A suspension of particles, dispersed in a gas.

A system or process for controlling the temperature, humidity, and sometimes the purity, of the air in a building or interior, with particular emphasis on cooling.

A small aquatic plant, often found on exposed areas of cooling towers.

Proteins produced in the blood in response to the presence of an antigen. By becoming attached to antigens on infectious organisms, antibodies can render them harmless or cause them to be destroyed.

Anti-corrosive chemicals protect metals by either passivating the metal (by the promotion of a thin metal oxide film i.e. anodic inhibitors), or by physically forming a thin barrier film by controlled deposition (cathodic inhibitors).

Any manmade water system that does not occur naturally, such as a hot water system.

An official examination and inspection to verify the efficacy of a system. In the case of a Legionella audit, this includes an evaluation of the existing risk management system, assessment reports and other protocols to ensure compliance with the law.

A single-celled organism (and plural of bacterium).

A chemical which destroys life by poisoning. Can include pesticides, herbicides or fungicide. There are two types of biocide: oxidizing and non-oxidizing.

A complex structure adhering to surfaces that are regularly in contact with water, consisting of colonies of bacteria and usually other microorganisms such as yeasts and fungi. Biofilms can form on solid or liquid surfaces and are typically resistant to conventional methods of disinfection. The slimy coating that can be found in pipes and tanks, and algal mats on bodies of water are examples of biofilms.

Water discharged from a plumbing system to control the concentration of salts or other impurities in the circulating water; usually expressed as a percentage of recirculating water flow.

A heat exchanger which heats water indirectly by circulating is over a heating coil or multiple coils. The source of heat can be water or steam, heated by an external heat source, contained within a pipe immersed in the water.

A unit of measurement used in microbiology indicating the number of microorganisms present in a water sample. It is normally measured by the number of colony-forming units (CFU) present in one millilitre of water.

A chemical element commonly used for water purification, chlorine is an oxidising biocide.

Installation of plant, pipes and fittings in which cold water is stored, distributed and subsequently discharged.

The concentration factor (CF) 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.

A cooling tower is a heat rejection device which rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers vary in size from small roof-top units to very large hyperboloid structures often associated with power plants. The vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioners.

Special chemicals which protect metals by either making the surface of a metal less reactive by way of a thin metal oxide film (anodic inhibitors) or by forming a physical barrier film by controlled deposition (cathodic inhibitors).

A dead end or blind end is a pipe which is closed at one end and therefore no water passes through.

A length of water system pipework leading to a fitting through which water only passes infrequently, when there is draw off from the fitting. A dead leg provides the potential for stagnation.

The process of removing or neutralising pathogenic bacteria in a water or other system.

The definition of a dip slide, as described in the HSE approved code of practice and guidance for the control of legionella bacteria in water systems, 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.

Dipslides are the most common method of measuring and monitoring microbial activity within many systems and commonly cooling systems.

A cleaning or cleansing process which removes infection..

Pipework which distributes water from hot or cold water storage to one or more fittings/appliances.

Hot and cold water intended for personal hygiene, culinary, drinking water or other domestic purposes.

Drift is water that is carried out of a cooling tower by the force of the air moving through the tower. It is water which recirculates, containing all the minerals, chemicals and bacteria that are in the tower. Drift is an uncontrolled loss, expressed as a percentage of the recirculating flow in a tower. 0.008% is a typical level of drift in a cooling tower in good condition.

A heat exchanger in which a refrigerant is cooled by a combination of air movement and water spray.

The reduction in temperature resulting from the evaporation of a liquid, which removes latent heat from the surface from which evaporation takes place. This process is used in industrial and domestic cooling systems.

The process of draining and cleaning a cooling system, including all associated pipework.

Organic growth or other deposits on heat transfer surfaces causing loss in efficiency.

The ratio of system volume to purge rate.

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

HMO stands for a house in multiple occupation, also known as a house of multiple occupancy.

An acronym which stands for heating, ventilation and air-conditioning.

The Langelier Saturation Index (LSI) helps to determine the scaling potential of water and is based on the study of the carbonate equilibrium in water.

The bacterium (Legionella pneumophila) which causes Legionnaires' disease, flourishing in air conditioning and central heating systems.

An inspection and examination to evaluate a Legionella risk management system, assessment reports and other protocols to ensure compliance with the law.

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.

The collection and laboratory testing of samples from a water system in order to monitor the presence of bacteria, both general (aerobic) bacterial species and Legionella bacteria.

Any illness caused by exposure to Legionella.

A severe form of pneumonia caused by Legionella bacteria.

A less severe form of Legionnaires’ disease, also caused by Legionella bacteria.

Water added to a cooling water system to compensate for wastage (e.g. via system leaks), evaporative loss and bleed.

Any organism too small to be viewed by the unaided eye, including bacteria, protozoa and some fungi and algae.

A biocide that functions by mechanisms other than oxidation, including interference with cell metabolism and structure.

A food source for micro-organisms.

Special chemical agents capable of oxidising organic matter e.g. cell material, enzymes or proteins associated with microbiological populations, resulting in death of the micro-organism.

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.

Heat treatment to destroy micro-organisms.

An adjective to describe an aquatic system of passively floating, drifting organisms, primarily comprising microscopic algae and protozoa.

The visible discharge of air and moisture from a cooling tower or other cooling system. This can include condensation and aerosols and as such represents a potential legionella risk.

Collection of cooling water at the base of a cooling tower.

Pontiac fever is a less severe form of Legionnaires’ disease, often affecting younger people.

Personal protective equipment is used to protect personnel during hazardous operations.

The period during which a chemical is retained in the system.

Identifying and assessing the risk from Legionella from work activities and water sources on premises and determining any necessary precautionary measures.

Crystalline deposits that form on plumbing system surfaces or pipework. Scale normally results from a build-up of unwanted minerals, commonly calcium carbonate.

Chemicals used to control the build-up of scale.

A schematic, or schematic drawing, is a representation of the elements of a system using abstract, graphic symbols rather than realistic pictures.

The first and last taps on the water re-circulating system. For a cold water storage system it is the nearest and furthest taps from the cold water storage tank.

A group of bacteria containing a common antigen, sometimes including more than one serotype, species, or genus.

An adjective to describe aquatic micro-organisms fixed to a surface, normally as part of a biofilm.

A circulation pump fitted to a hot water system to overcome the temperature stratification of the stored water.

A mucus-like sticky matter, produced by some micro-organisms, which covers a surface.

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.

Also known as a hot tub or Jacuzzi, a spa pool is a self-contained body of warm water designed for sitting in (not whole body immersion). The water is re-circulated and kept between 30 - 40°C and is usually not drained between use, being continually filtered and cleaned. A hydro-jet circulation, with or without an air induction bubble system, is also used to agitate the water.

Standing water which ceases to flow and is therefore liable to microbiological growth.

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

See Pond

Heat treatment used to disinfect a system.

A thermostatic mixing valve (TMV) is a valve in which the temperature at the outlet is pre-selected and controlled automatically by the valve.

TVC gives a quantitative idea about the presence of microorganisms such as bacteria, yeast and mould in a sample. To be specific, the count actually represents the number of colony forming units (cfu) per g (or per ml) of the sample.

A water heater, also referred to as a calorifier, is used for the transfer of heat to water in a vessel, the source of heat being contained in a pipe or coil immersed in the water.