A suspension in a gaseous medium of solid particles, liquid particles or solid and liquid particles having negligible falling velocity or a suspension of tiny water particle or droplets of water suspended in air as a fine spray or mist.

Air Conditioning

A form of air treatment in which temperature, humidity, and air cleanliness are controlled within desired limits.

Air Washer

A device for the intimate mixing of water and air, the air leaving the device at a humidity approaching saturation.


Usually microscopic aquatic plants, which require light to grow. Often found at the edges of cooling towers.


Proteins 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.

Anti – Stratification pump

A pump fitted usually to a vertical calorifier to reduce thermal layering within the calorifier and therefore to reduce the risk of proliferation of Legionella in the bottom. The pump is usually designed to take water from the top of the calorifier and pump it to the bottom at predetermined time intervals, usually when the system is otherwise lying idle.


A microscopic unicellular or multicellular organism.

Ball Valve

A valve used to maintain a liquid level in a tank by means of a hollow ball floating on the surface of the liquid.


A community of bacteria and other microorganisms, embedded in a protective layer with entrained debris, attached to a surface.


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.


Treatment of water with chlorine release agent or hypochlorite, to kill micro-organisms. See also Hypochlorous acid.

Chlorine Demand

The quantity of chlorine used up in oxidising the oxidisable material (both dissolved and suspended) present in the water before a reserve can be established.


A fixed open topped water container e.g. a water tank.

Cold Water System

An installation that includes plant, pipes and fittings in which cold water is stored, distributed and subsequently discharged.

Combined Water Heater

A rectangular or circular storage water heater (usually heated by electric immersion element) usually pre insulated and having their own built-in cold feed tank. Usually provide domestic hot water to local outlets. Hot water storage capacities range from 25- 210 litres.

Cooling Tower

A device for removing heat from a system by the evaporation of recirculating water. Heat is transferred from the system into the recirculating water by means of a heat exchanger. The water is circulated to the cooling tower, in which it is discharged through a stream of air.
This results in the evaporation of a small quantity of water into the air stream, increasing the humidity of the air towards saturation point. Latent heat of vaporisation is taken from the body of recirculating water, cooling it to a temperature approaching the wet bulb temperature.
The cooled recirculating water is collected in a pond or sump from which it is pumped back to the heat exchanger to provide further cooling and thus complete the circuit.


The destruction of a metal by chemical or electrochemical reaction with its environment.

Dead End

A length of pipe closed at one end through which no water passes such that stagnant conditions develop forming an environment, which may be favourable to the proliferation of bacteria. A pipe is considered to be a dead end or blind end if its length is greater than or equal to 1.5 times its internal diameter.

Dead leg

Pipes leading to a fitting through which water only passes when there is draw off from the fitting. A pipe is considered to be a dead leg if its length is greater than or equal to 1.5 times its internal diameter.


Any one or a combination of materials that have settled on the waterside surfaces of a system, e.g. the base of a storage tank, or a heat exchanger. The deposit may consist of corrosion products, lime scale, sludge, live or dead organic matter, silt or even water-soluble salts left behind by evaporation at points of leakage.


A process which destroys or irreversibly inactivates micro- organisms and reduces their number to a non-hazardous level.

Distribution Circuit

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


A chemical which loosens inorganic or organic material adhering to surfaces and lifts the material into suspension in the liquid.

Domestic Water Services

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

Fire Sprinkler

A network of piping which sprays water over an area when a fire occurs.

Free Chlorine

The amount of chlorine freely available to act as a disinfectant in the water. Note that its disinfection properties are strongly affected by the pH of the water and decline rapidly with increasing alkaline conditions.

Free residual chlorine

When chlorine is dosed to a water system, there is usually a demand caused by the presence of oxidisable material (both dissolved and suspended) in the water. The free residual level of chlorine is the free chlorine that is left after the demand has been satisfied.


Low form of plants without chlorophyll that reproduce by spores.

General Corrosion

Corrosion in a uniform manner across an entire surface.

Heat Exchanger

A device for transferring heat from the medium being cooled to recirculating water. The most common designs are shell- and-tube heat exchangers and plate heat exchangers.

Hot Water System

An installation that includes plant in which water is heated and pipes and fittings through which it is distributed and subsequently discharged. Hot water systems do not include cold water feed tanks or cisterns.

Hypochlorous Acid

The active biocidal agent in the chlorination reaction.

Incubation Period

Time taken for micro-organisms to grow in a culture medium to the point at which the number of colonies is counted to indicate the microbiological status of the system from which the culture medium sample was taken.

Immuno Suppressant

Drugs which suppress the body’s natural defences.


Material that reduces a normal tendency to cause an effect (e.g., scale or corrosion). Usually used to describe chemicals that almost eliminate corrosion through the formation of a protective film on a metal surface, or prevent scaling by interference with the scaling mechanism.

Legionnaires Disease

A form of pneumonia or pneumonia like illness caused by becoming infected with Legionella bacteria, usually by inhalation.


The genus legionella belongs to the family Legionellaceae and has over 40 species.
These are ubiquitous in the environment and found in a wide spectrum of natural and artificial waters.


Type of aerobic bacteria, which are found predominantly in aquatic environments, and can readily grow in warm waters. (Singular of Legionellae).

Legionella pneumophila

One of the most common species of Legionella known to cause Legionnaires’ disease.


An illness caused by exposure to Legionella bacteria.


Water (raw or treated) added to the circulating water in an evaporative cooling system to replace that lost by windage, bleed-off and leakage. It is sometimes expressed as a percentage of circulating water rate, but is more often expressed as volume per unit time, e.g. litre/minute.


Mainly microscopic single celled organisms including bacteria, protozoa, and some algae and fungi. These can grow in a system when conditions are suitable.


Planned observation of a variable.

Monoclonal antibody

An antibody which has developed to combat a particular organism.

Multiplication temperature

Optimum temperature at which a bacterium grows and divides to create more bacteria.


A food source for micro-organisms.

Occupied Zone

An enclosure in which human activity occurs.


Person who operates the plant or system.


Heat treatment to destroy pathogens usually at high temperature.


A numerical indication of the degree of acidity or alkalinity of a solution. The scale is logarithmic and runs from 0 to 14. Low numbers are acidic and high numbers are alkaline. 7 is neutral.

Pontiac Fever

A disease caused by the bacterium Legionella. A respiratory illness less severe than Legionnaires’ disease.


Point of Use Water Heater. Usually installed to provide hot domestic water at remote points beyond the recirculation system.


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) for solids dissolved in water.


Growth or extension by the multiplication of cells.


Lowest and simplest form of animal life, which reproduce by fission or conjunction, include amoebas, flagellates, ciliates etc.

Risk Assessment

In the case of Legionnaires’ disease, the identification and assessment of risk of legionellosis from the association between water sources and human activities on and around premises, and the determination of any necessary precautionary measures to eliminate or minimise the risk identified.

Sentinel Taps

For a recirculating hot water service, 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.


A sub-group of the main family.

Serological Test

Identifying the sub-group in the laboratory, usually by antibody-antigen reactions.

Service Hot Water

Hot water intended for hygienic or culinary use.

Shunt Pump

A circulation pump fitted to hot water service/plant to overcome the temperature stratification of the stored water.


A mucus-like exudate produced by some microorganisms, which covers a surface.


A general term for soft mud-like deposits found on heat transfer surfaces or in other sections of a cooling system.

Sodium Hypochlorite

An alkaline solution that releases hypochlorous acid when diluted by dosing to a water system.


A group of micro-organisms whose members differ only in minor detail.


Stored Point of Use Heater, greater than 15 litres of hot water.


A pipe system installed in a building having at frequent intervals spray nozzles protected by connections made of fusible alloy which, in the event of fire, melt and release water for automatic fire fighting.


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


A coarse filter usually positioned upstream of the water pump to protect it from debris.


The thermal layering of water in a calorifier caused by lack of sufficient flow. The result is that the hottest water is in the top layers and the coldest is in the bottom layers, usually where the cold feed water enters. Vertical Calorifiers are often susceptible to stratification. The problem can be solved by installing an anti-stratification pump.


Depressed chamber below or alongside the collection basin underneath a cooling tower, designed to facilitate pump suction.

Susceptible Persons

People who are much more likely to contract a disease.

Thermal Disinfection

Heat treatment to disinfect a system.

Thermal Insulation

A material having low heat transfer characteristics, used to reduce undesirable heat transfer.


Thermostatic mixing valve in which the temperature at the outlet is pre- selected and controlled automatically by the valve.

Total Chlorine

The sum of the free chlorine and the combined chlorine.


Total dissolved solids, the quantity of solids dissolved in the water.


The Total Viable Count or the total number of bacteria in a sample capable of growing on a specific growth media at a specific temperature (per volume or area) in a given sample.

Vermin Screen

A mesh device fitted to the overflow of a cold water storage tank to prevent ingress of vermin.