WATER SYSTEM HYGIENE AND SAFETY
Monochloramine – a new approach to disinfection
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 non- pneumonic 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 side- effects, 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
64 Health Estate Journal June 2023
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
97 pm H O 103 ± 3° 169.3 pm Cl O O Cl 117.6° N H H
Figure 1: Chemical structures of common disinfectants (hypochlorous acid and chlorine dioxide) compared with monochloramine.
Cl 147.3 pm
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 (NH2
Cl) 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.
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