A new Legionella disinfectant residual study commissioned by the Association of Metropolitan Water Agencies (AMWA) has found no significant relationship between disinfectant residual levels in public water systems and Legionella occurrence in federal office buildings.
The study examined Legionella sampling results from federally operated buildings and compared them with contemporaneous disinfectant residual data reported by the public water systems supplying those facilities.
Researchers concluded that there was no consistent association between higher disinfectant residual concentrations within distribution systems and the presence of Legionella in building water samples.
Legionella disinfectant residual study examines federal building data
The study, titled Legionella Occurrence in Buildings: Implications of Public Water System Disinfectant Residual, was commissioned by AMWA and the American Water Works Association and published in the May–June 2026 edition of AWWA Water Science.
The research used baseline drinking water quality testing data collected by the US General Services Administration (GSA), covering thousands of federal facilities.
Researchers cross-referenced Legionella sampling results from selected GSA-operated buildings with disinfectant residual measurements from the water utilities serving those locations.
The findings suggest that increasing disinfectant residual levels within distribution systems may not significantly reduce Legionella occurrence at building taps.
Findings could influence future EPA drinking water regulations
The research comes ahead of anticipated revisions to the US Environmental Protection Agency’s Microbial and Disinfection Byproducts (MDBP) Rules, expected to be proposed in 2027.
Previous recommendations from a National Drinking Water Advisory Council working group included establishing higher federal minimum disinfectant residual requirements of up to 0.5 mg/L for free chlorine and 0.7 mg/L for total chlorine in chloraminated systems.
AMWA has previously recommended a minimum numeric residual level of 0.2 mg/L for chlorine and total chlorine, arguing that higher levels could introduce additional compliance challenges without demonstrated public health benefits.
According to the study authors, the analysis found no clear disinfectant residual threshold that consistently correlated with lower Legionella occurrence within buildings.
Researchers call for greater focus on building water management
The study’s authors suggest that future policy approaches should focus more directly on building water management practices rather than increasing disinfectant concentrations throughout distribution networks.
Tom Dobbins, CEO of AMWA, said the research provides new evidence that higher disinfectant residual levels are not meaningfully associated with Legionella readings at the tap.
Chad Seidel, President of Corona Environmental Consulting and a co-author of the study, said future policy development should prioritise building-centred approaches to managing Legionella risks.
Legionella bacteria can proliferate within building plumbing systems, particularly where water stagnation, temperature conditions and system design create favourable growth conditions.
Water utilities and building operators increasingly recognise that controlling Legionella requires a combination of treatment, monitoring and building-specific water management strategies.
Read more drinking water and water quality news in the Drinking Water and Water Quality sections of H2O Global News.
FAQs
What is Legionella?
Legionella is a type of bacteria that can grow in water systems and may cause Legionnaires’ disease when contaminated water droplets are inhaled.
What did the Legionella disinfectant residual study find?
The study found no significant relationship between higher disinfectant residual levels in public water systems and lower Legionella occurrence in sampled federal buildings.
Why are disinfectant residual levels important?
Disinfectant residuals help maintain microbiological protection throughout drinking water distribution systems by limiting bacterial growth during water transport.
Could the findings affect future drinking water regulations?
Yes. The study may help inform future EPA decisions regarding minimum disinfectant residual requirements under proposed revisions to the MDBP Rules.
