A paper I am in the process of getting published discussed issue in water distribution systems.  Biofilms are a major part of the topics, but legionella is a hidden pathogen that only really gets noticed when people get sick.  It is a cause of pneumonia, which generally manifests from aerosol form – in showers.  Utilities do not test for legionella, its not thought by some water distribution operators to be an issue except from air.  However, public health experts are suggesting that legionella may be a hidden problem promoted by biofilms in water. The number of cases has increased substantially since 2000.  There are a couple indicators of a potential issue.  First, water temperatures greater than 17C appear to be required.  Low chlorine residuals are a problem yet, for systems using chloramines, a minimum of 0.5 mg/L will suffice while in free chlorine residual systems, as little as 0.1 mg/L.  Stagnant water is a problem.  Hydraulic flushing of the distribution system systematically will help remove deposits that might harbor legionella.  Nitrification may be an indicator, which means a pH between 8 and 8.6 should be avoided.

These are basically the same findings as for nitrification and biofilms.  What it indicates is that the tubercles and rusty water may harbor far more significant organisms than once thought.  Time to take biofilm and nitrification control more seriously.