Skip links and keyboard navigation

Chlorine dosing

Chlorine dosing is intended to establish a free chlorine residual in a water distribution system. A free chlorine residual is effective against most bacteria (including Legionella), harmless to humans (when within guideline values) and can be used in hot, warm and cold water distribution systems.

Chlorine dosing should not be confused with chlorine dioxide treatment.

Important

Chlorine dosing introduces new risks into your water distribution system and should be carefully considered prior to any installation and/or operation.

Chlorine is a powerful oxidising agent and concentrated chlorine solutions should be handled and used with appropriate containment by competent personnel with suitable personal protective equipment.

Only chlorine suitable for use in drinking water should be used. Pool chlorine is not suitable.

Installation of chlorine dosing equipment should be undertaken by a qualified and competent professional, such as a water treatment specialist.

Chlorine and free chlorine residual

Chlorine is a disinfectant commonly used by water utilities. Free chlorine residuals of between 0.5‑2.0 mg/L provide effective continuous disinfection in water distribution systems.

Facilities that receive water from a registered drinking water service provider should ideally receive a free chlorine residual of between 0.2-0.5 mg/L in their water supply.

However the drinking water service provider is not required to maintain a minimum free chlorine residual within the water supply and factors such as pipe length and chlorine demand mean that the actual concentration of free residual chlorine may be less than 0.2 mg/L when the water reaches your facility.

Chlorine, in the form of hypochlorous acid, is the active residual. It is reactive and not particularly penetrative, which may result in the re-colonization of your water distribution system with Legionella when the residual falls below an active concentration.

Chlorine dosing

When chlorine is dosed into a facility’s water distribution system routinely it is injected continually in order to maintain a constant free chlorine residual.

Chlorine is usually dosed as a concentrated solution of sodium hypochlorite which, when dissolved in water, forms hypochlorous acid. Hypochlorous acid is the active disinfectant form of chlorine often described as ‘free’ or ‘active’ chlorine.

The combined concentration of hypochlorous acid and hypochlorites is often described as ‘total chlorine’.

An important component of chlorine dosing 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.

Installation, operation and on-going maintenance

Installation of chlorine dosing equipment should be undertaken by a qualified and competent professional, such as a water treatment specialist.

Selection of dosing point(s) into a water distribution supply will require consideration of your water distribution system and should be undertaken by an individual who has a sound knowledge of the facility’s water distribution system and/or a specialised and qualified professional, such as a licenced plumber or hydraulic engineer.

Operation and on-going maintenance of chlorine dosing equipment should be undertaken by a qualified and appropriately licenced water treatment specialist.

Measuring and recording chlorine levels

Should be recorded in accordance with your water risk management plan and include details of where, when, concentration of chlorine, by whom and any additional comments.

Free and total chlorine levels can be measured with test strips (roughly) or more accurately with a photometer. The difference between the free and total chlorine concentrations at the correct pH is a measure of how much chlorine is being actively used in the disinfection process.

In an effective disinfected system, the values for free and total chlorine should be similar indicating all organic material and microorganisms in your water distribution system have been disinfected and that there is active chlorine in the water.

Chloraminated water supply

The Mt Crosby Water Treatment Plant in South East Queensland, supplies a large proportion of the drinking water used in Brisbane and Ipswich. Unlike most other Queensland drinking water supplies, Mt Crosby water is chloraminated, rather than chlorinated.

This means that some SEQ drinking water retailers are exclusively supplying chloraminated drinking water, some supply only chlorinated water while some supply chloraminated water for part of the year and chlorinated water at other times.

If your facility is in SEQ, it is highly recommended you ask your drinking water service provider, which type of disinfectant is provided in your drinking water supply because this will affect any proposed chlorine dosing to be undertaken.

Chlorinating a chloraminated water distribution supply should be carefully considered, and if implemented, closely monitored. This is because it may result in the production of the potentially harmful disinfection by-products, such as dichloramine and trichloramine and may also result in ineffective disinfection.

For this reason chlorinating a chloraminated water distribution supply should only be undertaken by a qualified and appropriately licenced professional.

Chlorine concentrations in a drinking water supply should not exceed the Australian Drinking Water Guideline (ADWG) value of 5 mg/L. If chlorine dosing increases the free chlorine residual level above 5 mg/L, the water distribution system must be flushed before it can be returned to use as a drinking water supply.

Caution

  • Chlorine becomes less soluble with increasing water temperature.
  • Chlorine disinfection is compromised by ultra-violet (UV) disinfection.
  • 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 and should therefore be closely monitored.
  • Chlorine degrades over time and ‘use by’ dates should be observed.

Resources

Last updated: 1 June 2021