WATER SYSTEM HYGIENE AND SAFETY
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
3 3 (c)
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: n The chemistry of the disinfectant. n The nature of the microbiological contaminant.
n 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
Bulk fluid Biofilm
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 by- product) 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.
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
2 2 11
24 hours (Monochloramine) + 20 min (Free Chlorine) + 8 hours (Free Chlorine) + 20 hours (Free Chlorine)
0 0 -1250 -1000 -750 -500 -250 Distance (µm)
Fig 2: Biofilm penetration of free chlorine and monochloramine determined by microelectrodes (Lee 2011).
0 250 500
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.
June 2023 Health Estate Journal 65
Monochloramine (or free chlorine) (mg Cl2
/L)
Substratum
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