10.2.1 Short-term evaluation of operational monitoring

Operational monitoring is carried out throughout the water system, including source water and catchment, treatment processes, and the distribution system. All operational monitoring results should be promptly reviewed against any established criteria, objectives and previous results to assess whether the system is operating under normal conditions or whether there is an increase in the level of challenge, or the preventive measures and barriers are not performing effectively.

When target criteria and/or critical limits (for critical control points) have not been met, operational staff need to remain aware that the water supply system may not be functioning effectively, and assess the immediate or future risk of supplying unacceptable and possibly unsafe water to consumers.

Box 10.1 sets out priorities for attention when operational characteristics are found to deviate from established operational criteria.

Box 10.1

Priorities for attention where operational criteria are not met

The response to deviations from any established operational criteria is risk-based, and will vary depending on the operational characteristic in question and the impact of the deviation on the provision of safe drinking water. As a guide, however, priorities for attention include:

1. deviations that could have a significant impact on critical control point performance; for example:

   • clarifier turbidity deviating from operational target criteria, affecting downstream filtration and resulting in inadequate filter performance;

   • colour of water through a UV plant exceeding the design specification, resulting in ineffective disinfection;

2. deviations that could have a direct impact on the microbial safety of the water; for example, reduction in chlorine residual in the distribution system, especially for water supply systems where Naegleria is considered a high risk, or where there is evidence of recontamination or breakdown of distribution system integrity;

3. deviations that could contribute to consumers receiving drinking water containing a chemical characteristic above a health-based guideline value;

4. deviations that could contribute to consumers receiving aesthetically poor drinking water;

5. deviations that could have a direct impact on not meeting the microbial health-based targets.

Critical control points (CCPs)

Of all operational monitoring, monitoring at CCPs is the most critical for assuring drinking water safety. Monitoring at CCPs should occur frequently, preferably continuously, using online analysers, and these analysers should be alarmed at both the target criteria and the critical limits, so that operational staff are alerted promptly of adverse results and effective operational control can be maintained.

Where alarm systems exist, a protocol for alarm response should also be established which considers the length of time an alarm condition can exist before a field response is initiated (to avoid callouts based on background “noise”) and the actions to be taken if a field response is initiated. This may be influenced by factors such as the quality of the source water (low risk / high risk), existence of other barriers to contamination, and at which specific barrier or process the alarm occurs.

Target criteria breach

Any breach of target criteria should be regarded as a warning or indication of a change in system status and possibly the start of a trend towards loss of control of the process, which may ultimately result in a breach of a critical limit. Investigation and appropriate corrective actions to resolve any potential problems should immediately be undertaken to ensure a critical limit is not breached.

Possible corrective actions for deviations from target criteria at CCPs include:

• inspection of the water supply system for faults;

• manual backwashing of filters;

• alteration of plant flow rate to reduce loading;

• use of an alternative raw water source;

• increasing disinfectant dose;

• adjusting process control;

• inspection and calibration of monitoring equipment;

• engagement of backup equipment;

• increased monitoring and observation.

Box 10.2 provides an example of the short-term evaluation of filtration performance and the corrective action that should be taken when the target criterion for turbidity has not been met.

Box 10.2

Short-term evaluation of filtration performance

For the filtration critical control point example detailed in Box 9.5, the target criterion for turbidity at each filter was set at <0.2 NTU and a critical limit for turbidity was set at 0.5 NTU.

If the turbidity from an individual filter has exceeded 0.2 NTU continuously for longer than the pre-determined delay period, an alarm should alert the operator that the target criterion has been breached and target filtration performance is not being achieved. The operator should promptly assess the filtration process and investigate the cause of the alarm. If the exceedance is during normal operation, immediate corrective actions should be implemented to achieve target performance. This may include:

• visual inspection of the filter to identify abnormalities;

• reviewing turbidity trends for all individual filters;

• confirming that upstream processes (e.g. coagulation) are operating normally;

• assessing raw water quality for unusual loadings;

• checking filter flow rates;

• manual backwashing of the filter; and

• reducing the hydraulic load on the filter.

If the exceedance of the target criteria is the result of a backwash event, the operator should keep the filter performance under close surveillance to confirm that plant operation returns to normal as expected, and ensure that the critical limit of 0.5 NTU is not breached. If an alarm indicates that the critical limit is exceeded, this should result in the filter being immediately taken off line until operation is satisfactorily back within specification.

After corrective action has been taken, its effectiveness needs to be verified. This usually entails additional monitoring. Secondary impacts of the corrective action, and the need for adjustments or additional action further along in the supply system, should also be considered.

Exceedance of, or deviation from, a target criterion at a critical control point would not generally require notification of the relevant health authority or drinking water regulator, provided the corrective action successfully prevents a breach of a critical limit.

Critical limit breach

Breaching of a critical limit indicates that control of a process has been lost, probably resulting in an unacceptable health risk. The relevant health authority or drinking water regulator should be notified without delay, corrective action should be taken immediately to resume control and normal operation of the process, and implementation of an emergency response plan should be considered. The emergency plan may include:

• plant shutdown;

• immediate collection and review of all relevant results (e.g. if filtered water turbidity exceeds limits, this should include source water quality operation of downstream disinfection plants);

• water diversion and/or reliance on an alternative supply;

• reduction in flow and the holding of unsafe water in pipelines for disposal;

• additional treatment elsewhere in the system (e.g. secondary disinfection, spot dose, booster disinfection);

• mains flushing, cleaning and localised disinfection;

• increased sampling and monitoring of relevant operational and drinking water quality characteristics downstream throughout the distribution system;

• implementation of a boil-water advisory in consultation with the relevant health authority or drinking water regulator, if microbial contamination is suspected.

Critical operational processes with online, continuous monitoring of performance can be equipped with alarm systems set at critical limits which, when breached, trigger an automatic immediate shutdown of the treatment plant. This mitigates the risk of producing water with an unacceptable level of associated health risk (e.g. supply of undisinfected water) to consumers. Where possible, the water transfer system may also be shut down or diverted, to ensure that unsafe water is not supplied to consumers.

When any critical limit is breached, rapid response and investigation are essential to ensure that consumers’ health is protected and supply is maintained. It may also be necessary to issue a public advisory, depending on available knowledge of the situation, the rapidity and effectiveness of the actions taken in response to the breach, and whether drinking water of unacceptable quality has been or will be supplied to consumers. This decision will be made in consultation with the relevant health authority or drinking water regulator.

When the system is back under control, the root cause of the barrier breach should be investigated and improvements made, based on the outcome of the investigation (see Section 3.6.2).

Other operational monitoring – catchment to consumer

In addition to evaluating data at critical control points, results from other operational monitoring activities throughout the system should also be promptly reviewed against established target criteria and objectives, and previous results, to assess whether:

• the system is operating under expected normal conditions or there is an increase in the level of challenge;

• the preventive measures and barriers are performing effectively; and/or

• the monitoring results indicate a trend in performance that may be associated with:

  • poor maintenance;

  • insufficient backwashing;

  • clogging of filters;

  • increased chlorine demand; or

  • poor calibration of monitoring equipment.

Results from observational monitoring activities would also be assessed. Any reports of barrier breaches, such as damage to tank roofs, backflow or cross-connections, are significant and require immediate attention. Other observations of concern, such as increased human activity in a catchment, floc blanket in poor condition, “boiling” in filter beds when backwashing, reduction in chlorinator maintenance, or failure to meet targets for testing of backflow prevention devices, while they may not have an immediate impact on water quality, should nevertheless be addressed promptly to bring performance back to established requirements and target criteria.

The potential impact of poor performance or failure of an upstream barrier on the performance or integrity of downstream barriers should also be assessed.