| ACOP L8 |
Approved Code of Practice & Guidance L8, The Control of Legionella Bacteria in Water Systems published by The Health & Safety Commission. |
| Aerosol |
A suspension in a gaseous medium of solid particles, liquid particles or solid and liquid particles having negligible falling velocity. |
| 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. |
| 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. |
| Bacteria |
(Singular bacterium) a microscopic, unicellular (or more rarely multicellular) organism. |
| 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. |
| 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. |
| 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. |
| CFU/ml |
Colony forming unit per millilitre. |
| CFU/l |
Colony forming unit per litre. |
| Cold Water Service (CWS) |
Installation of plant, pipes and fittings in which cold water is stored, distributed and subsequently discharged. |
| 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 the comparison of either the chloride or magnesium hardness concentration. |
| 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). |
| CWS Tank |
Cold water storage tank. |
| 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 a draw-off from the fitting. |
| Destratification pump |
A circulation pump fitted to hot water service/plant to overcome the temperature stratification of the stored water. |
| 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. |
| Disinfection |
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 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. |
| 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. |
| Fill/Packing |
That portion of a cooling tower which constitutes its primary heat transfer surface; sometimes called ‘packing’ or ‘pack’. |
| Fouling |
Organic growth or other deposits on heat transfer surfaces causing loss in efficiency. |
| Half-life |
Ratio of system volume to purge rate. |
| 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). |
| 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. |
| Pontiac fever |
A disease caused by species of legionella, an upper respiratory illness less severe than Legionnaires’ disease. |
| Make-up water |
Water which is added to a cooling water system to compensate for wastage (e.g. via system leaks), evaporative loss and bleed. |
| Micrograms per litreµg.L-1 |
Numerically equivalent to parts per billion in water. |
| Micro-organism |
An organism of microscopic size including bacteria, fungi and viruses. |
| Milligrams per litre mg.L-1 |
Numerically equivalent to parts per million in water. |
| Non-oxidising biocide |
A non-oxidising biocide is one that functions by mechanisms other than oxidation, including interference with cell metabolism and structure. |
| Nutrient |
A food source for micro-organisms. |
| Oxidising biocide |
Agents capable of oxidising organic matter, e.g. 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. |
| Pasteurisation |
Heat treatment to destroy micro-organism usually at high temperature. |
| Planktonic |
Free floating micro-organisms in an aquatic system. |
PPB
(Parts per billion) |
A measure of dissolved substance given as a number of parts there are in a billion parts of solvent. It is numerically equivalent to micrograms per litre with respect to water. |
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/1 with respect to water. |
| Pond/Sump |
Collection of cooling water at the base of a cooling tower. |
| Retention time |
Time a chemical is retained in the system. |
| 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. |
| Sero-group |
A sub-group of the main species. |
| Sentinel taps |
For 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. |
| Sessile |
Aquatic micro-organisms adhering to a surface normally as part of a biofilm. |
| 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. |
| Slime |
A mucus-like exudate which covers a surface produced by some micro-organisms. |
| Stagnation |
The condition where water ceases to flow and is therefore liable to microbiological growth. |
| 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. |
| Thermal disinfection |
Heat treatment to disinfect a system. |
| Thermal stratification |
The layering of water due to differences in temperature and thus density. |
| Thermostatic mixing valve |
Mixing valve in which the temperature at the outlet is pre-selected and controlled automatically by the valve. |
Total visible counts
(TVC) |
The total number of culturable bacteria (per volume or area) in a given sample (does not include legionella). |
| Risk assessment |
Identifying and assessing the risk from legionellosis from work activities and water sources on premises and determining any necessary precautionary measures. |
| 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. |
| > |
Greater than. |
| < |
Less than. |
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