Legionella Control Glossary

Aerosol - Definition

An aerosol is defined as a free floating suspension of fine solids, liquids or both in a gaseous form. It can be breathed deep into the lungs and carry harmful bacteria including Legionella.

Air-conditioning - definition

Air conditioning is defined as the mechanism used to change the conditions or property of indoor air quality. It is mainly used to control comfort parameters such as temperature and relative humidity to create comfortable indoor air quality conditions.

Algae - definition

Algae is a genus name for an extremely large and diverse group of the very simple, mainly autotrophic organisms that comprise both unicellular and multi-cellular forms. Algae commonly colonise internal walls of storage tanks, cooling towers and water containing vessels that are exposed to sunlight.

Bacteria - definition

Bacteria (singular bacterium) comprises a very large family group of unicellular and multi-cellular microorganisms normally a few micrometers in length. Bacterium has a wide range of shapes including spirals spheres and rods. Bacteria were including in the first life forms of earth.

Biocide - definition

A biocide can be defined as either a chemical substance or microorganism that can be used to prevent, make harmless or control another harmful organism by either chemical or biological interaction.

Biofilm formation - definition

A biofilm is defined as a coating of microorganisms where the cells stick to each other and onto a surface. Biofilm is commonly described as slime forming bacteria and often include species such as Pseudomonas.

Cooling Tower blow-down or bleed-off

Water drained or discharged from a water system due to the build up of salts or other impurities where such build up is undesirable. Cooling towers are bleed regularly for example to reduce salts and impurities that can act as food source for microbial growth including Legionella bacteria.

Calorifier - definition

A piece of equipment that is utilised to transfer heat to water in a contained unit by indirect heat transfer of heat contained in a coil or pipe work which is then immersed into the cold water supply. Any such water containing vessel can therefore be defined as a calorifier.

Colony Forming Units or CFU - Meaning

A unit of measurement used to indicates the number of microorganisms identified in a water sample. It is commonly measured by the number of colony forming units (CFU) present in a millilitre of water.

Cold Water Service (CWS) - definition

A cold water service includes the installation of equipment, pipe work and fittings that are then used to store distribute and supply fresh water for use. Domestic cold water service (DCWS) includes water supplied for domestic water use only.

Concentration factor - definition

This is the result of a comparison calculation made between the levels of total dissolved solids in a cooling tower circuit against the total dissolved solids in a cooling towers make up water supply for example. A term cycles of concentration is often used in the industry and reflects a comparison between indicator constituents of two bodies of water.

Cooling Tower- definition

A cooling tower is defined as a piece of equipment used to transfer waste heat to the atmosphere. They commonly use either the engineered process of evaporation by fluid or air to create cooling and help discharge the waste heat.

Corrosion - definition

Corrosion is defined as the gradual destruction or degradation of a material. Metals are prone to corrosion by chemical reaction with oxygen in the environment.

Corrosion inhibitors - definition

A corrosion inhibitor comprises a chemical substance that if introduced to a liquid or gas contain in a vessel will reduce the level or rate of degradation to the vessel in which the liquid or gas is stored.

Dead ends and blind end - definition

A length of pipe that has been capped, closed, or cannot be opened at one end resulting in no water being able to pass through the pipe. This results in water stagnation and subsequent bacterial growth or proliferation.

Dead leg - definition

A length of pipe fitted with an operable valve that is never opened or used.

Dip slide(s) - definition

A dip slide is a piece of equipment used to analyse the microbial content of dirty water. It includes a sterile case and dip stick with a culture media or agar which is used by immersing the stick into the body of water and then replacing it in its sterile container. The whole unit is then placed in an incubator to facilitate bacterial growth. The resulting growths are then read against a chart to provide an indication of water quality.

Disinfection- definition

Disinfection is the process used to destroy or render microorganisms irreversibly useless in order to reduce their numbers to a level that is unlikely to cause significant harm or is considered non hazardous. Disinfection does not necessarily kill all microorganisms, especially resistant spores. It is less effective than sterilisation which is intended to kills all types living life.

Distribution circuit or system - definition

The system of pipes and equipment used to move water from one point to another where water can then be used at one more appliance, system or fitting within a building.

Drift - definition

Drift in the water industry is defined by the calculation of water lost from a system or cooling tower in liquid droplet form that has become suspended in the system exhaust and is then released into the atmosphere. Drift eliminators are fitted to reduce such water loss for example.

Drift eliminator - definition

Drift eliminators comprise a piece of equipment which is designed to prevent water droplets passing through it. Whilst most do not completely stop all water droplets from passing they come various different designs that have varied levels of efficiency in the task they are designed to achieve. Technically would be better named drift reducers.

Evaporative condenser - definition

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

Evaporative cooling- definition

Evaporative cooling is the process by which evaporation of a liquid into the surrounding air is used to cool an object or liquid that is in contact with it. Latent heat is the amount of heat required to evaporate the liquid as it is drawn into the air.

Cooling Tower Fill Pack or Packing- definition

The area of the cooling tower that is designed to encourage the process of heat transfer or exchange from the water or liquid to the atmosphere. The pack is normally used to break up the body of water so as to increase surface water area and therefore speed up the heat exchange process

Half–life- definition

The half life is the calculated ratio between the system volume and the rate of system purge.

Hot water service (HWS) - definition

A hot water service includes the installation of equipment, pipe work and fittings that are then used to store distribute and supply fresh hot water for use. Domestic hot water service (DHWS) includes water supplied for domestic water use only.

Legionnaires’ disease - definition

A potentially fatal bacterial pneumonia infection caused by Legionella bacteria.

Legionella - definition

A pathogenic type of gram negative aerobic bacteria that can cause Legionnaires‘ disease.

Legionella audits - definition

Independent verification and validation of the control measure put in place to manage the risk of Legionella. The audit will normally cover aspects of the system, records, management, training, etc to ensure all compliance and safety risks are being suitably managed.

Legionellae - definition

The group name of Legionella bacteria. There are in excess of 50 difference species of which some are known to be fatal to humans. Legionallae bacteria are prevalent in both natural and built environment aquatic systems.

Legionella longbeachae - definition

One of the Legionella species in the Legionellae family. It was identified from in a patient who contracted Legionella in Long Beach California. This species of Legionella is often found in soils and potting compost and has understood to be the cause of Pontiac fever.

Legionella pneumophila - definition

One of the Legionella species known to cause 90% of infections in humans.

Legionella sampling - definition

The action of collection a water sample using aseptic techniques in line with BS7592

Legionellosis - definition

An umbrella term used to cover all of the illness caused by the difference species of Legionella bacteria. Legionellosis covers legionnaires disease, Pontiac Fever, Lochgoilhead fever etc.

Make-up water - definition

Make up water is the water added to a cooling tower or equipment to replace water that has been lost of wasted due to leaks, evaporation, bleed off or discharge.

Micro-organism - definition

A microorganism is a living organism that is of a minute size. The varied list can comprise single cell, cell clusters, or multi-cellular complex organisms with varied effects some of which can be beneficial and some harmful.

Non-oxidising biocide - definition

Non-oxidising biocides comprise chemicals or microorganisms that become effective by means of disrupting or interfering with the target organisms structure or metabolisms which then renders the target organism harmless or effective.

Nutrient - definition

Any type of food source that bacteria or microorganisms can use to stimulate the reproduction or growth process.

Oxidising biocide - definition

A substance or chemical that can oxidise (burn through) or degrade organic matter including microbial cells and other cell material including enzymes and proteins meaning micro-organism are killed when they come into contact with such substances. Chlorine, bromine and chlorine dioxide is the most commonly used oxidising biocides. Silver hydrogen peroxide is a slightly newer oxidising biocide being used in water services today.

Pasteurisation - definition

The process of heating water to a temperature that will kill majority of bacteria present including Legionella. A Temperature of 70°C will offer good pasteurisation and Legionella control.

Plank tonic - definition

A micro-organism that is free floating in a body of water or aquatic environment.

Pontiac fever - definition

An upper respiratory infection that is caused by Legionella bacteria although doesn’t develop into pneumonia.

Cooling Tower Pond or Sump - definition

The storage vessel that holds the body of water collected at the base of the cooling tower after it is used as part of the cooling process or before it is re-circulated to a distribution system for cooling.

Retention or Contact time - definition

The length of time a chemical remains in the system or length of time a chemical needs to be maintained in a system to clean it sufficiently.

Risk Assessment - definition

The identification and quantification of the level of risk posed to health and safety by a system, procedure or process. Legionella risk assessment covers the identification and rating of the level of risk a water system has in relation to the likelihood of infection.

Scale Formation - definition

Hard deposits that form on system surfaces or pipe work and tap outlets as a result of unwanted build up of calcium carbonate minerals. Significant scale can provide areas for bacteria growth, a food source for bacterial growth and reduce efficiency and operational function of service.

Scale Inhibitors - definition

Chemicals or substances used to control scale formation and build up on surfaces or within a system. Scale inhibitors can work by either disruption the precipitation process and or preventing the formation of scale crystals to prevent build up.

Sentinel Taps - definition

The Sentinel taps are defined as the first and last taps on a water distribution system. For hot systems the nearest and further form the water heater or calorifier. For cold water systems the sentinel will be the nearest and furthest taps from the cold water storage tanks.

Seldom used taps or outlets. - Definition

Taps or outlets that are used less than once per week and there create a risk of water stagnation in the distribution pipe work.

Legionella –sero-groups- definition

The sero-group name is the sub group names for a particular legionella species. For example: Legionella Pneumophila, Sero group 1, sub group Pontiac is the main species and strain responsible for 90% of human infections.

Sessile bacteria - definition

The types of bacteria that stick to surfaces including internal surfaces of tanks and pipe work. Sessile bacteria can eventually create a biofilm or coat of bacteria over an entire surface of a material. Pseudomonas is a sessile bacterium principally.

Shunt Pump - definition

A shunt pump is a pump or anti stratification pump is commonly fitted to calorifier water heaters to transfer hot water from the top of the water heater to the base on a periodic basis. Shunt pumps or anti stratification pumps are used to try and reduce the risk of significant bacterial build up in the base of the hot water heater.

Sludge / Silt deposits - definition

This is a generic term to describe soft silt or mud likes accumulations often identified at the base of water storage tanks, water heaters and cooling tower ponds. Such deposits can create areas for bacterial growth and will also provide a nutrient source for bacteria growth and proliferation.

Water Stagnation - definition

Stagnation occurs when water is prevented from flowing in any direction. Such conditions encourage bacterial growth and particularly the growth of Legionella.

Strainers / Filters - definition

Strainers or course filters are used to remove or separate any debris or constituents in a body of water to reduce the risk of such debris effecting the efficient operation of a system. Various grades and types of strainers and filters existing and are used. The type used often depends on the required quality of water required to maintain efficiency or prevent damage to sensitive components or pumps for example.

Thermal disinfection - definition

When heat is used to kill bacteria or micro-organisms living on a surface or in solution.

Thermostatic mixing valve - definition

A blending valve or device where cold water is mixed into the hot water supply to deliver a tap outlet with warm water set to a predefined temperature.

Total viable counts (TVC) - definition

The TVC count represents the total number of viable or living bacteria in a set volume of water or over a set area. TVC counts excluding Legionella and certain other species.

Windage - definition

The water that is lost from a cooling tower due to a forced draught of air or wind including both water that is lost around the base of the cooling tower as well as that lost to cooling tower drift.