Water Treatment Glossary


ACID A chemical compound whose aqueous solution turns blue litmus paper red, reacts with and dissolves certain metals to form salts, and reacts with bases to produce salts and water. They are capable of transferring a hydrogen ion (proton) in solution.
ACUTE Describes a severe and often dangerous condition in which relatively rapid changes occur.
ACUTE TOXICITY Adverse health effects occurring within a short time period of exposure to a single dose of a chemical or as a result of multiple exposures over a short time period, e.g. 24 hours.
AEROSOL A colloidal suspension in a gaseous medium of solid particles, liquid particles or solid and liquid particles having negligible falling velocity.
AFFF, AQUEOUS FILM-FORMING FOAM, Fire-fighting foam which flows on burning liquid as a film, providing rapid knock-down.
ALCOHOL-RESISTANT FOAM Foam for use against fires involving liquids miscible with water, e.g. alcohol, acetone.
ALGAE A small, usually aquatic, plant which requires light to grow, often found on exposed areas of cooling towers.
AIR CONDITIONING A form of air treatment whereby temperature humidity and air cleanliness are all controlled within limits determined by the requirements of the air-conditioned enclosure.
ALKALINITY An expression of the total basic anions, including hydroxyl, carbonate and bicarbonate in a solution.
AMINE Chemical used to control corrosion in condensate systems. It is classified as neutralizing or filming.
ANION A negatively charged atom or group of atoms, or a radical which moves to the positive pole (anode) during electrolysis.
ANOXIA Deficient supply of oxygen to tissues.
ANTIBODIES Substances in the blood which destroy or neutralise various toxins or components of bacteria known generally as antigens. The antibodies are formed as a result of the introduction into the body of the antigen to which they are antagonistic as in all infectious diseases.
ANTIGEN A foreign substance (usually a protein) that stimulates formation of antibody.
ASPHYXIA The result of a diminished supply of oxygen to the blood and tissues and interference with the respiratory function. Simple anoxia may be caused by ‘inert gases’, e.g. nitrogen, and some flammable gases, e.g. methane. Toxic anoxia may be caused by certain substances, e.g. carbon monoxide and hydrogen cyanide, which interfere with the body’s ability to transfer or utilize oxygen in the tissues. Rapid unconsciousness and death can occur in either case.
ASTHMA Periodic attacks of wheezing, chest tightness and breathlessness resulting from constriction of the airways.
ATOM The smallest unit of an element incapable of further subdivision in the course of a chemical reaction.
ATOMIC NUMBER The number of protons in an atomic nucleus.
ATOPY Hypersensitivity where tendency to allergy is inherited.
AUTO-IGNITION TEMPERATURE The minimum temperature required to initiate or cause self-sustained combustion of material in the absence of any external source of energy. (Values may change significantly with geometry, gas/vapour concentration, and presence of catalyst.) Any ignition source must be at a temperature of, or greater than, the ignition temperature of the specific substance.
BACTERIA (singular bacterium) a microscopic, unicellular (or more rarely multicellular) organism.
BASE A chemical compound whose aqueous solution turns red litmus paper blue and is capable of accepting or receiving a proton from another substance. They react with acids to form salts and water.
BATNEEC Term used in the Environmental Protection Act and other legislation. Certain polluting processes are required to use the Best Available Techniques Not Entailing Excessive Cost (BATNEEC) to reduce the environmental impact of a prescribed process as far as possible. Environment Agency inspectors determine what constitutes BATNEEC for each application and are to change the definition as improved technologies or techniques become available.
BIOCHEMICAL OXYGEN DEMAND (BOD) Official term given to measure how polluting organic industrial effluent is when it is discharged into water. This effluent is feed for bacteria which consume oxygen, making it more difficult for plant and fish life to survive. The lower the BOD level, the less polluting the effluent.
BIOCIDE A substance which kills micro-organisms.
BIOFILM A community of bacteria and other micro-organisms, embedded in a protective layer with entrained debris, attached to a surface.
BLACK LIST The Black List was introduced by the EC in Directive 76/464/EEC on dangerous substances released into water as list I. It contains substances selected mainly on the basis of their toxicity, persistence and accumulation in living organisms and in sediment.
BEST PRACTICABLE ENVIRONMENTAL OPTION (BPEO) Organizations may be encouraged to undertake systematic decision processes with a view to seeking the BPEO that provides the most benefit (or least damage) to the environment, at an acceptable cost.
BLEVE, BOILING LIQUID EXPANDING VAPOUR EXPLOSION Instantaneous release and ignition of flammable vapour upon rupture of a vessel containing flammable liquid above its atmospheric boiling point.
BLOW-DOWN/BLEED-OFF Water discharged from the system to control the concentration of salts or other impurities in the circulating water; usually expressed as a percentage of recirculating water flow.
BLOWING AGENT Chemical liable to decomposition at low temperature to produce a large volume of gas.
BOTTOM BLOWDOWN A portion of the boiler water that is intermittently sent to drain to remove sludge and other suspended matter.
CALORIFIER An apparatus used for the transfer of heat to water in a vessel by indirect means, the source of heat being contained within a pipe or coil immersed in the water.
CARBONIC ACID A weak and unstable acid, H2CO3, existing only in solution. Carbonic acid is formed as carbon dioxide in steam dissolves in condensate. This is the primary cause of condensate system corrosion.
CARCINOGEN An agent (whether chemical, physical or biological) capable of increasing the incidence of malignant neoplasms. Defined in Regulation 2 of the Control of Substances Hazardous to Health Regulations 1999 as: any substance or preparation which if classified in accordance with the classification provided for by Regulation 5 of the Chemicals (Hazard Information and Packaging for Supply) Regulations 1994 would be in the category of danger, carcinogenic (category 1) or carcinogenic (category 2) whether or not the substance or preparation would be required to be classified under those Regulations; or any substance or preparation: listed in Schedule 1, or arising from a process specified in Schedule 1 which is a substance hazardous to health.
CARRY OVER The entrainment of liquid water, along with dissolved impurities, in steam leaving a boiler. This results in loss of heat transfer, along with deposition in steam-using equipment.
CATION A positively charged atom or group of atoms, or a radical which moves to the negative pole (cathode) during electrolysis.
CHEMICAL BOND Strong forces of attraction holding atoms together in molecules or crystalline salts.
CHLORINE An element used in disinfection.
CHLOROFLUOROCARBONS (CFCS) Organic substances containing chlorine and fluorine which were
initially thought to be harmless and found extensive use, e.g., as propellants because they are largely non-flammable. Some CFCs have since been found to be one of the main sources of atmospheric ozone depletion and a greenhouse gas. Until recently they were used extensively as aerosol propellants, solvents, refrigerants and in foam making. Many countries have now agreed to eliminate CFCs as soon as possible.
CHLORINATED HYDROCARBONS Hydrocarbons containing chlorine atoms, e.g. trichloroethylene. Some of these chemicals accumulate in the food chain and do not readily degrade. Some plastics which contain certain chlorinated hydrocarbons release dioxins into the air, when burnt at low temperatures.
CHRONIC Occurring for a prolonged period.
CHRONIC TOXICITY Adverse health effects resulting from repeated daily exposures to a chemical for a significant period.
CLASS A FIRE A fire involving solids, normally organic, in which combustion generally occurs with the formation of glowing embers.
CLASS A POISON (USA) A toxic gas/liquid of such a nature that a very small amount of the gas, or vapour of the liquid, in air is dangerous to life.
CLASS B FIRE A fire involving liquids or liquefiable solids.
CLASS B POISON (USA) Any substance known to be so toxic that it poses a severe health hazard during transportation.
CLASS C FIRE A fire involving gases or liquefied gases in the form of a liquid spillage, or a liquid or gas leak.
CLASS D FIRE A fire involving metals.
CNS DEPRESSANT Substances, e.g. anaesthetics and narcotics, which depress the activity of the central nervous system. Symptoms following exposure include headache, dizziness, loss of consciousness, respiratory or cardiac depression, death.
COLD WATER SERVICE (CWS) Installation of plant, pipes and fitting in which cold water is stored, distributed and subsequently discharged.
CONDENSATE The water that is formed when steam cools and changes from a gas to a liquid.
CONDUCTIVITY Represents the electrical current carrying capacity of a water. It is used as a means of indirectly measuring the total dissolved solids concentration of a water. Conductivity x 0.7 = TDS
CONFINED SPACE A space which is substantially, although not always entirely, enclosed and where there is a reasonably foreseeable risk of serious injury from hazardous substances or conditions within the space or nearby. The risks may include flammable substances; oxygen deficiency or enrichment; toxic gases, fume or vapour; ingress or presence of liquids; free-flowing solids; presence of excessive heat. For the purpose of the Confined Spaces Regulations 1997 a ‘confined space’ means any place, including any chamber, tank, vat, silo, pit, trench, pipe, sewer, flue, well or other similar space in which, by virtue of its enclosed nature, there arises a reasonably foreseeable specified risk.
COOLING TOWER An apparatus through which warm water is discharged against an air stream; in doing so part of the water is evaporated to saturate the air and this cools the water. The cooler water is usually pumped to a heat exchanger to be reheated and recycled through the tower.
CONCENTRATION FACTOR Compares the level of dissolved solids in the cooling water with that dissolved in the make-up water (also known as cycle of concentration). Usually determined by comparison of either the chloride or magnesium hardness concentration.
CONTACT DERMATITIS Inflammation of the skin due to exposure to a substance that attacks its surface.
CONTROLLED WASTE All household, industrial or commercial waste of any quantity or description.
CORROSIVE A substance that chemically attacks a material with which it has contact (body cells, materials of construction).
CORROSION INHIBITORS Chemicals which protect metals by: (a) passivating the metal by the promotion of a thin metal oxide film (anodic inhibitors); or (b) physically forming a thin barrier film by controlled deposition (cathodic inhibitors).
COSHH (CONTROL OF SUBSTANCES HAZARDOUS TO HEALTH) The Control of Substances Hazardous to Health Regulations 1999 establish the responsibilities of employers with regard to all substances which pose a health hazard in the workplace.
CRYOGEN A substance used to obtain temperatures far below freezing point of water, e.g. < – 78OC.

CVCE (CONFINED VAPOUR CLOUD EXPLOSION) Explosion of a gas or vapour which is initially ‘confined’ within a vessel, building, piping, etc. CYCLES OF CONCENTRATION The ratio of dissolved solids in boiler water to the dissolved solids in the feedwater or make-up water.

DANGEROUS SUBSTANCES (UK) Defined substances which may be hazardous to the fire services in an emergency. (Dangerous Substances (Notification and Marking of Sites) Regulations 1990.) Defined substances over which control is exercised for conveyance in all road tankers or in tank containers >3 m3 capacity. (The Carriage of Dangerous Goods by Road Regulations 1996.) Defined substances covered by a comprehensive system to inform consumers of potential dangers and to reduce the hazard when carried by road. The Chemical (Hazard Information and Packaging for Supply Regulations 1994). Defined substances, including all toxic gases, all flammable gases, asbestos and most hazardous wastes, for which carriage in packages or in bulk is controlled. (The Carriage of Dangerous Goods by Road Regulations 1996).
DETONATION Explosion in which the flamefront advances at more than supersonic velocity.
DEAD END/BLIND END A length of pipe closed at one end through which no water passes.

DEADLEG Pipes leading to a fitting through which water only passes when there is draw-off from the fitting.
DIP SLIDE(s) A dip slide is a means of testing the microbial content of liquids. It consists of a plastic carrier bearing a sterile culture medium which can be dipped in the liquid to be sampled. It is then incubated to allow microbial growth. The resulting microbial colonies are estimated by reference to a chart.
DISCHARGE CONSENTS Permission to discharge trade effluent directly into controlled waters is given by the National Rivers Authority in the form of a discharge consent which will specify amounts and conditions. Discharges to public sewers are controlled by discharge consents by one of the ten Water Service Companies.
DISINFECTION A process which destroys or irreversibly inactivates micro-organisms and reduces their number to a non-hazardous level.
DISSOLVED SOLIDS Solids in true solution in ionic form in water that cannot be removed by filtration. The presence is due to the solvent action of water in contact with minerals in the earth. Expressed as total dissolved solids (TDS).
DISTRIBUTION CIRCUIT Pipework which distributes water from hot or cold water plant to one or more fittings/appliances.
DOMESTIC WATER SERVICES Hot and cold water intended for personal hygiene, culinary, drinking water or other domestic purposes.
DRIFT Circulating water lost from the tower as liquid droplets entrained in the exhaust air stream; usually expressed as a percentage of circulating water flow but for more precise work it is parts of water per million by weight of air for a given liquid to gas ratio.
DRIFT ELIMINATOR More correctly referred to as drift reducers or minimisers – equipment containing a complex system of baffles designed to remove water droplets from cooling tower air passing through it.
DRY STEAM Steam containing no moisture. Commercially dry steam contains not more than 0.005% moisture.
DUST Solid particles generated by mechanical action, present as airborne contaminant (e.g. <75 micron in size).

DUTY OF CARE The concept of the duty of care for waste is set out in Section 34 of the Environmental Protection Act (1990) which states that it is the duty of any person who imports, produces, carries, keeps, treats or disposes of controlled waste to keep that waste properly under control. ECOTOXICOLOGY The study of toxic effects of chemical and physical agents on living organisms as well as human beings, especially on populations and communities within defined ecosystems.

ENDOTHERMIC REACTION A chemical reaction resulting in absorption of heat.

ENVIRONMENT AGENCY The Environment Agency provides a comprehensive approach to the protection and management of the environment by combining the regulation of land, air and water. Its creation is a major and positive step, merging the expertise of the National Rivers Authority, Her Majesty’s Inspectorate of Pollution, the Waste Regulation Authorities and several smaller units from the Department of the Environment.

EPIDEMIOLOGY The study in populations of health factors affecting the occurrence and resolution of disease and other health-related conditions. ERYTHEMA Reddening of skin, inflammation.

EVAPORATIVE CONDENSER A heat exchanger in which refrigerant is condensed by a combination of air movement and water sprays over its surface.

EVAPORATIVE COOLING A process by which a small portion of a circulating body of water is caused to evaporate thereby taking the required latent heat of vaporisation from the remainder of the water and cooling it. EXOTHERMIC REACTION A chemical reaction in which heat is released and, unless temperature is controlled, may lead to runaway conditions. FEEDWATER Water introduced into a boiler during operation. It includes make-up and returned condensate. FIBROSIS Scarring, usually of lung tissue.

FILL/PACKING That portion of a cooling tower which constitutes its primary heat transfer surface; sometimes called ‘packing’ or ‘pack’.

FILMING AMINE An organic chemical that forms a water repellent film on system metal when steam condenses. The film controls corrosion in the condensate system.

FIRE POINT The minimum temperature at which a mixture of gas/vapour and air continues to burn in an open container when ignited. The value is generally above the flash point.

FLAMMABLE RANGE The concentrations of flammable gas or vapour between the LEL and UEL at a given temperature.

FLASH POINT The lowest temperature required to raise the vapour pressure of a liquid such that vapour concentration in air near the surface of the liquid is within the flammable range, and as such the air/vapour mixture will ignite in the presence of a suitable ignition source, usually a flame. (Open cup values are approximately 5.5o to 8.3oC higher than the closed cup values.)

FOAMING The formation of bubbles that have sufficiently high surface tension to remain as bubbles beyond the water surface. This interferes with the natural steam process and can result in carryover.

FOG (MISTS) Liquid aerosols formed either by condensation of a liquid on particulate nodes in air or by uptake of liquid by hygroscopic particles. FOULING Organic growth or other deposits on heat transfer surfaces causing loss in efficiency.

FUME Airborne solid particles (usually < 0.1micron) that have condensed from the vapour state.

HALF LIFE Ratio of system volume to purge rate.

HARDNESS The total of a water’s calcium and magnesium ion content. The total concentration is reported as calcium carbonate. Hardness is sometimes referred to as carbonate and non-carbonate hardness. Carbonate, also referred to as temporary, hardness is that portion of the total hardness that combines with carbonate and bicarbonate ions. The remainder of the hardness is that which combines with sulphate or other anions and is known as non-carbonate or permanent hardness.

HAZARD The inherent property of a substance capable of causing harm (e.g. toxicity, radioactivity, flammability, explosivity, reactivity, instability). In a broader context anything that can cause harm, e.g. electricity, oxygen-deficiency, machinery, extreme temperature.

HAZARDOUS WASTE An unofficial class of industrial wastes which have to be disposed of with particular care. In the UK the closest definition is for ‘special wastes’. Certain toxic organic wastes, such as PCBs, have to be burned in high-temperature incinerators.

HEAVY METALS A group of metals which are sometimes toxic and can be dangerous in high concentrations. The main heavy metals covered by legislation are cadmium, lead, and mercury. Industrial activities such as smelting, rubbish burning, waste disposal and adding lead to petrol increase the amount of toxic heavy metals in the environment.

HOT WATER SERVICE (HWS) Installation of plant, pipes and fittings in which water is heated, distributed and subsequently discharged (not including cold water feed tank or cistern).

HUMIDIFIER FEVER A flu-like illness caused by inhalation of fine droplets of water from humidifiers that have become contaminated. HYDROCARBONS Organic compounds that contain only hydrogen and carbon. The major sources of hydrocarbons in the atmosphere are vehicle emissions (unburned fuel) and gas leaks. Contributes to acid rain.

HYGIENE STANDARD See OES, MEL, TLV. INERTING Depression of the flammable limits of a flammable gas/vapour–air mixture by the addition of an inert gas, e.g. nitrogen, carbon dioxide, or similar mixtures, to render it non- flammable. ION An isolated electron or positron, or an atom or molecule, which by loss or gain of one or more electrons has acquired a net electric charge.

IONIZING RADIATION The transfer of energy in the form of particles or electromagnetic waves of a wavelength of 100 nanometers or less or a frequency of 3 to 1015 hertz or more capable of producing ions directly or indirectly. ISOTOPE One of two or more atoms having the same atomic number but different mass number.

IPC (INTEGRATED POLLUTION CONTROL) Under this new integrated approach to pollution control land, water and air are to be treated collectively rather than as separate environmental media. Industries are given consents to pollute with the effect on all three media being taken into consideration. IPC was introduced by the Environmental Protection Act (1990) and an EC system of Integrated Pollution, Prevention and Control is being introduced.

JET FIRE Fuel burning as a flame when flammable gas or vapour issues from a pipe, or other orifice, and burns on the orifice.

KINETICS The branch of physical chemistry concerned with measuring and studying the rates and mechanisms of chemical reactions.

LANDFILL Disposal of waste in the ground. This method is commonly used for both domestic waste and more hazardous chemical waste. Landfill sites used for difficult and potentially-dangerous wastes are now engineered, managed and monitored to prevent poisons leaking out. LC50 The calculated concentration of a substance that causes death in 50% of a test population under prescribed conditions in a prescribed period of time (normally expressed as ppm or mg/m3 for gases, mg/1 for liquids). LD50 The calculated dose of chemical (mg per kg body weight) causing death in 50% of test population. (The species of animal, route of administration, any vehicle used to dissolve or suspend the material, and the time period of exposure should be reported.)

LEACHATE Liquid that leaks from waste disposal sites. (In a broader sense liquid, e.g. solution, removed from a solid by a solvent, such as water.) LEGIONNAIRES’ DISEASE A form of pneumonia caused by legionella bacteria.

LEGIONELLAE The genus legionella belongs to the family legionellaceae which has over 40 species. These are ubiquitous in the environment and found in a wide spectrum of natural and artificial collections of water. LEGIONELLA Type of aerobic bacterium which is found predominantly in warm water environments. (singular of legionellae).

L. PNEUMOPHILA One of the causative organisms of Legionnaires’ disease. LEGIONELLOSIS Any illness caused by exposure to legionella.

LEL (LOWER EXPLOSIVE, OR FLAMMABLE, LIMIT) The minimum concentration of a gas, vapour, mist or dust in air at a given pressure and temperature that will propagate a flame when exposed to an efficient ignition source. Generally expressed as % by volume for gases and vapours, and as mg/m3 for mists or dusts.

LPG (LIQUEFIED PETROLEUM GAS) Petroleum gas stored or processed as a liquid in equilibrium with vapour by refrigeration or pressurization. The two LPGs in general use are commercial propane and commercial butane supplied to product specifications, e.g. BS 4250. (These, or mixtures thereof, comprise LPG for the purpose of the Highly Flammable Liquids and Liquefied Petroleum Gas Regulations 1972.)

M ALKALINITY Also called total alkalinity. This is the measure of the total of bicarbonate, carbonate and hydroxyl ions in a water.

MAKEUP WATER Water which is added to a cooling water system to compensate for wastage (eg via system leaks), evaporative loss and bleed. MAKE-UP The water added to a boiler system to compensate for that lost through steam production, blowdown, etc.

MAJOR ACCIDENT An occurrence (including in particular, a major emission, fire or explosion) resulting from uncontrolled developments in the course of operation of any establishment and leading to serious danger to human health or the environment, and involving one or more dangerous substances. Requirements with respect to the control of major accident hazards involving dangerous substances apply to defined establishments under the Control of Major Accident Hazards Regulations 1999.

MASS NUMBER The sum of the number of protons and neutrons in the nucleus of an atom.

MEL, MAXIMUM EXPOSURE LIMIT (UK) The maximum concentration of an airborne substance (averaged over a reference period) to which employees may be exposed by inhalation under any circumstances. (Listed in ‘Occupational exposure limits’, EH40/–HSE.)

METAL FUME FEVER Non-specific, self-limiting illness resembling an attack of influenza caused mainly by exposure to fumes of zinc, copper, or magnesium and less frequently due to exposure to other metal fumes. Exposures occur from molten metals, e.g. in smelting, galvanizing, welding. METALWORKING FLUID Fluid applied to a tool and workpiece to cool, lubricate, carry away particles of waste and provide corrosion protection. Generally comprising neat mineral oils, or water-based materials, or a mixture of the two. Fluids may also contain emulsifiers, stabilizers, biocides, corrosion inhibitors, fragrances and extreme pressure additives. MICRO-ORGANISM An organism of microscopic size including bacteria, fungi and viruses.

MINERAL OIL Oil derived from petroleum. Includes a wide range of hydrocarbons from light oils, kerosene and gas oils, to the heavier fuel and lubricating oils.

MOLECULES Groups of atoms held together by strong chemical forces and forming the smallest unit of a compound. The atoms may be identical, e.g. H2 or different, e.g. H2O.

MULTIPLE CHEMICAL SENSITIVITY An acquired disorder characterized by recurrent symptoms, referable to multiple organ systems, occurring in response to many chemically-unrelated compounds at doses far below those established in the general population to cause harmful effects. No single widely accepted test of physiologic function can be shown to correlate with symptoms.

MUTAGEN A chemical or physical agent that can cause a change (mutation) in the genetic material of a living cell. NARCOSIS Drowsiness or sleepiness. NATURAL GAS Flammable gas consisting essentially of methane with very minor proportions of other gases. Flammable limits approximately 5–15%. Odourized for commercial distribution within the UK.

NEUTRALISING AMINE An alkaline organic chemical that neutralizes the acidity of condensate to control corrosion.

NON-OXIDISING BOCIDE A non-oxidising biocide is one that functions by mechanisms other than oxidation, including interference with cell metabolism and structure.

NRA (NATIONAL RIVERS AUTHORITY) The National Rivers Authority were the body responsible for the management of water resources and the control of water pollution in England and Wales. They are now part of the Environment Agency.

NUTRIENT A food source for micro-organisms. ODOUR THRESHOLD The minimum concentration of a substance at which the majority of test subjects can detect and identify the substance’s characteristic odour.

OES, OCCUPATIONAL EXPOSURE STANDARD (UK) The concentration of an airborne substance (averaged over a reference period) at which, according to current knowledge, there is no evidence that it is likely to be injurious to employees if they are exposed by inhalation, day after day. (Specified by HSC in Guidance Note EH40.)

OXIDIZING AGENT Compound that gives up oxygen easily or removes hydrogen from another compound. It may comprise a gas, e.g. oxygen, chlorine, fluorine, or a chemical which releases oxygen, e.g. a nitrate or perchlorate. A compound that attracts electrons.

OXIDISING BIOCIDE Agents capable of oxidising organic matter, eg cell material, enzymes or proteins which are associated with microbiological populations resulting in death of the micro-organisms. The most commonly used oxidising biocides are based on chlorine or bromine (halogens) which liberate hypochlorous or hypobromous acids on hydrolysis in water. The exception is chlorine dioxide, a gas which does not hydrolyse but which functions in the same way.

OXYGEN ATTACK Corrosion or pitting in a boiler system caused by dissolved oxygen.

OXYGEN DEFICIENCY Depletion of oxygen content in an atmosphere to below the normal 21%. Exposure to <18% must not be permitted. Concentrations 6% to 10% oxygen can lead to loss of consciousness. OXYGEN ENRICHMENT Increase in oxygen content of air to above the normal 21%. Enrichment within a room to >25% can promote or accelerate combustion.
OXYGEN SCAVENGER A chemical added to boiler feedwater to remove dissolved oxygen.
OZONE A reactive form of oxygen the molecule of which contains 3 atoms of oxygen. In the ozone layer it protects the earth by filtering out ultra-violet rays. At ground level, as a constituent of photochemical smog, it is an irritant and can cause breathing difficulties.
OZONE LAYER A thin layer of ozone that lies about 25 kilometres above the earth in the stratosphere. Forms a protective screen against harmful radiation by filtering out ultra-violet rays from the sun.
P ALKALINITY A measure of half the carbonate and all of the hydroxyl ions in a solution. It is determined through titration using phenolphthalein indicator.
PARAOCCUPATIONAL EXPOSURE Exposure of workers to an airborne contaminant from a nearby process or operation not forming part of their jobs. Also termed ‘neighbourhood exposure’.
PASTEURISATION Heat treatment to destroy micro-organism usually at high temperature.
PCBS (POLYCHLORINATED BIPHENYLS) Toxic synthetic chemicals with excellent heat resistance and low electrical conductivity properties. Now little used but considerable quantities remain in old electrical equipment. Produces dioxins and polychlorinated dibenzo-furans when burned below 1200C. PCBs are toxic and bio-accumulative.
PERCUTANEOUS ABSORPTION Absorption via the skin, e.g. due to local contamination or a splash of chemical.
PERMIT-TO-WORK A document needed when the safeguards provided in normal production are unavailable and the manner in which a job is done is critical to safety. Identifies conditions required for safe operation.
pH The hydrogen ion concentration of a water stated on a scale from 0 to 14. A pH of 7 is neutral; a pH below 7 indicates an acidic solution and a pH above 7 indicates an alkaline solution.
PLANKTONIC Free floating micro-organisms in an aquatic system.
PNEUMOCONIOSIS A group of lung diseases of a chronic fibrotic character due to the inhalation and retention in the lungs of a variety of industrial dusts. The main diseases are asbestosis, silicosis, coalworkers’ pneumoconiosis and mixed-dust pneumoconiosis; less common pneumoconioses are associated with talc, clay or aluminium.
PPM Parts per million: a measure of dissolved substances given as the number of parts there are in a million parts of solvent. It is numerically equivalent to milligrams per litre mg/l with respect to water.
POND/SUMP Collection of cooling water at the base of a cooling tower.
POOL FIRE A fire involving a flammable liquid spillage onto ground or onto water, or within a storage tank or trench. The pool size depends upon the scale and local topography. Fire engulfment and radiant heat pose the main risks.
PRACTICABLE Capable of being done in the light of current knowledge and invention.
PRECIPITATE To separate materials from a solution through the formation of insoluble matter by chemical reaction.
PRESCRIBED DISEASE A disease prescribed for the purpose of payment of disablement benefit under the Social Security Act 1975 and the Social Security (Industrial Injuries) (Prescribed Diseases) Regulations 1985 and subsequent amendments. (Conditions due to physical agents, biological agents and miscellaneous conditions are classified in addition to conditions due to chemical agents.)
PRESCRIBED PROCESS Industrial process which requires an official authorization because of the likelihood of causing pollution under the provisions of the Environmental Protection Act (1990).
PRESCRIBED SUBSTANCE A substance controlled by Section I of the Environmental Protection Act (1990) because of its potential to pollute. Different substances are prescribed for release to different environmental media.
PRESSURE SYSTEM Defined in the Pressure System Safety Regulations 2000 as a system containing one or more pressure vessels of rigid construction, any associated pipework and protective devices; the pipework with its protective devices to which a transportable gas container is, or is intended to be, connected; or a pipeline and its protective devices which contains or is liable to contain a relevant fluid, but does not include a transportable gas container. Here ‘relevant fluid’ is steam; any fluid or mixture of fluids which is at a pressure of >0.5 bar above atmospheric pressure, and which fluid or a mixture of fluids is a gas, or a liquid which would have a vapour pressure of >0.5 bar above atmospheric pressure when in equilibrium with its vapour at either the actual temperature of the liquid or 17.5oC; or a gas dissolved under pressure in a solvent contained in a porous substance at ambient temperature and which could be released from the solvent with the application of heat.
PRE-TREATMENT Term frequently used to define mechanical treatment of water, e.g. softening or reverse osmosis, prior to its use in a process – also called external treatment.
PRIMING The discharge of steam containing excessive quantities of entrained water
PULMONARY OEDEMA Production of watery fluid in the lungs.
PYROPHORIC SUBSTANCE A material that undergoes such vigorous oxidation or hydrolysis (often with evolution of highly-flammable gases) when exposed to atmospheric oxygen or to water, that it rapidly ignites without an external source of ignition. This is a special case of spontaneous combustion.
RAW WATER The water supplied to a plant or facility before external or internal treatment is applied.
REASONABLY PRACTICABLE The implication that the quantum of risk is balanced against the sacrifice or cost in terms of money, time and trouble necessary to avert that risk. If the risk outweighs the sacrifice or cost, additional precautions are necessary.
RECYCLING The use of materials, usually after further processing, which otherwise would be thrown away. Becoming common practice in industry, especially with expensive commodities such as chemical solvents although many products require a commercial subsidy in order to make recycling viable.
RED LIST The Red List was drawn up by the UK Government in 1989 in response to international conferences of the states bordering the North Sea. The aim was to reduce inputs of Red List substances by 50% by 1995, from 1985 levels. All those substances listed on the Red List are included on the EC Black List. Authorizations to discharge Red List substances are dealt with by the Environment Agency under Integrated Pollution Control (IPC).
REDUCING AGENT A material that adds hydrogen to an element or compound; a material that adds an electron to an element or compound.
REPORTABLE DISEASE (UK) A disease which must be reported to the authorities when linked to specified types of work. (The Reporting of Injuries Diseases and Dangerous Occurrences Regulations 1995.)
RESPIRABLE DUST That fraction of total inhalable dust which penetrates to the gas exchange region of the lung (usually considered to be in the range 0.5 micron–7 micron).
RESPIRATORY SENSITIZER (ASTHMAGEN) A substance which can cause an individual’s respiratory system to develop a condition which makes it ‘over-react’ if the substance is inhaled again. Such an individual is ‘sensitized’; over-reaction is then likely to occur at concentrations of the substance which have no effect on unsensitized persons and lead to characteristic symptoms, e.g. rhinitis (a runny nose), conjunctivitis or in severe cases asthma or alveolitis.
RETENTION Time a chemical is retained in the system.
RISK The likelihood that a substance will cause harm in given circumstances.
RISK ASSESSMENT Identifying and assessing the risk from legionellosis from work activities and water sources on premises and determining any necessary precautionary measures.
SAFE SYSTEM OF WORK A formal procedure resulting from systematic examination of a task to identify all the hazards. Defines safe methods to ensure that hazards are eliminated, or risks controlled.
SCALE A dense, crystalline deposit form by precipitated material. It usually forms on boiler tube surfaces where heat transfer occurs.
SCALE INHIBITORS Chemicals used to control scale. They function by holding up the precipitation process and/or distorting the crystal shape, thus preventing the build-up of a hard, adherent scale.
SEALED SOURCE A source containing any radioactive substance whose structure is such as to prevent, under normal conditions of use, any dispersion of radioactive substances into the environment, but it does not include any radioactive substance inside a nuclear reactor or any nuclear fuel element.
SENSITIZATION DERMATITIS Inflammation of the skin due to an allergic reaction to a sensitizer.
SENSITIZER A substance that causes little or no reaction in a person upon initial exposure, but which will provoke an allergic response on subsequent exposures.
SENTINEL TAPS For a hot water services – the first and last taps on a recirculating system. For cold water systems (or non-recirculating hot water systems), the nearest and furthest taps from the storage tank. The choice of sentinel taps may also include other taps which are considered to represent a particular risk.
SERO-GROUP A sub-group of the main species.
SESSILE Aquatic micro-organisms adhering to a surface normally as part of a biofilm.
SHUNT PUMP A circulation pump fitted to hot water service/plant to overcome the temperature stratification of the stored water.
SLIME A mucus-like exudate which covers a surface produced by some micro-organisms.
SLUDGE A general term for soft mud-like deposits found on heat transfer surfaces or other important sections of a cooling system. Also found at the base of calorifiers and cold water storage tanks.
SLUDGE (BOILER): A soft, water-formed sedimentary deposit that can usually be removed by bottom blowdown.
SICK BUILDING SYNDROME A group of symptoms more common in workers in certain buildings and which are temporarily related to working in them. Symptoms include lethargy, tiredness, headache; also sore/dry eyes, dry throat, dry skin, symptoms suggestive of asthma, blocked or runny nose. Cause is multifunctional but does include agents encountered in the workplace.
SMOKE Particulate matter (usually <0.5 m in diameter) in air resulting usually from combustion, including liquids, gases, vapours and solids. SODIUM SULPHITE The most commonly used oxygen scavenger in steam boiler systems.

SOLVENTS Liquids that dissolve other substances. Chemical solvents are used widely in industry: e.g. by pharmaceutical makers to extract active substances; by electronics manufacturers to wash circuit boards; by paint makers to aid drying. Solvents can cause air and water pollution and some can be responsible for ozone depletion.

SOFT WATER Water containing relatively low concentrations of calcium and magnesium ions (typically less than 5 ppm).

SOFTENER A device for removing hardness from water. Ion exchange softeners operate by exchanging sodium ions for calcium and magnesium ions. (also known as a Base Exchange water softener).

SPECIAL WASTE Controlled waste which is subject to special regulations regarding its control and disposal because of its difficult or dangerous characteristics. The UK definition of special waste is similar, but not identical, to the EC’s hazardous waste.

SPONTANEOUS COMBUSTION Combustion that results when materials undergo atmospheric oxidation at such a rate that the heat generation exceeds heat dissipation and the heat gradually builds up to a sufficient degree to cause the mass of material to inflame.

STAGNATION The condition where water ceases to flow and is therefore liable to microbiological growth.

STEAM EXPLOSION Overpressure associated with the rapid expansion in volume on instantaneous conversion of water to steam.

STRAINERS A coarse filter usually positioned upstream of a sensitive component such as a pump control valve or heat exchanger to protect it from debris.

SUBSTANCE HAZARDOUS TO HEALTH As defined in Regulation 2 of the Control of Substances Hazardous to Health Regulations 1999, a substance which is listed in Part 1 of the approved supply list as dangerous for supply within the meaning of the Chemicals (Hazard Information and Packaging for Supply) Regulations 1994 and for which an indication of danger specified for the substance in Part V of that list is very toxic, toxic, harmful, corrosive or irritant; a substance for which the Health and Safety Commission has approved a maximum exposure limit or an occupational exposure standard; a biological agent; dust of any kind except of a substance within para. (a) or (b) above, when present at a concentration in air equal to or greater than: 10 mg/m3, as a time-weighted average over an 8-hr period, of total inhalable dust, or 4 mg/m3, as a time-weighted average over an 8-hr period of respirable dust; a substance, not covered by (a) or (b) above, which creates a hazard to the health of any person which is comparable with the hazards created by substances mentioned in those sub- paragraphs.

SUSPENDED SOLIDS Solids not in true solution in water, rather in particulate from capable of being removed by blowdown.

SYNERGISTIC When the combined effect, e.g. of exposure to a mixture of toxic chemicals, is greater than the sum of the individual effects.

SYSTEMIC POISONS Substances which cause injury at sites other than, or as well as, at the site of contact.

TERATOGEN A chemical or physical agent that can cause defects in a developing embryo or foetus when the pregnant female is exposed to the harmful agent.

THERMAL DISINFECTION Heat treatment to disinfect a system. THERMODYNAMICS The study of laws that govern the conversion of one form of energy to another.

THERMOSTRATIC MIXING Mixing valve in which the temperature at the outlet valve is pre-selected and controlled automatically by the valve.

TLV-C, THRESHOLD LIMIT VALUE – CEILING (USA) A limit for the atmospheric concentration of a chemical which may not be exceeded at any time, even instantaneously in workroom air.

TLV-STEL, THRESHOLD LIMIT VALUE – SHORT TERM EXPOSURE LIMIT (USA) A maximum limit on the concentration of a chemical in workroom air which may be reached, but not exceeded, on up to four occasions during a day for a maximum of 15 minutes each time with each maximum exposure separated by at least one hour.

TLV-TWA, THRESHOLD LIMIT VALUE – TIME WEIGHTED AVERAGE (USA) A limit for the atmospheric concentration of a chemical, averaged over an 8-hr day, to which it is believed that most people can be exposed without harm.

TOTAL DISSOLVED SOLIDS (TDS): The total of all substances dissolved in ionic form in a water.

TOTAL INHALABLE DUST Airborne material capable of entering the nose and mouth during breathing and hence available for deposition in the respiratory tract.

TOTAL VIABLE COUNT (TVC) The total number of culturable bacteria (per volume or area) in a given sample (does not include legionella).

TOXIC WASTE Poisonous waste, usually certain organic chemicals such as chlorinated solvents.

TRADE EFFLUENT Any waste water released from an industrial process or trade premises with the exception of domestic sewage.

UEL, UPPER EXPLOSIVE (OR FLAMMABLE) LIMIT The maximum concentration of gas, vapour, mist or dust in air at a given pressure and temperature in which a flame can be propagated.

UVCE (UNCONFINED VAPOUR CLOUD EXPLOSION) Explosion which may occur when a large mass of flammable vapour, normally >5 tonnes, after dispersion in air to produce a mixture within the flammable range is ignited in the open. Intense blast damage results, often causing ‘domino effects’, e.g. secondary fires.
VALENCY The number of potential chemical bonds that an element may form with other elements.
VOCs (VOLATILE ORGANIC COMPOUNDS) Gaseous pollutants whose sources include vehicle emissions and solvents.
WDAs (WASTE DISPOSAL AUTHORITIES) Body responsible for planning and making arrangements for waste disposal in their area with the waste disposal companies and for providing household waste dumps under the Environmental Protection Act 1990.
WET STEAM Steam containing entrained boiler water.
WINDAGE Physical loss of water from a cooling tower caused by draught of air or wind – water is lost around the base of the cooling tower as a result of cross winds as opposed to drift.