Quantitative Microbial Risk Assessment Framework Incorporating Water Ages with Legionella pneumophila Growth Rates

Water age in drinking water systems is often used as a proxy for water quality but is rarely used as a direct input in assessing microbial risk. This study directly linked water ages in a premise plumbing system to concentrations of Legionella pneumophila via a growth model. In turn, the L. pneumophila concentrations were used for a quantitative microbial risk assessment to calculate the associated probabilities of infection (P_inf) and clinically severe illness (P_csi) due to showering. Risk reductions achieved by purging devices, which reduce water age, were also quantified. The median annual P_inf exceeded the commonly used 1 in 10,000 (10^-4) risk benchmark in all scenarios, but the median annual P_csi was always 1-3 orders of magnitude below 10^-4. The median annual P_csi was lower in homes with two occupants (4.7 × 10^-7) than with one occupant (7.5 × 10^-7) due to more frequent use of water fixtures, which reduced water ages. The median annual P_csi for homes with one occupant was reduced by 39-43% with scheduled purging 1-2 times per day. Smart purging devices, which purge only after a certain period of nonuse, maintained these lower annual P_csi values while reducing additional water consumption by 45-62%.