As featured in Waterline Spring 2024
Monochloramine
A new approach to disinfection of water systems
By Frank Butterworth, Technical Director at Goodwater Ltd
This article discusses the issue of Legionella 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 the author explains the chemical properties of monochloramine that make it stable and less corrosive than other chlorine-based disinfectants, its ability to penetrate biofilms, as well as it’s long term residual disinfectant properties.
The article also highlights the importance of controlling ammonia formation rates when using monochloramine as a supplemental disinfectant. Finally, the article emphasizes the importance of choosing the right disinfectant and the location of installation to ensure effective control.
Legionella are ubiquitous bacteria that can be present in natural and artificial water environments and can survive under a range of environmental conditions. Their ability to colonize artificial water systems poses a serious concern for public health as they can cause a range of pneumonic and non-pneumonic respiratory illnesses collectively referred to as legionellosis.
Monochloramine can be considered one of the best solutions available 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 to other chlorine based disinfectants such as hypochlorite and chlorine dioxide. It is effective at concentrations of 2.0mg/l – 3.0mg/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 formulas, hypochlorous acid (HOCl – the dissolved form of chlorine) and monochloramine (NH2Cl) have similar chemical structures which are different to that of chlorine dioxide (ClO2).
Fig.1 Chemical structures of common disinfectants (hypochlorous acid and chlorine dioxide) compared to Monochloramine.
Monochloramine is a weaker oxidiser than both chlorine dioxide and hypochlorite and this is due to the nitrogen atom bound to a chlorine atom rather than the oxygen atom.
Chlorine (Hypochlorous acid & hypochlorite). Can be considered the least effective chlorine-based disinfectant requiring at least 2.0mg/l – 3.0mg/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 (DBP’s) such as trihalomethanes (THM’s) is high.
Chlorine Dioxide can be an effective disinfectant at concentrations as low as 0.1mg/l – 0.5mg/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 materials respectful of the usual chlorine based disinfectants and at the application levels of 2.0mg/l – 3.0mg/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 – causing 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 they can 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. Leading to effective disinfection of the underlying bacteria within the buildup 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.
Fig 2. Biofilm penetration of free chlorine and monochloramine determined by microelectrodes (Lee 2011).
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 that are as low as 0.50 mg/l and 1.00 mg/l respectively.
Even at concentration approaching 2.0mg/l -3.0mg/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 to control and diminish the ammonia formation rates when monochloramine is used as a supplemental disinfectant.
Fig 3. Cold and hot water diagram installation.
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 to the extent of control delivered.
A key factor that affects the disinfection residuals and hence 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 are.
• The chemistry of the disinfectant
• The nature of the microbiological contaminant
• 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, leading it to be 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 a better dosage control because the water is always circulating into the system even when nobody is using hot water (i.e. during overnight hours).
Microbiologically, Legionella colonizes primarily in warm water, therefore treating all the cold-water system can be seen as being wasteful.
Economically, treating the entire building’s water distribution system would increase the equipment and operational costs, since bigger disinfection units have to be installed and larger volumes of reagents are going to be consumed. Another thing to note is storing bigger volumes of chemical on site can come with increased health and safety risks. If wishing to treat the hot water system only, users may wish to reduce their hot water storage temperatures, once Monochloramine has controlled microbiological growth which in turn leads to energy savings and increased operational stability. This could potentially also allow removal of TMVs that can act as a source of contamination thus reducing service 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.
BPR (UK)
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). They are only one of two European companies listed on the GB Article 95 List for Product Type PT05 – Drinking Water, therefore Monochloramine use as a biocide is entirely acceptable in the UK.
Conclusion
Legionella bacteria, which can cause pneumonic and non-pneumonic respiratory illnesses, can be eliminated by monochloramine at concentrations of 2.0mg/l – 3.0mg/l within a few weeks of continuous application. Monochloramine is a stable and mild oxidant. Has the best compatibility with the typically used materials 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 to other common disinfectants, it is a weaker oxidizer due to the nitrogen atom bound to a chlorine atom rather than the oxygen atom, and it exhibits high penetration through biofilms.
Biofilms can become a protective barrieragainst disinfectant residuals, creating a challenge for traditional disinfectants.
Table 1: Literature survey on the efficacy of Monochloramine for Legionella remediation.
After many studies and trials on the use of monochloramine following a 4-step process of validation:
1. Verification of efficiency using laboratory studies.
2. Anecdotal field reports of efficiency from individual institutions.
3. Controlled field trials in individual institutions.
4. Successful applications in multiple institutions over a prolonged period.
It can be deemed that Monochloramine is an ideal disinfectant for use in healthcare facilities, where the safety of patients and personnel is of upmost importance.
The table above demonstrates positive remediation results have been obtained by application of monochloramine. Only recently has monochloramine been well understood and used more widespread across the globe. This could explain why it took some time for monochloramine to affirm itself as a superior biocide for secondary disinfection of building system.
Author Biography
Frank Butterworth is Technical Director at Goodwater Ltd, one of the UK’s leading Water Treatment Companies, with over 30 years’ experience in water treatment, water hygiene, water treatment plant design, project management and compliance across the industry.
Frank has worked extensively both in the UK and overseas gaining a wealth of experience as a multidisciplined chemist engineer.
Email: [email protected]
Phone: 0118 973 5003
