Glossary of Terms

Disclaimer
Please note this article is for general interest and research purposes only and does not purport to give professional advice. This article should not be considered as training material or used for any other purpose.

Aerosol
Small liquid particles having negligible falling velocity (mists developed by showering etc)

Algae
A small aquatic plant which requires light to grow (often found in exposed parts of cooling towers)

Antibodies
Substances in blood which neutralize or destroy toxins or components of bacteria known as antigens.

Bacteria
A microscopic, normally single cell organism (e.g. legionella)

Biocide
A substance which kills micro-organisms.

Biofilm
A complex community of bacteria and other micro-organisms, embedded in a protective layer with entrained debris, attached to a surface (often seen as a slime found on the internal aspects of pipe work and fittings that provides a habitat for legionella.)

Blow-down (Bleed off)
Water purged from the system to control the concentration of salts or other impurities in circulating water; often expressed as a percentage of the recirculating water.

Calorifier
A device used to heat water, the source of heat being contained in a pipe or coil immersed in the water to be heated.

Chlorine
A chemical element used for disinfection.

Cold water service (CWS)
Plant, pipes and fittings installed to store and distribute cold water around a building and subsequently discharged.

Cooling tower
An apparatus designed to reject heat from air conditioning systems or industrial processes. Often associated with large outbreaks of Legionnaires’ disease.

Concentration factor
Compares the level of dissolved solids in cooling water with that dissolved in make-up water. Usually determined by comparison of either the chloride or magnesium hardness concentration.

Corrosion inhibitors
Chemicals which protect metals by:
Anodic Inhibitors - promoting a thin metal oxide film to passivate the metal.
Cathodic Inhibitors - physically producing a barrier film by controlled deposition.

Dead end / blind end
Pipework permanently closed at one end through which no water can pass.

Deadleg
Often referred to as branch pipes, leading from common water supplies to outlets such as showers. Water only passes when the outlet is used. (Often associated with outbreaks).

Dip slide
Used for testing the microbial content of water. Normally, consists of a plastic carrier bearing a sterile culture medium which can be dipped into liquids. It is then incubated to allow microbial growth.

Disinfection
A process to destroy or inactivate micro-organisms and reduce numbers to safer levels.

Dispersant
A chemical which loosen organic material adhering to surfaces.

Domestic water services
Hot and cold water for personal hygiene, or other domestic uses.

Drift
Water lost from a cooling tower, normally in the form of water droplets and aerosols.

Drift eliminator / reducer
Baffles designed to be fitted to cooling towers to reduce drift, they do not eliminate drift.

Evaporative Condenser
A heat exchanger in which refrigerant is condensed by a combination of air movement and water sprays.

Fill/ Packing
The part of a cooling tower which constitutes the primary heat transfer surface.

Fire sprinkler
A network of piping which releases water when a fire occurs.

Flushing – outlets
Manual flushing of conventional outlets (e.g. showers) which are not in regular weekly use should be flushed for several minutes at least weekly, and more frequently where highly susceptible occupants are present. Records of flushing should be maintained as lapses in flushing have shown a critical increase in legionella at the outlet.

Fouling
Organic growth or other deposits found on the internal aspects of pipes and fittings, often associated with heat transfer surfaces.

Half-life
Ratio of system volume to purge rate. (Not associated with self-purging showers)

Hot water service
Plant, pipes and fittings installed to heat, store and distribute hot water around a building and subsequently discharged

Incubation period
Time taken for bacterium to grow and divide into two.

Immunosuppressant drugs
Drugs which suppress the body’s natural defences.

Legionnaire’ disease
A form of pneumonia caused by legionella bacteria.

Legionella
Primarily, a water born bacteria, there are over 40 species of legionella which belong to the family legionellaceae. (Legionella is singular of legionellae)

Legionella pneumophila
Probably the most causative organisms of Legionnaires’ disease.

Legionellosis
An illness caused by legionella.

Monitoring
Continuous observation of a variable.

Pontiac fever
A legionella infection with flu like symptoms.

Make up water
Water added to a cooling system to compensate for wastage, evaporative loss and bleed off.

Micro-organism
An organism of microscopic size including bacteria, fungi and viruses.

Non-oxidising biocide
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 which is associated with microbiological populations, resulting in the death of micro-organisms which it comes into contact with.

Pasteurisation
Heat treatment aimed at killing micro-organisms at high temperatures. Often difficult to achieve in large water systems .

Planktonic
Free floating micro-organisms in water.

Plume
Visible water vapour discharged from a cooling tower.

ppm
Parts per million: a measure of dissolved substances given as the number of parts there are in a million parts of solvent. Numerically equivalent to milligrams per litre mg/l with respect to water.

Pond / sump
An open vessel containing a collection of cooling water at the base of a cooling tower.

Retention time
Time a chemical is retained in the system

Scale inhibitor
Inhibits formation of scale

Sero-group
A sub-group of the main species.

Sentinel taps
The first and last taps on pipe work in water systems.

Sessile
Aquatic micro-organisms adhering to a surface, normally as part of a biofilm.

Showers – conventional
Characterised by a single water pipe connected to the shower head.

Showers - drain down valves
A device which is not recommended for legionella control in showers. It was once mistaken that draining water from the blended water pipe connected between the mixer valve and a shower head would offer some type of control. These devices are proven to be ineffective against legionella.

Showers – self purging
Characterised by two water pipes connected to the shower head; a conventional blended water feed pipe, and a purge pipe connected to drain services to dispose of stagnant water found in the fitting and associated dead legs; purges to drain immediately before use.

Shower - spray plate
Water is often delivered from a disc with fine apertures to produce spray patterns; conventional spray plates are of particular concern as it has been shown they can provide ideal conditions for legionella to proliferate. Conventional showers should be regularly cleaned and disinfected

Shunt pump
A circulation pump fitted to hot water heaters / storage vessels to reduce temperature stratification of stored water.

Slime
A mucus-like biomass that often covers a surface, produced by some micro-organisms.

Stagnation
The condition where water stays static and is therefore liable to microbiological growth (often found in deadlegs)

Strainers
Filters positioned upstream of a sensitive component such as a mixer valve, pump etc, to protect it from debris.

Thermostatic mixing valve
A valve which determines the water temperature delivered by an outlet.

Total viable counts (TVC)
The total number of culturable bacteria (per unit of volume or area) in a given sample – does not include legionella.

Risk assessment
Identifying the areas of risk from legionellosis and determining precautionary measures.

Windage
Water loss from a cooling tower caused by cross winds.

Article by:
Legionella Control Journal - www.lcj-online.co.uk