Frank Butterworth, Technical director at water hygiene, treatment, and equipment specialist, Goodwater, discusses the various strategies that have been used over the years to combat the growth and proliferation of Legionella in healthcare water systems. He particularly focuses on the advantages, in combating the bacterium, of monochloramine as a secondary disinfectant in buildings’ water systems, compared with using other chlorine-based disinfectants.
Legionellae are ubiquitous bacteria which can be present in water systems and can cause respiratory illnesses. Various strategies have been used to combat these bacteria over the years, but most disinfectants have drawbacks, such as corrosion/material compatibility issues, or toxic by-products. Monochloramine has been presented as a promising solution. In this article I will explain the chemical properties of monochloramine that make it stable and less corrosive than other chlorine-based disinfectants, its ability to penetrate biofilms, and its long-term residual disinfectant properties. I will also highlight the importance of controlling ammonia formation rates when using monochloramine as a supplemental disinfectant. Finally, the article will emphasise the importance of choosing the right disinfectant, and the location of installation, to ensure effective control.
Legionellae can be present in both natural and artificial water environments, and can survive under a range of environmental conditions. Their ability to colonise artificial water systems poses a serious concern for public health, as they can cause a range of pneumonic and nonpneumonic respiratory illnesses collectively referred to as legionellosis.
There are a number of strategies available to combat Legionella and protect water systems, but most of them – while having acknowledged disinfectant power – can also be responsible for significant sideeffects, both on water system conditions (corrosion) and on human health, due to the potential generation of toxic disinfection by-products (DBPs). Monochloramine can be considered one of the best solutions available, both due to its ability to reach and kill bacteria by penetrating biofilm, and to maintain its effectiveness in low-flow systems over extended periods, even at the extremities of the system.
What is monochloramine?
Monochloramine is a chlorine-based disinfectant generated from hypochlorous acid and an ammonium salt in a controlled reaction that favours the pH values of typical drinking water systems. Due to its chemical composition and structure, monochloramine is a stable, mild oxidant, especially when compared with other chlorine-based disinfectants such as hypochlorite and chlorine dioxide. It is effective at concentrations of 2 mg/l – 3 mg/l, and can eliminate bacterial colonisation within a few weeks of continuous application
Monochloramine is considered to have the best compatibility with the typically used materials of construction, meaning it is less corrosive to pipework systems and their components. Its stability is also far superior to other chlorine-based disinfectants, making it an effective disinfectant in complex building services installations such as hospitals or hotels.
Chemistry
The key to monochloramine’s stability and materials compatibility is a factor of its chemical properties. As suggested by their chemical formulae (see Figure 1), hypochlorous acid (HOCl – the dissolved form of chlorine) and monochloramine (NH2Cl) have similar chemical structures, which are different to that of chlorine dioxide (ClO2). Monochloramine is a weaker oxidiser than both chlorine dioxide and hypochlorite, and this is due to the nitrogen atom bound to a chlorine, rather than the oxygen atom.
Chlorine (hypochlorous acid and hypochlorite) can be considered the least effective chlorine-based disinfectant, requiring at least 2 mg/l – 3mg/l to achieve effective disinfection. Under these conditions chlorine is highly corrosive to both metallic and plastic components. Its effectiveness is strongly pH-dependant, and the potential for the formation of toxic disinfection by-products (DBPs) such as trihalomethanes (THMs) is high.
Chlorine dioxide can be an effective disinfectant at concentrations as low as 0.1 mg/l – 0.5 mg/l, but under these conditions can be corrosive to both plastic and metallic systems. There are also issues maintaining effective concentrations in hot water systems, as it is effectively just chlorine dioxide gas in solution. Monochloramine can be considered as the most material-respectful of the usual chlorine-based disinfectants, and at the application levels of 2 mg/l – 3 mg/l it does not exhibit the corrosiveness of either chlorine dioxide or chlorine. Due to its stability, it is less prone to ‘gassing off’ than chlorine dioxide or chlorine.
Biofilm penetration
The chemistry of monochloramine is the cause of its high stability and low reactivity – resulting in enhanced penetration of biofilms. Biofilms are aggregates of microbial cells which are accumulated at a solid-liquid interface, encased in a matrix of hydrated extracellular polymeric substances (EPS). The EPS structure not only promotes the binding of organic and inorganic compounds, enhancing the localised availability of nutrients, but also offers a protective environment against disinfectant residuals, creating a protective barrier.
When disinfectants reach the biofilm, they can either react with, or diffuse through, the biological material of EPS, reaching the inner layers of the biofilm where bacteria are located. Free chlorine and chlorine dioxide both react with the EPS and external biofilm layers, being consumed before reaching the inner regions of the biofilm, where most of the bacteria settle and colonise. Being a milder oxidant, monochloramine can effectively penetrate the EPS and upper layers of the biofilm without being consumed. This results in effective disinfection of the underlying bacteria within the build-up of biofilms.
Materials corrosion
Corrosion is one of the biggest concerns when a secondary disinfection system is installed. High corrosion rates result in high maintenance costs to replace corroded pipework and other building water system equipment. Corrosion rates are influenced by several different factors – and the type of disinfectant used is one of them. Thanks to its lower oxidation potential and its stability, monochloramine demonstrates itself to be far less aggressive towards all pipe materials than traditional disinfectants.
Scientific studies have demonstrated that chlorine and chlorine dioxide can be aggressive towards commonly used pipework materials at concentrations as low as 0.5 mg/l and 1 mg/l respectively.
Even at concentrations approaching 2 mg/l – 3 mg/l, monochloramine does not directly react with metals, but free ammonia (a monochloramine potential byproduct) could increase the metal release rates, especially with copper plumbing systems. The key is then to have a system that is capable of controlling and reducing the ammonia formation rates when monochloramine is used as a supplemental disinfectant
Installation location
Supplementary disinfection is applied in building water systems in order to ensure a disinfectant residual in the building’s water services. The type of disinfectant chosen plays a crucial part in the extent of control delivered. One of the key factors to affect the disinfection residuals, and hence the Legionella remediation process, is the temperature of the water system to which the disinfectant is applied.
The disinfectant is often fed into the main cold-water supply to the building, or perhaps more locally into the domestic hot water distribution loop. Factors which influence this decision include:
The chemistry of the disinfectant.
The nature of the microbiological contaminant.
The capital and maintenance costs of the delivery system
Higher system temperatures can affect the stability and efficacy of a disinfectant. This is particularly the case with chlorine dioxide, which, as a gas dissolved in water, makes it particularly susceptible to ‘gassing off’
If the disinfectant is injected in the mains or boosted cold water services, part of the treated water will flow through the system water heaters, and the ‘thermal shock’ to which the biocide is subjected could cause breakdown of the disinfectant molecule, reducing its concentration, and increasing the potential formation of harmful disinfection by-products
Delivering the disinfectant directly into the domestic hot water loop helps to control the formation of DBPs, and ensure better dosage control, because the water is always circulating in the system, even when nobody is using hot water (i.e. during overnight hours).
Microbiologically, Legionella colonises primarily in warm water, and thus treating the entire cold water system can be seen as wasteful. In financial terms, treating the entire building’s water distribution system would increase both equipment and operational costs, since larger disinfection units have to be installed, and larger volumes of reagent will be consumed. It is also important to note that storing larger volumes of chemical on site can come with increased health and safety risks. If wishing to treat only the hot water system, once monochloramine has controlled microbiological growth, users may wish to reduce their hot water storage temperatures, in turn culminating in energy savings and increased operational stability. This could potentially also allow removal of TMVs that can act as a source of contamination, thus reducing servicing and maintenance requirements.
The legislation – What is a biocide?
A biocide is defined as a chemical substance intended to destroy, deter, render harmless, or exert a controlling effect on, any harmful organism. In practice, a disinfectant dosed into any kind of water with the aim of preventing the proliferation of microorganisms (such as Legionella) that can potentially represent a risk to health is classified as a biocide
Following the end of the EU exit transition period on 31 December 2020, the EU Article 95 list of biocidal active substance suppliers is no longer applicable in Great Britain (England, Wales, and Scotland). Under the GB Biocidal Products Regulation (GB BPR), Great Britain now maintains its own independent list of biocidal active substance suppliers, referred to as the GB Article 95 List. Biocidal active substance and product suppliers that were included on the EU Article 95 List on 31 December 2020 were automatically added to the GB Article 95 List.
Sanipur, a manufacturer of commercial monochloramine dosing units, submitted its own Biocide Dossier for monochloramine (generated from sodium hypochlorite and an ammonium source). It is only one of two European companies listed on the GB Article 95 List for Product Type PT05 – Drinking Water; thus monochloramine use as a biocide is entirely acceptable in the UK.
Conclusions
Legionella bacteria, which can cause pneumonic and non-pneumonic respiratory illnesses, can be eliminated by monochloramine at concentrations of 2 mg/l – 3 mg/l within a few weeks of continuous application. Monochloramine is a stable and mild oxidant, and offers the best compatibility with the materials typically used in water distribution systems, making it less corrosive to pipework systems and their components, unlike other chlorine-based disinfectants
The chemical structure of monochloramine is the key to its stability and material compatibility. Compared with other common disinfectants, it is a weaker oxidiser due to the nitrogen atom being bound to a chlorine atom, rather than the oxygen atom, and it exhibits high penetration through biofilms. Biofilms can act as a protective barrier against disinfectant residuals, creating a challenge for traditional disinfectants
Many studies and trials have been undertaken on the use of monochloramine following a four-step validation process:
1. Verification of effectiveness using laboratory studies;
2. Anecdotal field reports of its effectiveness from individual institutions;
3. Controlled field trials in individual institutions;
4. Successful applications in multiple institutions over a prolonged period
The results of such studies indicate that monochloramine is an ideal disinfectant for use in healthcare facilities, where the safety of patients and staff is of upmost importance.
Table 1 shows that positive Legionella remediation results have been obtained via the application of monochloramine. Only recently has monochloramine been well understood and used more widely across the globe. This could explain why it took some time for monochloramine to affirm itself as a superior biocide for secondary disinfection of buildings’ water systems.
Frank Butterworth
Frank Butterworth, Technical director at Goodwater, has over 30 years’ experience in water treatment, water hygiene, water treatment plant design, project management, and compliance, across the industry. He has worked extensively both in the UK and overseas, gaining a wealth of experience as a multidisciplined chemist engineer.