Disinfectant Residuals and Biofilm Control

What a residual can tell you

Residual monitoring can help confirm whether a lawfully supplied 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, disinfection by-products and microbiological evidence. A result within an operating range answers a chemistry question at that sampling point and time. It does not show which organisms are attached upstream, whether they are viable, or whether deposits and low-flow sections are under control.

Avoiding over-interpretation

A residual at one point does not demonstrate conditions throughout complex pipework or on every surface. Raising or lowering a residual in response to one result can also affect disinfection by-products and community selection. These relationships should be reviewed together, not used to make unsupported efficacy claims or copied into a product dose.

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.

Mediterranean network evidence: temperature, free chlorine, ecology and DBPs

A 2026 study of an operational drinking-water network in Valencia, Spain (Gonzalez et al., doi 10.1007/s11274-026-05101-x) followed bulk water and in-situ biofilm devices for one year. Reported water-temperature measurements were 20.6 °C in June and 25.4 °C in September. Those warmer sampling periods coincided with chlorine dosing above 0.5 mg/L as Cl₂ and higher trihalomethanes, which reached a maximum of 48 µg/L. The network analyte was free residual chlorine. The study reported significant correlations between temperature and THMs, pH and metal concentrations, while its multivariate analysis associated seasonal shifts in water and biofilm communities with temperature, THMs, free residual chlorine and metals.

The ecology did not move as a single response to residual. Pseudomonas and Acidovorax formed part of the core microbiome, early three-month biofilms were richer in Acidovorax and Rhodocyclus-group organisms, and older biofilms had greater contributions from Pseudomonas, Dechloromonas and Caulobacter. September biofilms aligned with higher temperature, THMs and manganese, while June samples aligned more closely with free residual chlorine and Alphaproteobacteria. These are observational associations in one network, not proof that one variable caused another.

The treatment train must be kept explicit. Chlorine dioxide was used for primary disinfection at the treatment works, followed by granular activated-carbon filtration, then UV and chlorination for final disinfection. The residual measured and discussed in the distribution network was free chlorine, not chlorine dioxide. The paper is therefore evidence about seasonal interpretation in a chlorinated network. It is not chlorine dioxide distribution-residual evidence, an efficacy comparison, or a dosing basis for any product.

How to read the four signals together

Plot temperature, free-disinfectant residual, DBPs and microbiological or community results on the same timeline, with sampling location, flow state and operational changes. A warmer-period rise in dose can coincide with a lower residual, different biofilm composition and increased DBPs, but a shared timeline does not by itself show which factor caused another. Looking only at the residual can hide those relationships; looking only at DNA can mistake genetic material for living organisms; looking only at a DBP can miss the hydraulics and precursor conditions behind it.

Use trends to frame investigation rather than infer efficacy. Confirm unexpected findings with the appropriate method, inspect or sample surfaces where the risk justifies it, and review temperature, stagnation, deposits and treatment changes with the responsible person. This guide does not prescribe a residual or product dose.

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, lawful GB BPR route and label directions independently of this educational guide.

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

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