Hyperchlorination is the use of high doses of chlorine to disinfect water systems. It is a process that many jurisdictions recommend for decontamination if Legionella is found. In most instances it is the alternative to pasteurisation. However  it also brings with it some problems that need considering.

Hyperchlorination has been used to decontaminate cooling towers for decades. ‘Shot’ or ‘slug’ dosing with sodium hypochlorite (the active ingredient of bleach) to between 20 and 50 mg/L is a relatively easy process and will knock off Legionella and most other microorganisms very quickly. The effects will last for a couple of weeks. It is also a strategy for decontaminating building water systems. Once again the effects will last for a few weeks – probably longer than cooling towers as there is less organic material to ‘mop it up’.

Chlorine as a disinfectant

Chlorine is a very reactive molecule. In water it forms hypochlorous acid which is particularly active in mildly alkaline conditions. It is an oxidising agent that will ‘react’ with metals and any organic material to produce chloride salts. This ‘reactivity’ is not at all specific. So in the right situation it will react with any metals or organic material it contacts. Obviously the benefit of this is that it will ‘oxidise’ bacteria. Literally burning holes in their cell surface. The downside is that it may react with things you don’t really want it to.

Chlorine is the mainstay of disinfection for water utilities globally. It has undoubtedly been a game changer in providing safe drinking water to millions of people. Utilities using chlorine to disinfect usually aim for delivering water with between 0.5 and 1.0 mg/L at your meter. At this concentration it is non-corrosive and still has some antimicrobial activity. Of course no disinfectant is perfect and, though safe to drink, the water entering your premises still has plenty of live bacteria and protozoa in it. A recent report from Adelaide shows that Legionella and Mycobacteria survive, and even multiply all the way down the pipelines from the treatment plant. This occurs whether chlorine or monochloramine is in use.

Disinfection is the single process that has had the greatest impact on drinking water safety. There is clear
evidence that the common adoption of chlorination of drinking water supplies in the 20th century was
responsible for a substantial decrease in infectious diseases. Disinfection will kill all bacterial pathogens
and greatly reduce numbers of viral and most protozoan pathogens. Combined with protection of water
sources from human and livestock waste, disinfection can ensure safe drinking water. In the absence of
complete protection of source water, filtration is likely to be required to improve the removal of viruses
and protozoa. Source: Australian Drinking Water Guidelines

The Hyperchlorination process

So water with some living bacteria and other bugs enters your building. Once in their new home stagnation, warming of the water and a build up of nutrients sets off multiplication. Without a management plan then it is highly likely that Legionella and other pathogens will multiply to numbers that can cause disease. A response to detecting elevated numbers of Legionella is to disinfect using hyperchlorination. Typically this means raising concentrations of chlorine in the water to between and 10 and 15 mg/L. At this concentration it is not a good idea to drink the water. Washing in it may cause skin irritation or rashes.

When hyperchlorination is happening people must not use the water. Of course this is very disruptive in places like health care and aged care facilities. Sometimes it means evacuating the occupants  temporarily. The water then has to be flushed out of the system and replaced with fresh water. It is advisable to chlorinate all of the system in this process – hot, warm or cold. The bacteria enter in the cold supply, so all three are sources of bacteria without proper management.

Hyperchlorination problems

Aside from being very disruptive and expensive on water there are other problems to consider. The active form of chlorine in water is hypochlorous acid. For this acid to form and be active it needs slightly alkaline water. Dosing large quantities (more than 5mg/L) of chlorine tends to acidify the water. Once water is acidic the chlorine becomes more and more corrosive. So unless it is under careful control hyperchlorination can be quite damaging to a water system.

Combine hyperchlorination with chemical flushing and the effects can be even more dramatic. Exposing metal surfaces by chemical flushing makes them more accessible to chlorine and more likely to be damaged. Add to this mixture that the reacted chlorine becomes chloride. Chloride is an excellent conductor of electric charge. All a system needs is some mixed metals (copper / lead / zinc / Iron), which is typical of older buildings, and an electric cell can be created. This is known as galvanic corrosion. It acts like a battery, rapidly degrades metal components and is difficult to stop. It also impacts on the quality of the drinking water in the building by releasing metals.

Some water supplies are treated with monochloramine. Hyperchlorination of water containing monochloramine creates disinfection by-products that smell and taste. The US EPA recommend lower doses of chlorine to reach ‘break-point’ chlorination as a way around this problem.

An Alternative

Chlorine is an effective disinfectant in water systems. Maintaining concentrations between 0.5 and 2mg/L is enough to control most bacteria. At these concentrations the water is still safe to drink or bathe in. Water utilities use this range as the target for getting safe water to your meter. So maintaining similar levels once the water is inside a building will maintain the safety of the water. These lower levels of chlorine are as effective as hyperchlorination. However they won’t cause corrosion. They can work with monochloramine if applied by knowledgeable water treatment professionals. Lower concentrations can disinfect or decontaminate without the disruption or damage of larger doses.

Further Reading

Australian Drinking Water Quality Guidelines 6 2011 Version 3.2 Updated February 2016.

EnHealth (2015) Guidelines for Legionella Control.

US EPA Office of Water EPA 810-R-16-001 (2016) Technologies for Legionella Control in Premise Plumbing Systems: Scientific Literature Review

Whiley, H et al  (2014) The presence of opportunistic pathogens, Legionella spp., L. pneumophila and Mycobacterium avium complex, in South Australian reuse water distribution pipelines. J. Water and Health 13(2):553-61