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Superchlorination

Superchlorination, also known as hyperchlorination, temporarily increases the free chlorine residual in a water distribution system. A high free chlorine residual (i.e. above 5 mg/L) is effective against most bacteria (including Legionella) and can be used in hot, warm and cold water distribution systems, although in hot water systems chlorine degrades rapidly.

In practice, superchlorination is best applied to cold water systems while thermal treatment is preferable for hot water systems.

When to undertake superchlorination

Superchlorination should be undertaken in accordance with your water risk management plan (WRMP) and may be required:

  • as part of the commissioning process for new water infrastructure
  • for remediation of affected infrastructure following a detection of Legionella or other microbial hazard that has been assessed as being of high risk to patients or residents
  • based on the complexity of the plumbing infrastructure of your facility, in areas where biofilm growth is suspected (e.g. low flow pipe sections), on a scheduled basis (e.g. every six months).

As significant biofilm growth can reduce the efficacy of superchlorination, if there is evidence of significant biofilm in a water distribution system, it may be necessary to conduct a chemical clean of the system before superchlorination in order to improve the effectiveness the treatment.

Who undertakes superchlorination

Superchlorinated water is hazardous and so only a specialised, suitably qualified and competent professional (such as an appropriately trained plumber or water treatment specialist) should undertake superchlorination.

An important component of superchlorination is maintaining a pH of 7.5 which ensures effective disinfection. Chlorine is less effective as a disinfectant in water with pH less than 7.5 or above pH 8.0. Most drinking water in Queensland is supplied within this range. Where it is not within this range, the competent professional should consider pH adjustment using appropriate chemical dosing.

The facility’s water risk management team and a workplace health and safety officer should be consulted before carrying out superchlorination to ensure that appropriate safeguards are in place to protect patients, residents and workers.

On-going maintenance of the chlorine dosing equipment used for superchlorination should be also undertaken by a suitably qualified and appropriately licenced professional.

Where to undertake superchlorination

Superchlorination should be undertaken at the following locations:

  • during commissioning the entire water distribution system should be superchlorinated in accordance with AS3500.1:2015 Appendix J unless an alternative, effective sanitation process is used
  • at appropriate dosing point(s) upstream of any  locations where Legionella or other microbial hazards have been detected within your water distribution system.

Depending on the size and complexity of the water distribution system, superchlorination may be applied to an entire facility, or only a limited number of outlet(s) in a localised area (e.g. single wing, ward or cluster of rooms).

Steps

Main steps for superchlorination should meet the following criteria:

  • Ensure all appropriate safeguards are in place before commencing superchlorination (see above).
  • Dose chlorine solution so as to obtain a free chlorine residual of at least 10 mg/L at each downstream sample location, and maintain the residual continuously and at a reasonable rate of water flow for at least 10 minutes. This may require dosing of chlorine at a level much greater than 10 mg/L for a long duration until the distal outlets are able to reach a consistent, measurable residual of 10 mg/L. (Note that the Australian Drinking Water Guideline for chlorine in drinking water is 5 mg/L. Therefore no water with chlorine above this level should be supplied to outlets used by staff, patients or residents).
  • Each sample location should have a record sheet attached to it. This should not be removed until all work is completed or, in the case of a detection, after resample results are shown to be clear of Legionella.
  • Once a residual of 10 mg/L is reached at the furthest point in the water distribution system, and is maintained for at least 10 minutes in free-flowing water, keep this elevated level of chlorine held within the piping system for at least 2 hours by shutting off all taps.
  • Following this duration of at least 2 hours, flush the system to remove the high chlorine solution.
  • Repeat the above chlorination a minimum of 2 times (3 total cycles of treatment are recommended).
  • All affected outlets are to be flushed following this practice.
  • After completion, it is highly recommended that the facility maintain a measurable, residual level of chemical disinfectant at all water taps and showers. This residual can either come from the water being supplied by the local water provider, or from a chlorine dosing system installed within the healthcare facility.
  • Acceptable drinking water disinfectants, and how they are to be used to provide acceptable levels of disinfectant residual, are described in Part IV (Information sheet 1.2) of the Australian Drinking Water Guidelines (ADWG).
  • If chlorine or chloramines are used as the residual disinfectant, professional equipment should be used to measure it, such as a digital colorimeter or spectrophotometer.
  • Superchlorinated water can be quite aggressive (potentially causing lead and copper to be released into the water from pipes and fittings) and so, in all facilities, especially those older than 20 years, plumbing should be checked to ensure it is in an acceptable operating condition and free of excessive corrosion or wear. If the plumbing is not in good condition superchlorination may not be suitable and alternative approaches to sanitation should be explored.
  • If the chlorine treatment was done in response to a Legionella detection, the outlet should be resampled 3–7 days post treatment to determine if the contamination has been removed. If it has not been achieved, repeat superchlorination of the affected outlets.

Chloraminated town water supply

Large parts of South East Queensland (SEQ) receive drinking water that has been chloraminated rather than chlorinated. Chloramination involves the drinking water service provider dosing both chlorine and ammonia to form chloramines which, while being less effective as disinfectants, are considered to persist longer in extensive drinking water distribution systems, such as those in SEQ.

It is important to know the type of disinfectant provided in your drinking water supply before undertaking superchlorination.

Superchlorinating a chloraminated water distribution supply may initially reduce the effectiveness of chlorine and may also result in the production of the potentially harmful disinfection by-products, dichloramine and trichloramine. A water quality specialist should be consulted before undertaking chlorination of chloraminated water.

Effects of superchlorination on your water distribution system

Duration of the effect of superchlorination varies and may last between 2–4 weeks.

Chlorine is corrosive at concentrations above 5 mg/L (corrosiveness increases incrementally with the concentration of chlorine dosed) and at pH below 7.0. It may cause damage to components of the water distribution system such as pipework, seals and taps and cause metals (especially copper and lead) to be released from these components.

Measuring and recording chlorine levels during superchlorination

Free chlorine residual levels should be recorded in accordance with your WRMP and include details of where, when, concentration of chlorine, by whom and any additional comments.

Free and total chlorine levels can be measured using test strips during superchlorination but these are not suitable for routine chlorine monitoring.

Caution

  • As superchlorination temporarily increases the chlorine concentration of the drinking water supply to above the drinking water guideline of 5 mg/L, after superchlorination, the water distribution system should be thoroughly flushed with water having a routine level of disinfectant before it can be returned to use as a drinking water supply.
  • Chlorine concentrations in a drinking water supply should not exceed the ADWG value of 5 mg/L.
  • Chlorine is a powerful oxidising agent and should be handled and used with appropriate containment by competent personnel with suitable personal protective equipment.
  • Chlorine becomes less soluble with increasing water temperature.
  • Chlorine disinfection is compromised by ultra-violet (UV) disinfection and so during superchlorination UV treatment (if it exists) should be turned off.

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Last updated: 1 June 2021