Quantitative Microbial Risk Assessment to Evaluate Failure Assumptions and Excess Treatment Requirements for Direct Potable Reuse

As communities look for new ways to strengthen their water supplies, direct potable reuse (DPR) is becoming an increasingly important option. DPR uses advanced treatment to purify recycled water before introducing it directly into a drinking water supply, eliminating the need for an environmental buffer such as a reservoir or aquifer.

Without that buffer, utilities may have less time to detect and respond if a treatment process isn’t performing as expected. That raises an important question: How much additional treatment should DPR systems provide to protect public health in the event of a potential treatment failure?

A recent peer-reviewed study published in Water Research, “Quantitative Microbial Risk Assessment to Evaluate Failure Assumptions and Excess Treatment Requirements for Direct Potable Reuse,” examines that question. Co-authored by Carollo’s Amos Branch and Andrew Salveson, among others, the research explores how different assumptions about treatment failures affect pathogen-reduction requirements for DPR systems.

Using Quantitative Microbial Risk Assessment for Direct Potable Reuse

Quantitative microbial risk assessment (QMRA) estimates potential health risks from exposure to disease-causing microorganisms. For potable reuse, it can help determine how much pathogen reduction a treatment system needs to provide to meet established public health targets.

These requirements are typically expressed as log reduction values (LRVs). DPR facilities use multiple treatment barriers to achieve their required LRVs, but additional treatment may also be incorporated to provide protection if one of those barriers temporarily underperforms.

The study examines how much of this additional treatment may be appropriate if added as redundancy to provide protection against different hypothetical failure scenarios.

How Treatment Failures Affect DPR Requirements

Treatment failures can vary considerably. A process might experience a small performance decline for a few minutes, while a more significant disruption could last much longer.

The researchers evaluated three key variables: how severe a failure is, how long it lasts, and how frequently it occurs.

The analysis found a clear relationship: more conservative failure assumptions led to higher pathogen-reduction requirements. Assuming failures will be larger, longer, or more frequent means a DPR system would need additional treatment to continue meeting health-based risk targets.

Because higher treatment requirements can influence facility design, complexity, cost, and project feasibility, the authors emphasize that selecting failure assumptions is an important risk-management decision rather than simply a modeling exercise.

Supporting Risk-Informed Direct Potable Reuse Planning

The researchers also developed a simplified model that approximates the amount of additional treatment needed under different failure scenarios. The simplified model provides regulators, designers and utilities with a practical screening tool without requiring a more computationally intensive analysis for every scenario.

Ultimately, the research demonstrates the importance of linking DPR treatment requirements to transparent, realistic assumptions about system performance. As DPR continues to expand, approaches like QMRA can help utilities and regulators develop treatment strategies that support robust public health protection while accounting for real-world operating conditions.

Read the full study in Water Research to explore the QMRA methodology, treatment failure scenarios, and findings in greater detail.