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

Abstract

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%.