Monochloramination has had a major impact on potable water in Australia. The longer residual time and greater stability than chlorine has allowed long distance transfer of safe water across great distances. In many areas monochloramine has replaced chlorine as a utilities choice of disinfection. But no disinfectant is a ‘cure-all’. Digging a little deeper into the current knowledge might help you decide whether this is the option for you.
Monochloramine is made by adding ammonia to chlorine. This is usually performed at specific injection points. Adding a little excess ammonia is a critical part of the process. Not enough ammonia and two chlorines will react with one ammonia and produce dichloramine, or even trichloramine. These two relatives of monochloramine are described by the Australian Drinking Water Guidelines as undesirable disinfection by-products (DBPs). So why has it been used extensively in Australia and globally?
Chlorine and monochloramine
Chlorine is highly reactive and ‘burns’ organic matter it contacts. Surprisingly it is also a naturally occurring disinfectant. The Oxyntic cells in your stomach lining produce hydrochloric acid to break down food and kill bacteria. Some cells in your immune system also produce chlorine (as hypochlorous acid) to kill bacteria. Sometimes this reactivity can be a problem.
Monochloramine is less reactive than chlorine. In the first instance this may sound like a bad thing. But not reacting too quickly is a conventional wisdom in everyday life. A disinfectant that does not react too quickly is able to last longer and penetrate further than its more reactive counterpart. For this reason monochloramine works very successfully on long water pipelines where chlorine would ‘burn out’ too quickly. This means it has a better residual activity.
Monochloramine is not as electrically charged as chlorine. Chlorine exists in water as hypochlorous acid and carries a large negative charge over it’s surface that attracts water molecules. Water carries weak positive charges that ‘hang on’ to the chlorine molecule. This makes the molecule ‘fat’ and it’s size prevents it from diffusing into things like slime and biofilm. Monochloramine is not as heavily charged which keeps it ‘slimmer’ and more able to diffuse into biofilm. This makes monochloramine more penetrant and better able to disrupt biofilm and slimes.
The combination of longer residual times and greater penetration makes monochloramine a very attractive alternative to chlorine. It is easy to measure both with the same equipment and experienced water treatment specialists can use them together if necessary. So why is monochloramine not the ‘cure-all’ we’ve been waiting for?
To ensure only monochloramine is produced in the reaction with chlorine an excess of ammonia is required. You can think of it like this. If there is more ammonia than chlorine then the ammonia will only ‘pick up’ one chlorine each. But if there is less ammonia it will ‘pick up’ two or even three chlorines.
NH3 = ammonia, HOCl = chlorine (hypochlorous acid) Source: ADWG 2016.
So what’s the problem with excess ammonia? Well, ammonia makes a very good fertiliser and many bacteria like it as a nitrogen source. So if there is an excess of ammonia in the water it tends to stimulate bacterial growth. In a building water system excess ammonia can lead to ‘nitrification’. Certain groups of common bacteria will go about using ammonia and converting it to nitrate which is an even better bacterial growth stimulant. Typically these bacteria form biofilms. Just think of putting ammonium or nitrate fertilisers on your lawn! Of course this will get biofilm growing. An awkward twist in this story is that many ‘nitrifying’ bacteria are also resistant to monochloramine. Some even break it down to get more ammonia. As well as this research demonstrates that although it is more penetrant than chlorine it is less effective as a disinfectant in a nitrifying biofilm.
Clearly, without control, the nitrification process is going to cause problems for the microbiology of the building water system. More monochloramine is not going to fix it. But wait there’s more!
Dichloramine and trichloramine are the by-products of chloramination. It is generally best to avoid their production due to odour and taste issues. Quite often the smell at swimming pools is thought to be chlorine. In most cases this is not true. It is actually chloramines and is an indicator that ‘break point’ chlorination is not happening. In this case the ammonia is coming from the bathers!
Monochloramine because it is the most stable and produces the lowest tastes and odours. Dichloramine is a stronger disinfectant than monochloramine, but is less stable and produces distinctive odours. Trichloramine is the least stable and produces offensive odours. (ADWQG 2013)
Chloramine DBPs can cause illness if they reach high concentrations in poorly maintained swimming pools. However there is no data that suggests they cause adverse health affects at the concentrations found in potable water system under normal circumstances.
A problem that can occur is chlorine disinfection of monochloraminated building systems. Systems with positive Legionella test results are required to be disinfected by many regulatory authorities. Most jurisdictions stipulate hyperchlorination or pasteurisation as the means of disinfection.
Source: Guidelines for Legionella Control, Government of South Australia 2013
Hyperchlorination will inevitably cause any monochloramine present to convert to the DBPs causing water taste and odour issues. This of course means dumping a lot of water and disruption to services. Using an approved alternative treatment can get around this problem saving water and keeping building services fully operational.
The Viable Non-Culturable state
The success of monochloramine in controlling Legionella in large buildings has been widely reported for the past two decades. Culture results have seemingly dropped to zero in relatively short periods. The critical point is ‘culture results’. Many bacteria are able to enter the Viable Non-Culturable (VBNC) state. This is usually in response to an environmental shock. Rapid changes in temperature, exposure to Ultra Violet light or chemicals can all induce this state. The bacteria are still alive and able to cause disease, but they won’t grow on laboratory culture media.
Whilst reports of successful Legionella control with monochloramine abound there is a significant body of evidence that all is not what it seems. Laboratory studies and field studies have shown that monochloramine induces the VBNC state in Legionella and other bacteria. Studies using methods to resuscitate the Legionella or detect them by molecular methods have shown clearly that a significant proportion of the Legionella population go VBNC when they meet monochloramine. This means that culture methods will give false negative test results indicating Legionella is not present even when it is. It is important to note that VBNC Legionella are also just as infectious as their culturable counterparts.
Ultimately this means that laboratory testing from chloraminated water may not detect Legionella or other bacteria. Even though they may be present and infectious.
No disinfectant is a ‘cure all’. More importantly unless the design and operation of a system is good no disinfectant will guarantee success. Understanding plumbing and hydraulics is the first step in choosing a disinfection strategy. In the right application with the right design monochloramine is the right disinfectant. However it is unwise to think that merely the choice of chemical will get the result you want.
Identifying these three factors – application, design and disinfectant is a critical component of your water safety and risk management strategy. Chemistry, microbiology, plumbing, and hydraulics must work together to get to a cost effective and practical solution.
Australian Drinking Water Quality Guidelines 6 2011 Version 3.3 Updated November 2016.
Coniglio, M.A. et al (2015) Monochloramine for controlling Legionella in biofilms: how much we know? Journal of Nature and Science, 1(2):e44
Government of South Australia (2013) Guidelines for the control of Legionella
Lee, W.H. et al (2011) Free Chlorine and Monochloramine Application to Nitrifying Biofilm: Comparison of Biofilm Penetration, Activity, and Viability. Environ. Sci. Technol. 45, 1412–1419
Turetgen, I. (2008) Induction of Viable but Nonculturable (VBNC) state and the effect of multiple subculturing on the survival of Legionella pneumophila strains in the presence of monochloramine. Annals of Microbiology, 58(1):153-156