When looking at your site for preventing Legionella it’s easy to underestimate the significance of dead-legs and little used areas of your site. Deadlegs can influence the outcome of poor results of Legionella samples when carrying out verification monitoring.

Understanding the results requires a little background information to explain what these numbers might means and so I will explain the ways in which Legionella operates within a domestic plumbing system by its very nature. Legionella is known to behave in one of two ways:

  • sessile which means to live on the surfaces such as internals of pipes, or,
  • planktonic which means to circulate and free moving around the system.

When the water in legs of pipes that are not moving they are creating an environment which has a declining chlorine residual and a sedentary environment which bio-film and microbes can colonise and reproduce.

Biofilm is a complex microbial ecosystem which may and often does facilitate Legionella growth.

Now imagine a section of pipe connected to a location of your building prior to a ward or residential shower at an age care home, each time you turn on the shower you risk taking biofilm and / or Legionella for the ride.

It’s advisable to remove areas which were from now defunct facilities or obsolete services but unfortunately it’s not always easy to spot the signs. Many can be ascertained through a survey process or diagnostic sampling which can give you a snapshot of the system.

When you review Legionella results there are patterns that can indicate a site or area which has become colonised by Legionella, a specific system may be affected, for example- only the hot water system is consistently showing the presence of Legionella. Or most frustrating of all, borderline failing or slightly excessive numbers of Legionella which are dotted around the sites with no rhyme nor reason, these can often be attributed to water stagnation in one form or another. You may decide to sample for total plate count (HCC, TPC, TVC or TBC) which is a really helpful indication of stagnation BUT does not always relate to Legionella growth.

As you become acquainted with your water system through sampling, patterns will form and you can begin to notice how the behaviour of the residents and staff influence Legionella growth.

Remember that if you have little-used areas such as event rooms with toilet facilities or activity rooms with sinks, these too can be classified at a dead-leg unless they are flushed weekly. Know the problematic areas on site which often get neglected and you can save time and money in term of re-sampling and performed remedials actions such as disinfection or flushing, not to mention the waste of water in the long-term. With stagnation, consistently flushing is more about prevention than reactive actions to fix the problem.