Risk-Based Microbial Treatment Targets​ for Water Reuse

Risk-based microbial treatment targets determine the appropriate level of treatment needed for fit-for-purpose water reuse to achieve a certain level of protection by considering the specific source of water and its desired end use. 

EPA released the Risk-Based Framework for Developing Microbial Treatment Targets for Water Reuse as a scientific resource for states, Tribes, and territories adopting water reuse. By summarizing the state of the science, this document presents microbial treatment targets for a range of scenarios, as well as information for entities to develop their own risk assessments. 

The report supports states and other entities on their path to ensuring safe, fit-for-purpose recycled water. Common water reuse scenarios were selected for the source waters and applications. The report summarizes reference pathogen densities in source waters, exposure estimates for common uses, dose-response models for selected reference pathogens, and established risk benchmarks to inform the calculation of microbial treatment targets using quantitative microbial risk assessment (QMRA).

Learn more by watching a 2025 webinar from EPA's Water Research Series!

On this page

• How Microbial Treatment Targets Are Calculated
• Microbial Treatment Target Tables
• Example Use Cases

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How Microbial Treatment Targets Are Calculated

1. Determine the reuse application

Define the end use and source water. End uses assessed in the report are potable use, unrestricted access landscape irrigation, and indoor non-potable use. Sources of water assessed are untreated municipal wastewater, untreated onsite wastewater, graywater, stormwater, and roof runoff. Other sources and end uses can be modeled using the provided framework if their associated input data is available elsewhere.

2. Select risk benchmarks

Determine a target level of risk for the treatment to achieve. Two benchmarks, both of which are considered protective of public health, are included in the report: infections and disability adjusted life years (DALYs). These have been previously used for related applications but risk assessors can also select different values when applying the framework.

The infection-based benchmark seeks to limit the number of infections so low that they can be considered negligible. A level of 1 in 10,000 infections per person per year was used during development of the Surface Water Treatment Rule, but it is not a regulatory requirement.

The DALY-based benchmark considers the degree of human health impairment resulting from infections. Different health outcomes are quantified based on impact to quality and quantity of life, with one DALY equivalent to one healthy year of life lost. The World Health Organization has defined 1 in 1,000,000 DALYs per person per year as a tolerable burden of disease for waterborne pathogens from drinking water. 

3. Identify modeling inputs

QMRA follows a four-step process: hazard identification, exposure assessment, dose-response, and risk characterization. The report provides best available assumptions for these parameters. If running the model with different selections, consider the following:

• Which pathogens best represent the most concerning risk for this water or exposure route?
• What is the pathogen density in the source water?
• How would people come into contact with the recycled water? What is the exposure volume?
• How likely is infection after exposure? What dose-response relationship should be used?

To simulate real-world variability and uncertainty, these models are run repeatedly using a Monte Carlo simulation. The report uses a threshold that the desired benchmark is achieved 95% of the time.

4. Determine treatment requirements

Log reduction targets (LRTs) are calculated based on the above inputs to identify the level of treatment required to make recycled water safe for its intended use. Targets are calculated on a logarithmic scale (e.g., LRT of 1 = 90% removal; LRT of 5 = 99.999% removal). 

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Microbial Treatment Target Tables

The microbial treatment target tables below are reproduced from the report. Each table represents one of three common end-use: potable reuse, landscape irrigation, and non-potable indoor use. The first column identifies the source of water: untreated municipal wastewater, untreated onsite wastewater, graywater, stormwater, and roof runoff. The microbial targets selected for removal represent classes of virus (e.g., norovirus), protozoa (e.g., Cryptosporidium, Giardia), and bacteria (e.g., Campylobacter, Salmonella). A subset of these may be chosen for a given application. Each microbial target includes treatment targets using infection-based or DALY-based health benchmarks. Values listed in table represent LRTs calculated using best available assumptions provided in the report.

Potable Reuse Log Reduction Targets

Source of Water	Norovirus		Cryptosporidium		Giardia		Campylobacter		Salmonella
	Infection	DALY	Infection	DALY	Infection	DALY	Infection	DALY	Infection	DALY
Untreated municipal wastewater	14.5	12.5	10.5	10.0	9.5	8.5	11.0	7.5	9.5	9.5
Untreated onsite wastewater	14.5	12.5	11.5	11.0	10.0	9.0	12.0	9.5	8.0	8.0
Graywater	13.0	11.0	9.0	8.5	8.0	7.0	9.5	7.5	5.5	5.5
Stormwater	13.5	11.5	9.5	9.0	8.5	7.5	10.0	6.5	8.5	8.5
Roof Runoff	n/a1	n/a1	NSD2	NSD2	5.5	4.5	9.0	6.5	8.0	8.0

Landscape Irrigation Log Reduction Targets

Source of Water	Norovirus		Cryptosporidium		Giardia		Campylobacter		Salmonella
	Infection	DALY	Infection	DALY	Infection	DALY	Infection	DALY	Infection	DALY
Untreated municipal wastewater	10.0	8.5	6.5	6.0	5.5	4.5	6.5	4.0	5.5	5.5
Untreated onsite wastewater	10.5	8.5	7.0	6.5	6.0	5.0	7.5	5.5	3.5	3.5
Graywater	8.5	6.5	4.5	4.0	3.5	2.5	5.5	3.0	1.5	1.5
Stormwater	9.0	7.5	5.5	5.0	4.5	3.5	5.5	3.0	4.5	4.5
Roof Runoff	n/a1	n/a1	NSD2	NSD2	1.5	0.5	5.0	2.5	3.5	3.5

Indoor Non-Potable Use Log Reduction Targets

Source of Water	Norovirus		Cryptosporidium		Giardia		Campylobacter		Salmonella
	Infection	DALY	Infection	DALY	Infection	DALY	Infection	DALY	Infection	DALY
Untreated municipal wastewater	10.5	9.0	7.5	7.0	6.5	5.5	7.5	5.5	6.5	6.5
Untreated onsite wastewater	11.5	10.0	7.0	6.5	6.5	5.5	7.5	5.5	4.0	4.0
Graywater	9.0	7.5	4.5	4.0	4.0	3.0	5.5	3.5	2.0	1.5
Stormwater	9.5	8.0	6.5	6.0	5.5	4.5	6.5	5.0	5.5	5.5
Roof Runoff	n/a1	n/a1	NSD2	NSD2	2.0	1.0	5.0	3.0	3.5	3.5

 ¹ n/a = pathogen not known to be present in source;   ² NSD = not sufficient data to compute target

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Example Use Cases

National Blue Ribbon Commission Recommended Pathogen Reduction Targets for Onsite Water Systems

The National Blue Ribbon Commission (NRBC) advances best management practices to support the use of onsite non-potable water systems within individual buildings or at the local scale. The 2025 report, Recommended Pathogen Reduction Targets for Onsite Water Systems, outlines the current state of the science on risk-based treatment targets for onsite water systems and provides guidance to determine the level of treatment, oversight and management for commercial, multi-family and mixed-use building for non-potable end uses. The NBRC recommended treatment targets reflect the most current understanding of reference pathogens and dose-response models.