Our building is colonized with Legionella.

Our site is a Victorian mansion with new buildings attached over the years. The main building is listed, which makes improvements very problematic. A registered water treatment company annually takes samples which return positive readings on Legionella. The company chlorinates the roof header tank and retakes samples. The result will return a lower reading but we just cannot seem to get Legionella out of the system. Recently the water treatment company reported in one part of the building a reading of 825 Legionella pneumophila serogroup (2-14) before chlorination, and a reading of 150 after chlorination. Another part of the building gave a reading of 300 cfu/mL before chlorination and 75 cfu/mL after chlorination. The company advised us to put the hot water on constant to kill the remaining bacteria. Further tests were taken afterwards and the results were still positive. A survey indicated that the run from the boiler is so great that the water loses temperature. We have improved the pumps, used stronger chemicals, chlorinated more frequently – all at a great cost. We have even fitted new boilers, but to no avail. Are we doing enough to satisfy our legal responsibilities/Health & Safety requirements? Or is our only answer to change all the old rusty pipework? This would cause a great problem, as it would put much of the building out of use for some time. The building is also listed, and any works require authorization.

Legionella commonly colonizes large buildings and even homes and residences. However, most people have drunk Legionella without any adverse effects. We advocate culturing of hospital water supplies, because patients  with underlying diseases congregate there. On the other hand, we do not advocate routine culture or disinfecting residential or building water supplies. So, the answer to your question is: We would not recommend routine culture of the water supply of this  mansion. Chlorination longterm will corrode your water system, and the risk to residents or workers within this building is low regardless of the presence of Legionella.