Disinfectant Residuals and Biofilm Control

What a residual can tell you

Residual monitoring can help confirm whether an authorised treatment is reaching selected points. Sampling locations, method and timing should be defined in the site programme.

A result should be interpreted alongside system changes, demand, temperature, flow and microbiological evidence.

Avoiding over-interpretation

A residual at one point does not demonstrate conditions throughout complex pipework or on every surface. It should not be used to make unapproved efficacy claims.

2026 evidence: incoming residual and Legionella in ~5,600 buildings

The largest building-level dataset on this question so far was published in AWWA Water Science on 21 May 2026 (von Hagen et al., doi 10.1002/aws2.70055). In 2024 the US General Services Administration tested water quality, including Legionella, at roughly 8,250 federally owned and leased buildings. The study analysed summary results from about 5,600 of them, detailed laboratory reports from 40, and the chlorine or chloramine residuals recorded by the public water systems supplying them. About 23.6% of sampled building water systems had at least one total-Legionella exceedance. Older buildings (pre-1980s), larger buildings and health and human-services facilities showed higher exceedance rates.

On the residual question, the authors found no clear numeric distribution-system residual threshold that was consistently linked to lower Legionella occurrence in buildings. State minimum-residual requirements were not associated with a statistically significant difference in the proportion of buildings with exceedances, and chloraminated and free-chlorine systems did not differ significantly at the system level. In the temporally matched analysis of seven supplies, residuals near buildings with exceedances were often, but not always, lower than near buildings without, and exceedances occurred in buildings whose supplies were meeting their state minimums.

What the building study does and does not establish

It establishes that the residual arriving from the distribution main is a weak predictor, on its own, of whether Legionella will be found at outlets inside a building. That is consistent with the point of this guide: a residual measured at one location describes the water there, not the condition of the building's pipework, dead legs, storage and outlets. Building age, size and use were stronger correlates than the incoming residual. The disinfectants studied were chlorine and chloramine, not chlorine dioxide, and the study measured what public water systems delivered, not what any building dosed internally.

It does not establish that residuals are unimportant, nor does it test any secondary or supplementary disinfection inside buildings. The authors list substantial limitations: summary data recorded exceedances rather than concentrations; national residual data (2012–2020) were not contemporaneous with the 2024 building sampling; only 57% of buildings could be matched to a supplier; and detailed laboratory reports were available for just 40 buildings. The setting is US federal buildings under US rules, so the figures do not transfer to UK duty-holder obligations under ACOP L8 and HSG274. Nothing in the study speaks to ChloroKlean products.

Evidence and uncertainty

Published biofilm studies are valuable for understanding mechanisms, but their conditions may not match a particular installation. Species, surfaces, deposits and operating conditions should be recorded when interpreting evidence.

For safety-critical systems, decisions should be documented through the relevant risk assessment and management plan.

Choosing the next question

A useful next step is to identify what is known, what is inferred and what needs verification. This avoids treating a general reference as a site diagnosis.

Where a product is considered, confirm the intended use, authorisation and label directions independently of this educational guide.

Use this sequence to frame investigation and control; it is not a dosing protocol.

Expert Insights

About the Reviewer

Frequently Asked Questions

Common questions about this topic, answered by our technical team.

Related Resources

Continue exploring our knowledge base and product information.