

Water meeting the relevant health and quality requirements for its intended use. Safety requires appropriate assessment and controls, rather than an assumption that all contaminants are absent.

Safe yield

A historically used estimate of a water source's sustainable withdrawal. The applicable definition should include environmental and social constraints, not only replenishment of storage.

Salinity

The amount of dissolved salts in water. Reporting may use mass concentration, mass fraction or practical salinity; conductivity-based estimates depend on composition and method.

Salt passage

The fraction of a specified feed solute appearing in membrane permeate on the stated concentration basis. Under the same definition, percentage salt passage equals 100% minus percentage rejection.

Sand filtration

Passage through a sand bed to remove particles, sometimes with biological activity. Slow sand and rapid sand filters operate differently; not every sand filter is a multilayer filter or removes dissolved contaminants.

Further reading: sand filtration.

Sanitisation

A controlled procedure to reduce microbial contamination of a system or surface to an acceptable level for its use. It does not necessarily provide sterilisation.

Saturated zone

The area below the water table where all open spaces are filled with water.

Saturation

An equilibrium condition relative to a dissolved substance or solid phase at specified conditions. A supersaturated solution can persist before precipitation occurs.

Scale

The precipitate that forms on surfaces in contact with water as the result of a physical or chemical change.

Screening

Use of screens to remove coarse floating and suspended solids from sewage.

Seawater reverse osmosis (SWRO)

RO designed for seawater salinity. It generally uses higher pressures than BWRO; pressure and energy requirements depend on feed and operating conditions.

Secondary treatment

Wastewater treatment, commonly biological, directed at biodegradable organic matter and associated solids after primary treatment. Nutrient removal and disinfection require specifically designed processes.

Further reading: wastewater treatment.

Sedimentation

Settling of solid particles in a liquid system due to gravity.

Sediments

Soil, sand, and minerals washed from land into water, usually after rain.

Semi-confined aquifer

An aquifer partially confined by soil layers of low permeability through which recharge and discharge can still occur.

Semipermeable

Describing a barrier that allows some components to pass more readily than others. Selectivity depends on the membrane and species; it does not imply total rejection of every dissolved solid.

Separate sewer

A system with separate sanitary and stormwater conveyance. Infiltration, illicit connections or capacity limits can still cause overloading or untreated releases.

Separation

The isolation of the various compounds in a mixture.

Septic tank

A tank that settles solids and provides partial anaerobic treatment of domestic wastewater. It normally forms part of an onsite system with further treatment or infiltration; it is not simply a storage tank.

Sequencing batch reactor (SBR)

A biological reactor operated in timed phases such as fill, react, settle and decant. Treatment functions occur sequentially rather than only in separate continuous-flow tanks.

Settleable solids

Those suspended solids in wastewater that will settle over a certain period of time and are removed in that way.

Settling

Downward movement of particles under gravity when their effective density exceeds that of the liquid. Rate depends on particle size, shape, density, fluid viscosity and flow conditions.

Sewage

Waste fluid in a sewer system.

Sewage contamination

The introduction of untreated sewage into a water body.

Sewage sludge

Solids and associated water separated during sewage treatment, including primary sludge and biological sludge. It is produced in treatment processes, not merely in a sewer.

Sewerage

The entire system of sewage collection, treatment, and disposal.

Silica

Silicon-containing material that may occur in dissolved, colloidal or particulate forms in water. Form, pH, temperature and concentration affect treatment and deposition risk.

Silt density index (SDI)

A test-based index of particulate/colloidal plugging tendency using a specified membrane test. SDI is not a direct measurement of silt concentration and does not describe every fouling mechanism.

Sludge

A water-rich suspension or semi-solid residue separated during treatment. It may contain biological solids, mineral precipitates or captured particles, depending on the process.

Sodium adsorption ratio (SAR)

An index comparing sodium with calcium and magnesium in irrigation water or soil extracts using equivalent-charge concentrations. Interpret it with salinity and soil conditions; it is not a drinking-water safety measure.

Sodium hypochlorite

A chlorine-releasing chemical solution used for disinfection and oxidation. Product strength decreases with storage and depends on conditions; dosing is commonly expressed as available chlorine.

Softening

The removal of calcium and magnesium from water to reduce hardness.

Soft water

Any water that does not contain large concentrations of the dissolved minerals calcium or magnesium.

Solidification

Treatment that converts a waste into a more solid form, often using binders. Stabilisation aims to reduce contaminant mobility or reactivity; neither necessarily destroys the contaminants.

Solids retention time (SRT)

The mass of solids retained in a biological system divided by the rate at which solids leave it, commonly expressed in days. Also called sludge age.

Solubility

The equilibrium amount of a substance that dissolves in a specified solvent under stated conditions. Temperature, pH and chemical form can be important.

Solute

Matter dissolved in a liquid, such as water.

Solvent

Substance (usually liquid) capable of dissolving one or more other substances.

Sparger

A device that introduces compressed air into a liquid.

Sparging

Injection of air below the water table to strip dissolved volatile organic compounds and to facilitate aerobic biodegradation of organic compounds.

Specific conductance

Electrical conductivity reported at a specified reference temperature, commonly 25 °C. It indicates ionic content but does not identify individual contaminants or directly measure TDS.

Specific energy consumption (SEC)

Energy used per unit product or treated-water volume, commonly kWh/m³. State the system boundary and whether auxiliary equipment and energy recovery are included.

Spring

Ground water seeping out of the earth where the water table exceeds the ground surface.

Sterilisation

A validated process intended to eliminate viable microorganisms, including bacterial spores. It is a different objective from routine drinking-water disinfection.

Stokes' law

A relation for the terminal settling velocity of small spherical particles in a viscous fluid under creeping-flow conditions. It has limitations for irregular, interacting or turbulent particles.

Strong-acid cation resin (SAC)

A cation exchanger with strongly acidic functional groups. Its service form determines whether it performs softening, demineralisation or another exchange duty.

Strong-base anion resin (SBA)

An anion exchanger with strongly basic functional groups. In the hydroxide form it is used in demineralisation; resin type and conditions affect weak-acid removal.

Sublimation

The transitions of water directly from the solid state to the gaseous state, without passing through the liquid state.

Surface tension

The elastic-like force in a body, especially a liquid, tending to minimize, or constrict, the area of the surface.

Surface water

All water naturally open to the atmosphere, concerning rivers, lakes, reservoirs, ponds, streams, impoundments, seas, estuaries and wetlands.

Suspended solids

Particles dispersed in water that are retained by a specified analytical filter. Suspended solids differ from dissolved solids; the operational boundary depends on the method.

Synergism

The combined action of several chemicals, which produces a total effect greater than the effects of the chemicals separately.

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T

T₁₀ (T10) contact time

The time by which 10% of water has passed through a contact process, determined or estimated by an accepted hydraulic method. It is used in many chemical-disinfection CT assessments.

Tertiary treatment

Treatment after secondary wastewater treatment, such as filtration, nutrient reduction or disinfection, selected for the required effluent quality. It does not imply a universal process or removal percentage.

Thermal pollution

A temperature change caused by a discharge or other human activity that adversely affects a receiving environment. Effects depend on the magnitude, duration and organisms present.

Titration

An analytical method in which a measured amount of reagent is added to a sample until a defined endpoint is reached, allowing the quantity of an analyte to be calculated.

Total chlorine

Free chlorine plus combined chlorine, measured and reported using a specified method, usually as mg/L Cl₂.

Total dissolved solids (TDS)

The dissolved residue determined by a specified filtration and drying method, commonly reported in mg/L. TDS differs from TSS; conductivity can estimate it only with an appropriate conversion.

Further reading: TDS and conductivity.

Total hardness (TH)

In most natural waters, the combined calcium and magnesium hardness, commonly expressed as mg/L as CaCO₃. It is distinct from alkalinity and TDS.

Total nitrogen (TN)

The combined nitrogen in the forms measured by the specified method, including organic nitrogen, ammonia, nitrite and nitrate. Report as mg/L as N.

Total organic carbon (TOC)

Carbon present in organic compounds measured by a specified analytical method, commonly mg/L as C. It does not identify individual compounds or directly measure biodegradability.

Total phosphorus (TP)

Phosphorus in the dissolved and particulate forms included by the analytical method, commonly measured after digestion and reported as mg/L as P.

Total solids

See Total solids (TS). The result depends on the analytical method and drying conditions.

Total solids (TS)

Residue remaining after a specified drying procedure on an unfiltered sample, including dissolved and suspended material. Commonly reported in mg/L.

Total suspended solids (TSS)

Solids retained on a specified filter and dried under defined conditions, reported in mg/L. TSS differs from turbidity and TDS.

Toxic water pollutants

Substances capable of causing adverse biological effects at relevant exposures. They can be natural or human-made; toxicity depends on chemical form, concentration, exposure and organism.

Transmembrane pressure (TMP)

Pressure difference across a membrane. In cross-flow systems it is often calculated as average feed/concentrate pressure minus permeate pressure; it does not subtract osmotic pressure.

Transmission lines

Pipelines that transport raw water from its source to a water treatment plant.

Transmissivity

For an aquifer, hydraulic conductivity integrated over its saturated thickness, commonly expressed in m²/day. It describes the capacity to transmit groundwater across that thickness.

Transpiration

Release of water vapour from plants, mainly through leaf stomata. Together with evaporation, it contributes to evapotranspiration.

Treatment plant

A facility using physical, chemical or biological processes to achieve specified water or wastewater quality. The required processes depend on the feed and intended use or discharge.

Trickling filter

A biological reactor where wastewater passes over media supporting a biofilm. Organic matter is biologically treated, and sloughed solids generally require downstream separation.

Trihalomethanes (THMs)

A group of compounds in which three hydrogen atoms of methane are replaced by halogens. The four commonly regulated drinking-water THMs contain chlorine and/or bromine and can form during disinfection of water containing organic precursors.

Tube settler

Inclined tubes installed in a settling unit to provide additional settling area and shorten particle settling distance. Collected solids move towards sludge-removal zones.

Turbidity

An optical measure of light scattering by material in water. Units such as NTU and FNU depend on the instrument method; turbidity is not identical to suspended-solids concentration.

Turbulent flow

Flow characterised by irregular velocity fluctuations and mixing. Whether flow is turbulent depends on factors including velocity, geometry, density and viscosity.

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U

Ultrafiltration (UF)

A membrane process retaining many colloids, macromolecules and microorganisms while generally passing dissolved salts. Virus retention varies with the membrane, integrity and operating conditions.

Ultrapure water (UPW; UP-water)

Water purified to very low levels of specified ionic, organic, particulate and microbial impurities. Required limits vary by application; high resistivity alone does not establish every aspect of purity.

Further reading: UP-water.

Ultraviolet oxidation

Photochemical treatment using UV, sometimes combined with oxidants, to transform susceptible chemicals. UV disinfection and UV-driven chemical oxidation are different treatment objectives.

Ultraviolet radiation (UV)

Electromagnetic radiation with wavelengths shorter than visible light. Suitable UV systems can inactivate microorganisms or support photochemical treatment; ordinary UV disinfection does not remove dissolved salts.

Unloading

The release of the contaminant that was captured by a filter medium.

Unsaturated Zone

The area above the water table where soil pores are not fully saturated with water.

Up-flow

An upward flow of water.

Urban run-off

Water from city streets domestic properties that carries pollutants into the sewer systems and receiving waters.

UV dose

UV energy delivered per unit area, commonly mJ/cm². Reactor performance depends on dose distribution, flow, UV transmittance, lamp condition and validated operating limits.

UV transmittance (UVT)

The percentage of incident UV light transmitted through a water sample at a stated wavelength and path length, commonly 254 nm and 1 cm. Lower UVT can reduce effective disinfection.

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V

Validation

Documented evidence that a process can achieve its specified performance within defined conditions. It differs from routine monitoring and verification of ongoing operation.

Vaporize

Conversion of a liquid into vapour.

Vapour

The gaseous phase of substances such as water.

Venturi

A passage with a constricted section that increases fluid velocity and reduces static pressure. It can be used for flow measurement or to draw gases or chemicals into a stream.

Viruses

Acellular infectious agents with DNA or RNA enclosed in a protein coat, sometimes with an outer envelope. They replicate within host cells; their small size and treatment resistance vary.

Viscosity

A fluid's resistance to deformation or flow. Dynamic and kinematic viscosity are different quantities; for liquid water, viscosity generally decreases as temperature rises over common treatment conditions.

Volatile organic compounds (VOCs)

Organic compounds that readily enter the gas phase under specified conditions. They may be natural or synthetic; health risks and treatment requirements differ between compounds.

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W

Waste activated sludge (WAS)

Biological solids intentionally removed from an activated-sludge system to control solids inventory and SRT.

Wastewater

The spent or used water from a home, community, farm, or industry that contains dissolved or suspended matter.

Wastewater infrastructure

The plan or network for the collection, treatment, and disposal of sewage in a community.

Water for injection (WFI)

A pharmaceutical water grade defined by the applicable pharmacopoeia. Chemical, microbial and endotoxin controls are distinct from requirements for ordinary potable or demineralised water.

Water monitoring

Planned measurement of water quality or quantity through continuous instruments, periodic sampling or both. Monitoring design depends on the decision being supported.

Water pollution

The presence in water of enough harmful or objectionable material to damage water quality.

Water quality

The physical, chemical, biological and, where relevant, radiological characteristics of water assessed against a specified use or environmental objective.

Water recycling

Treating and using water again for a defined purpose. The treatment needed can be substantial and must match the source quality, exposure routes and intended use.

Water reuse

Using treated water again for a defined purpose, such as irrigation, industrial processes or drinking-water supply. Required barriers and quality depend on the use and exposure routes.

Watershed

A land area from which water drains to a particular water body.

Water solubility

The maximum possible concentration of a chemical compound dissolved in water.

Water storage pond

An impound for liquid wastes designed to accomplish some degree of biochemical treatment.

Water supply system

The collection, treatment, storage, and distribution of water from source to consumer.

Water system

A context-dependent term for a natural hydrological network or an engineered collection, treatment and distribution system. State which meaning applies.

Water table

The surface in an unconfined aquifer where pore-water pressure equals atmospheric pressure. It marks the top of the saturated zone, excluding the capillary fringe.

Weak-acid cation resin (WAC)

A cation exchanger with weakly acidic functional groups, commonly used for selected dealkalisation or demineralisation duties. Capacity and performance depend strongly on feed chemistry and pH.

Weak-base anion resin (WBA)

An anion exchanger with weakly basic functional groups, commonly used to remove strong mineral acids after cation exchange. It does not provide the same weak-acid removal as an SBA resin.

Weir

A spill over device used to measure or control water flows.

Well

A deep hole with the purpose to reach underground water supplies.

Wetland

An area that is saturated by surface water or groundwater, with vegetation adapted for life under those soil conditions.

Wettability

The relative degree to which a fluid will spread into solid surface in the presence of other immiscible fluids.

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X

Xenobiotic

A chemical foreign to a particular organism or biological system. The term does not by itself establish persistence or toxicity.

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Y

Yield

The amount of useful output relative to an input or period. In sludge dewatering it can refer to cake production; in water treatment, state whether yield means recovered water or another product.

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Z

Zero liquid discharge (ZLD; zero discharge water)

A treatment approach designed to avoid a liquid effluent leaving a defined process boundary, recovering water and managing residual salts or solids. It does not mean that waste, emissions, water losses or environmental impacts disappear.

Zeta potential

An electrokinetic measurement which can be used for the control of coagulation processes.

Zone of saturation

The space in the soil below the water table in which all the pores are filled with water. The water in the zone of saturation is groundwater.

Zooplankton

Tiny aquatic animals eaten by fish.

Zwitterion

A molecule carrying both positive and negative formal charges while having no net charge. Its charge distribution depends on its chemical structure and pH; zwitterions are not necessarily macromolecules.

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