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  • James Howey

Setting Critical Limits

Setting the right critical limit is possibly the most important factor in managing water quality risk and ensuring the safe supply of water, whether it's drinking water or recycled water. When the critical limits are right, there is confidence that action will be taken to prevent the supply of potentially unsafe water.

Monitoring critical control points is the monitoring of operational performance of the critical treatment components in a water supply scheme and forms the basis of the operational monitoring program. It is continuous (or highly frequent, such as daily) and gives a real time indication of how well a treatment process is working. It will confirm that the system is working effectively and will detect an issue immediately, or within a timeframe that will allow for a preventive measure to be applied before non-compliant or unsafe water is supplied.

Examples of operational monitoring are:

  • turbidity monitored post filtration

  • confirming disinfection C.t. by monitoring chlorine residual, flow, pH and temperature

  • conductivity testing after reverse osmosis (RO)

  • transmembrane pressure in pressure filters.

The critical limit, is the point at which a process may not meet the required level of hazard removal. If operation is outside the critical limit, then the safety of the water produced can no longer be guaranteed. Critical limits usually include a concentration and timeframe for when action should be triggered. Once a critical limit is reached, action must be taken to prevent the supply of potentially unsafe water.

The ADWG says this about critical limits:

Critical limits are performance criteria that separate acceptability from unacceptability in terms of hazard control and water safety. They should be chosen carefully and should not be confused with target criteria. Critical limits may incorporate a numerical value as well as a consideration of time (e.g. failure to provide a minimum chlorine residual for a specified time).

Deviation from critical limits indicates loss of control of the process or activity and should be regarded as representing a potentially unacceptable health risk.

The water industry is continuously improving practice, and recently, relatively new frameworks have been introduced to improve water safety, including the Health Based Treatment Targets (HBT) framework for drinking water and WaterVal, the national framework for validating treatment technologies. Neither framework is fully implemented, but setting the right critical limits is the key principle of these frameworks.

The framework for identifying HBTs has always been a major component of water safety, through the application of Quantitative Microbial Risk Assessment (QMRA) techniques, especially in recycled water (Australian Guidelines for Water Recycling). The HBT framework, developed through WSAA, includes the identification of the pathogens reductions (log reduction values (LRVs)) required to make the water safe for the use, whether it be drinking water or a recycled water reuse. Included in this framework is testing the treatment infrastructure to prove that it can achieve the LRVs and to define the 'operational envelope' that will achieve the LRVs. This operational envelope is where the critical limits should be set. This will ensure that action is taken when we can no longer be certain that the system is achieving the LRVs.

The WaterVal framework, includes protocols for validating a range of treatment technologies and defining the operation envelope, providing options for different levels of validation. All options include defining the operational envelope. In developing the protocols, a significant amount of research went into correlating operational monitoring and pathogen log reduction values achieved by the treatment technology. Using the WaterVal methods, the operational range can be determined and the critical limits set, to trigger action when operation varies outside that range.

The Victorian Department of Health and Human Services has also developed a guideline for validating recycled water schemes, that provides detailed information on validation of operational performance.

The US EPA has also prepared detailed guidance on validating treatment technologies and the operational range.

For drinking water, the Australian Drinking Water Guidelines (ADWG) provide guidance on acceptable limits for filter performance and chlorine contact time for drinking water, however these were established prior to the development of the HBT Framework, which is now considered the standard.

Overview of the steps to set the critical limits

(this is not exhaustive - every scheme is different)

  1. Identify HBTs for treatment using HBT Manual for drinking water, AGWR for recycled water or undertaking a QMRA. Investigate chemical treatment requirements to achieve the appropriate final water quality.

  2. Review validation requirements and develop a plan

  3. Undertake testing to identify and validate the operational envelope

  4. Set critical limits, including parameter and timeframe

  5. Establish critical control points e.g. monitoring programs, corrective actions, reporting protocols, record keeping

  6. Review and observe trends in performance

  7. Revalidate in response to changes


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