LEGIONELLA - World Health Organization

it may be useful to subcontract this task to a specialist contractor. Independent assessment of
larger water systems will also help to identify design faults and areas that need servicing.
Routine servicing and replacement of components of the system should comply with manufacturers’
specifications or existing technical references, where available, and should be carried out by
properly qualified people. System layout and design specifications should be used to determine
the servicing and replacement requirements for the entire system.
Assess hazards and prioritize risks
Each system should be assessed individually, taking into account the proximity and susceptibility
of the population, and the mode of transmission from the water source. The potential risks
associated with the system should also be evaluated. This step involves understanding the
characteristics of the water system, the hazards that may arise and how they may create risks,
and the processes and practices that affect water quality.
Assess the system
This step involves assessing the existing system, including describing the system and preparing
a flow diagram. The aim of preparing a flow diagram is to increase the accuracy of the water
system evaluation and provide a conceptual understanding of the water supply process. The
diagram — a systematic representation of the sequence of steps or operations used in the
production or manufacture of a particular water item — can be used to show:
• pathways by which legionellae can be transferred to consumers
• points where controls are in place and where improvements might need to be made
• links, water flow direction and responsibilities in the water supply process; for example,
where the utility’s responsibility ends (e.g. at the consumer’s meter) and the consumer’s
begins (e.g. after receipt of water).
To avoid duplication, the diagram should cross-reference any supporting documentation that
covers finer details. Such documentation might include geographical information system
(GIS) layers and plumbing schematics, which could be used to identify stagnation points; for
example, maps showing key account holders, such as hospitals, schools and nursing homes.
3.3.2 Monitoring
The steps involved in monitoring— shown in Figure 3.2, and discussed below — are to:
• identify control measures
• monitor control measures
• validate effectiveness of WSP.
LEGIONELLA AND THE PREVENTION OF LEGIONELLOSIS