Health-Based Targets (HBT) for drinking water supply represents a quantitative method of assessing the safety of drinking water supply systems and are currently in consideration for inclusion into the Australian Drinking Water Guidelines. HBTs are community outcome focused, quantifying the pathogen hazard of drinking water supplies to the community by considering not just the rate of infection, but the severity and duration of illness.
The quantitative approach taken by the HBT process enables water providers to better understand the risk profile of their supply scheme. This ensures not only that capital works and operating processes are optimised for the delivery of safe water, but also that the areas of greatest opportunity are understood.
The HBT process is underpinned by four major activities:
Understanding Source Water Risk
Understanding risks to the source water starts with 'Sanitary Survey' of the catchment:
what human and agricultural activities are present in the wider and inner catchment areas?
is urban storm water or treated waste water discharged into the catchment?
what are the risks from on-site sewer systems in the catchment?
what catchment barriers are in place, including riparian vegetation, fencing and access to the water body?
This process isn't limited to surface waters. Ground water schemes will assess:
the nature of the strata, recharge areas and groundwater influence
characteristics of the bore, including water table depth and pump depth, sealing and fencing
The Sanitary Survey allows a 'Vulnerability Assessment' to be performed, which classifies the source water in accordance with the risks identified. The Vulnerability Assessment is verified with a 'Microbial Indicator Assessment', where the long-term susceptibility of the water body to E. coli is determined.
If your scheme doesn't monitor raw water for E. coli, the time to start is now! The HBT process requires two years of E. coli sampling data to ensure the validity of the Vulnerability Assessment. High risk catchments require a more involved Quantitative Microbial Risk Assessment to better characterise pathogens in the source water.
Understanding Treatment Processes
The 'Water Treatment Assessment' process requires assessment of the performance of each process element in the treatment train to determine the level of protection, or Log Reduction Value (LRV), that each process contributes to the treatment plant.
Detailed records of operational monitoring points and critical control points are crucial to this task. Each process should have appropriate online, continuous monitoring and records of instrument calibration.
It is important to assess the monitoring capabilities and record keeping processes of your plant early to ensure that sufficient data is available for analysis. In some cases, the plant will need to procure, install, commission and then begin recording of data - this can be a lengthy process!
Developing a Water Safety Assessment and Water Safety Improvement Plan
An understanding of the gaps in treatment is obtained by performing a ‘Water Safety Assessment’. From this assessment a ‘Water Safety Improvement Plan’ can be developed.
Shortfalls in LRV achieved by the plant may be due to underperforming plant or poorly selected control limits. Before succumbing to the urge to recommend a new barrier (such as a UV system or second stage filtration), it is worth assessing if optimisation of the plant can achieve a similar outcome:
is the coagulation/flocculation process optimised? Could optimisation of this process improve post-filter turbidity and hence the LRV that can be claimed?
were credits lost due to turbidity spikes during filter ripening? Could changes in the backwash or filter-to-waste process lower turbidity during ripening, and improve the LRV that can be claimed?
are chemical dosing systems poorly controlled, causing low-chlorine or high pH events?
were credits lost due to short-circuiting or cross-connections in the plant?
It is a common mistake during a gap analysis to focus on capital plant assets. Many schemes can benefit from operational improvements, including improved catchment management, operational monitoring and record keeping or operating procedures.
Implementing Health-Based Targets for your scheme may seem like a daunting challenge, but the key is better understanding your system using relevant information and good data. Start early! When getting prepared to implement HBT for your scheme, start by:
ensuring catchment monitoring is taking place to understand risks posed by human and agricultural activities
ensuring Microbial Indicator Assessment data is being recorded weekly (E. coli sampling of raw water)
performing an early Vulnerability Assessment to determine if a Quantitative Microbial Risk Assessment is required
establishing that your plant has sufficient instrumentation, and that calibration records are kept
An important aspect to remember is that HBTs support your drinking water quality risk management plan/system, it is not an addition to it.
For guidance through the process of implementing Health-Based targets, don't hesitate to call Viridis for assistance.