Direct potable reuse – a sustainable, safe and cost-efficient water supply solution

Historically, the preference for implementing PRW has been to employ indirect reuse via an environmental buffer, typically a reservoir, lake or aquifer. This buffer provides mixing of the PRW with water in the environment prior to abstraction for treatment and potable use. This dilution, detention time and the occurrence of natural processes are viewed as positives in providing additional mitigation of residual risk with PRW. Delivering PRW to these natural sources often requires significant additional infrastructure in the form of transfer pipelines and pumpstations to reach distant and elevated reservoirs or injection wells, or spreading basins to incorporate into the groundwater.

Direct potable reuse (DPR) is the process of supplying highly treated reclaimed water directly to a drinking water distribution system. However, Australia’s intensely unstable climate, technological advancements and the urgent need for sustainable water supply systems have prompted renewed discussion into the feasibility of DPR.

There are two approaches to DPR:

• The direct use of purified recycled water from an advanced water recycling/treatment facility (AWTF), utilising this water as a feed source for a drinking water treatment plant (WTP) (raw water augmentation), and;
• Supplying water directly into the potable water network (treated water augmentation, also sometimes referred to as “flange to flange”).

There are several potential benefits from directly integrating purified recycled water with the drinking water system, including:

• Reduced conveyance and transfer infrastructure costs. By supplying PRW directly to the WTP or via the main water distribution system, we remove the requirement and associated capital expenditure required to lay transfer pipelines to move the recycled water to an environmental buffer, such as a river, lake, reservoir or aquifer.
• Co-locating the source and demand. In theory, this integration into the water supply system can provide overall economic benefit by avoiding the need for some water supply assets, including drinking water transfer from distant sources and even water treatment assets where treated water augmentation is applied.
• Reduced risk of contamination arising from an environmental buffer. DPR removes the risk of an environmental buffer contaminating the extensively treated, high drinking quality purified recycled water. Such ‘contaminants’ could include algae, pathogens, turbidity in reservoirs and metals and salinity in aquifers.

Variability of the source wastewater and the (relatively) uncontrolled catchment present significant process challenges. Sudden changes in quality or chemical contamination of the raw wastewater can disrupt treatment processes, especially biological processes. From an operational standpoint, this makes achieving reliability exceedingly difficult. Without an environmental buffer or other management process, this presents a substantial risk.

In the absence of environmental buffers, the associated benefits of natural treatment, attenuation, and the delaying impact from out-of-spec water are unavailable. Hence, any such scheme must achieve extremely prominent levels of process reliability and operational robustness.

This requires increased investment by the sector in technologies that enable accurate and rapid real-time monitoring of feed quality, process conditions and treated water quality. Such technologies are crucial in building both confidence in direct potable use and providing operators with the ability to maintain process performance over time.

In response to these and other challenges associated with direct potable reuse we are seeing developments in the regulatory landscape intended to give communities greater confidence in direct DPR schemes. A prime example is proposed Californian legislation that includes requirements for additional pathogen log reduction capability and specific requirements for process risk management and operational skills to address this risk.

As we look to build water resilience globally, our conversations must ensure all options are being brought to the table and that all voices are included and heard on the journey. Direct potable use will play an increasing role, particularly in circumstances where no suitable environmental buffer exists or there are economic constraints. More importantly, the consideration of DPR (as well as IPR schemes) will become extra pronounced as Wastewater and Water Treatment Specialists and industry look towards more circular solutions that deliver long-term sustainability and make for resilient, productive and inspired communities.

At GHD, we use practical collaboration and the art of building meaningful, trusting partnerships with our clients. This enables us to manage and navigate through complicated and multifaced decision-making processes. By taking a human-centric approach and leaning into technological advancements, we can collectively realise the full potential of such critical water augmentation strategies and deliver sustainable, safe water supply solutions to communities facing water scarcity around the globe.

Catch up on the first perspective in this series Purified Recycled Water (PRW) – Helping to build thriving, water-resistant communities.

To learn more about how GHD can assist you in considering and developing PRW for your system, please reach out to our Wastewater and Water Treatment specialists in your region: