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Critical limits

Critical limits are monitoring values set for a particular parameter/control measure, monitored at a specific point, within a water distribution system. Exceedance of a critical limit should allow the water risk management team sufficient time to implement a corrective action appropriate to the level of risk.

Scope

Critical limits are maximum and/or minimum values set for a particular parameter/control and are considered “critical” as they define the bounds between which a control measure can be considered to be effective. A water management team may decide that at a particular point in a water distribution system a free chlorine residual above 0.5 ppm and below 2 ppm is desirable to ensure effective sanitation but prevent undesirable taste and odour at the water outlet.

If the critical limit is exceeded (or not met) then there may be harm and therefore action should be taken to bring the control back within its critical limits.

For every control measure there should be a procedure detailing the corrective actions to be performed and reporting protocol to be followed.

Some water management teams may decide to also set operating limits for particular criteria. Operating limits are more stringent than critical limits and can act as early warning signs that a control measure is trending towards the critical limit. The response to exceedance of an operating limit does not need to be as immediate or strong as for exceedances of critical limits, but would typically involve some process improvement to ensure that control is re-established and the risk of exceeding a critical limit is reduced.

Critical limit considerations

Temperature

Legionella bacteria grow best at temperatures between 20°C and 45°C. To prevent Legionella growth, it is advisable to keep the cold water temperature below 20°C and the hot water temperature above 60°C.

When setting a critical limit (maximum and/or minimum values)for temperature of a water system, it is important to consider the amount of time it takes for the temperature at an outlet to reach the required temperature.

To prevent scalds, temperature should be below 45°C. Hence, ensure you consider balancing the requirements of Legionella control with scald prevention.

Where a facility cannot maintain the cold water temperature below 20°C, this number should not be used as a critical limit. In this case other control measures must be relied upon to keep the risk from Legionella at an acceptable level.

Disinfection residual

Chlorine, in its various forms, is the most commonly used disinfectant for drinking water.

Disinfectant residual is the amount of disinfection available to inactivate Legionella and other water-related microbial hazards in your water distribution system.

In Queensland, your drinking water service provider must supply water that is free of detectable Escherichia coli (E. coli), but there is no requirement for a drinking water service provider to maintain a specified minimum disinfectant residual in the reticulation network.

While ideally the disinfectant residual in your incoming water should be approximately 0.5mg/L, this is not a parameter that is within the control of a healthcare facility so this cannot be a control measure and cannot have critical limits.

In order to suppress growth of Legionella bacteria in your water distribution system it is desirable for there to be a free chlorine/monochloramine residual of 0.2mg/L at the most distal outlet in your water distribution system.

The odour threshold for chlorine (the concentration at which most people can smell it) is approximately 0.6mg/L and some people may complain if chlorine is above this concentration. When determining your disinfection residual critical limits, consider that the maximum drinking water guideline value for chlorine is 5 mg/L.

Turbidity

Turbidity is the cloudiness of water caused by large numbers of individual particles, which individually are too small to be seen by the naked eye.

Turbidity is important both for aesthetic and health reasons as the more turbid the water is, the less effective most forms of disinfection will be.

Turbidity is measured with an instrument called a nephelometer, which detects the amount of scattering of light shone into the water. The units of turbidity are called Nephelometric Turbidity Units (NTU).

Clear water has a turbidity of less than 1 NTU, whereas a muddy river may have turbidity greater than 1000 NTU. Silt, microorganisms, plant materials and chemicals contribute to the turbidity of the water.

If some form of disinfection is used, it will be desirable to establish an upper critical limit for turbidity.

When determining your turbidity critical limit, consider:

  • At 1 NTU, turbidity can shield bacteria from disinfection, especially with ultraviolet disinfection
  • As turbidity increases to 5 NTU, this begins to become noticeable in a glass of water and may be unpleasant to drink. Taste and odour may also be affected.
  • At higher levels, the effects of turbidity depend on the cause (inorganic materials or suspended organic matter), but can include abrasion and physical wear on plumbing, staining of sinks and fixtures, discolouration of laundered fabrics and ineffective disinfection.
Last updated: 27 January 2017