Complex Relationships with Drinking Water Disinfectants

Effective operational management and use of in-premise hot and cold water systems is essential to reduce risk of waterborne pathogen colonisation, amplification and transmission.  The most commonly adopted primary control measures are maintaining hot and cold water temperatures and ensuring adequate volume throughput.  In some countries, including the UK, use of  secondary “top up” systemic biocides for municipal supplied influent water are allowed within buildings. Biocide is typically dosed or generated close to the point of influent water entry to the building(s).  Such systemic biocides include chlorine, chlorine dioxide, monochloramine, hydrogen peroxide, silver hydrogen peroxide and copper-silver ionisation. 

Biocides are more likely to be implemented in drinking water systems when it is not possible to sustain water temperatures, as part of a commissioning or refurbishment project, or in some cases in response to the presence of waterborne pathogens such as Legionella spp, Pseudomonas aeruginosa and non-tuberculous mycobacteria.  However, performance expectations following implementation of secondary systemic biocides are frequently not met.  Either initial microbial improvements are not sustained, or required disinfectant levels are problematic to maintain.  Such gaps in performance have been critically analysed in a recent review paper from Harriet Whiley’s research team at Flinders University, Australia.  Their paper (Xi et al) identifies common factors affecting and negatively impacting the efficacy of chlorine-based disinfectants with regard to control of Legionella spp.  In turn, this analysis helps Water Safety Groups better risk assess systemic disinfectants, suitability for  their specific pipework system and to understand how to appropriately measure efficacy of action.

Xi identified and screened 1890 published articles, selecting those that investigated chlorine-based biocides, factors influencing efficacy of biocidal action, Legionella and in-premise potable water systems.  A final selection of 117 relevant articles were included in their review.  In summary:

Study LocationStudy Numbers (%)
Field work,  monitoring real-life in-premise plumbing systems (healthcare facilities, hotels, residences, schools, offices, other public/commercial buildings)76/117 (65%):  43/76 studies were conducted in hospitals
Laboratory testing, using large scale model plumbing systems within controlled settings41/117 (35%)

Top 5 Factors Impacting Disinfection Efficacy Against Legionella

Field SettingLaboratory Setting
Disinfectant Concentration (excessive decay was noted in 87% of studies)Disinfection Concentration
Disinfection TypePresence of Biofilm
TemperatureDisinfectant Type
Legionella disinfectant resistanceLegionella disinfectant resistance

Maintaining disinfectant concentration was critical for efficacy yet multiple factors influencing the rate of decay were identified, not only related to chemical factors (disinfectant concentration) or physiochemical factors (organic carbon levels, presence of biofilms) but also on physical factors such as stagnation, temperature, water velocity, seasonal impacts, plumbing pipe material, size and complexity of pipework system.

Top 5 Factors Impacting Rate of Disinfection Decay in Plumbing Systems

Field SettingLaboratory Setting
StagnationMaterials in contact with water
Use of a water softenerTotal organic carbon
Building Floor (multi-story)Water velocity

In most studies, traditional plate culture methods were used to verify disinfectant effectiveness.  However, the use of a combination of detection methods including PCR/qPCR, flow cytometry and fluorescence alongside traditional culture techniques improved visibility of persistent pathogens.  Persistence of Legionella due to chlorine-based disinfectant tolerance was demonstrated in multiple studies, and worryingly the most resistance was shown by Legionella pneumophila serogroup 1 compared to serogroups 2-14.  Legionella spp exhibited a much higher resistance compared with coliform bacteria and, not surprisingly, those embedded within biofilm communities had a higher tolerance to disinfectant than those free living and unprotected in the planktonic phase.  Therefore, in order to understand the impact of systemic disinfectant on your microbial populations it is prudent to have a robust baseline of both plate culture based data plus an alternative detection method such as qPCR prior to disinfection.  With the baseline established a regular follow up post implementation of either shock or continuous secondary disinfection will enable visibility of Viable But Non-Culturable Cells, microbial population selection or potential rebound growth of resistant organisms.  The presence of biofilm increases Legionella tolerance to disinfectants, and Legionella can persist in highly chlorinated and chloraminated premise plumbing systems even in immature biofilms of between 18-30 days old.

Positive impact in effectiveness was demonstrated by consistent disinfectant concentration and temperature control 

Variation in the range of concentrations considered effective against Legionella was considerable, and also demonstrates that disinfection concentration alone does not elucidate control of waterborne pathogens.  Effective concentrations for chlorine ranged from 0.2 to 50 mg/L with 0.2 mg/L being the most frequently applied concentration. Hyperchlorination with chlorine concentrations of > 6 mg/L were employed in certain critical contamination cases but not always shown to be effective.  With monochloramine, the effective concentrations ranged from 1.3 mg/L to 3 mg/L with 2 mg/L being the most frequently applied concentration.  For chlorine dioxide the effective concentration ranged from 0.1 to 50 mg/L with 0.3 mg/L being the most frequently applied concentration.  The maximum concentration of 50 mg/L was employed as a “shock” dose.  It is important to note that 0.5 mg/L monochloramine and 0.1 mg/L chlorine have been shown to be effective in inactivating Legionella pneumophila in the laboratory setting. However where Legionella pneumophila is present within biofilms, concentrations of 0.3 mg/L and higher did not significantly impact the total number of cells within the biofilm.

Due to instability of disinfectants to demonstrate effectiveness in hot water, temperatures need to be consistently > 50 oC, ideally > 55 oC to ensure microbial control.  At higher temperatures there is an increased rate of disinfectant decay, and therefore other physical factors are reliant for success (e.g. throughput).  Whilst stagnation causes disinfectant decay, increased and excessive water velocity in pipes also accelerates disinfectant decay.  The higher shear stress leads to mobilisation of particulate debris and biomass from loose biofilm which then rapidly absorbs active disinfectant. 

Optimal disinfectant efficacy is also linked to pH.  The normal range for drinking water pH is between 6.5 (slightly acidic) and 8.5 (more alkaline), and typically around neutral at pH 7.  Chlorine is more effective at pH 7 or below, and chlorine dioxide appears to be more effective where pH is more alkaline, > pH 7.

Time to Re-Evaluate

Mapping the relationship and combined impact of these factors is complex. Many field studies fail to deliver expected sustainable waterborne pathogen control.  Data modelling can help predict required disinfectant residual levels within in-premise plumbing systems.  Having detailed information about water chemistries is not sufficient to support assessment, selection and implementation of a secondary biocide, rather a detailed understanding of operational temperatures, hydraulic balance, peripheral turnover and materials of construction are necessary to improve the likelihood of disinfectant success.  Manual, intermittent physical monitoring does not give sufficient detail or confidence that in-premise systems are running under optimal conditions.  Future use of real time remote sensors with ongoing machine learning and artificial intelligence may lead to a more robust water system performance. With evidenced and optimal operational parameters, Legionella and other waterborne pathogens present within in-premise plumbing systems are likely well managed and under those circumstances the use of systemic disinfectant may not be required.  Focus on “Engineering it Out” and the adage of Keep it Hot, Keep it Cold, Keep it Moving ensures sustainable impact.

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Xi H, Ross KE, Hinds J, Molino PJ, Whiley H. Efficacy of chlorine-based disinfectants to control Legionella within premise plumbing systems. Water Res. 2024 May 25;259:121794. doi: 10.1016/j.watres.2024.121794. Epub ahead of print. PMID: 38824796.

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