Science Inventory

Risk-based enteric pathogen reduction targets for non-potable and direct potable use of roof runoff, stormwater, and greywater


Schoen, M., N. Ashbolt, M. Jahne, AND J. Garland. Risk-based enteric pathogen reduction targets for non-potable and direct potable use of roof runoff, stormwater, and greywater. Microbial Risk Analysis. Elsevier B.V., Amsterdam, Netherlands, 5:32-43, (2017).


Communities face a challenge when implementing the use of alternative waters for non-potable and potable purposes given the lack of national microbial standards in the United States. Possible alternative waters include, but are not limited to: • Greywater: wastewater from bathtubs, showers, bathroom sinks, and clothes washing machines, excluding toilet and—in most cases—dishwasher and kitchen sink wastewaters; • Roof runoff (rainwater): precipitation collected from roof surfaces or other above ground collection surfaces not impacted by human activity; and • Stormwater: precipitation collected from ground level. In the absence of national guidance, states and communities have adopted standards based on fecal indictor concentrations in finished water (e.g., the NSF/ANSI Standard 350 for non-potable onsite reuse of greywater). However, the microbial standards for alternative waters based on finished water quality provide an unknown level of protection of human health for consumers. Pathogen treatment targets, referred to as pathogen log10 reduction targets (LRTs), are the difference between the log10-transformed pathogen concentrations pre-treatment and post-treatment. Pathogen reduction targets that are “risk-based” are calculated to achieve a specific level of health protection for consumers. To support the development of microbial LRTs for the management of alternative waters, we computed risk-based pathogen reduction targets for both non-potable and potable uses of alternative source waters.


This paper presents risk-based enteric pathogen log reduction targets for non-potable and potable uses of a variety of alternative source waters (i.e., locally-collected greywater, roof runoff, and stormwater). A probabilistic Quantitative Microbial Risk Assessment (QMRA) was used to derive the pathogen log10 reduction targets (LRTs) that corresponded with an infection risk of either 10−4 per person per year (ppy) or 10−2 ppy. The QMRA accounted for variation in pathogen concentration and sporadic pathogen occurrence (when data were available) in source waters for reference pathogens in the genera Rotavirus, Mastadenovirus (human adenoviruses), Norovirus, Campylobacter, Salmonella, Giardia and Cryptosporidium. Non-potable uses included indoor use (for toilet flushing and clothes washing) with occasional accidental ingestion of treated non-potable water (or cross-connection with potable water), and unrestricted irrigation for outdoor use. Various exposure scenarios captured the uncertainty from key inputs, i.e., the pathogen concentration in source water; the volume of water ingested; and for the indoor use, the frequency of and the fraction of the population exposed to accidental ingestion. Both potable and non-potable uses required pathogen treatment for the selected waters and the LRT was generally greater for potable use than non-potable indoor use and unrestricted irrigation. The difference in treatment requirements among source waters was driven by the microbial quality of the water – both the density and occurrence of reference pathogens. Greywater from collection systems with 1000 people had the highest LRTs; however, those for greywater collected from a smaller population (∼ 5 people), which have less frequent pathogen occurrences, were lower. Stormwater had highly variable microbial quality, which resulted in a range of possible treatment requirements. The microbial quality of roof runoff, and thus the resulting LRTs, remains uncertain due to lack of relevant pathogen data.

Record Details:

Product Published Date:04/03/2017
Record Last Revised:05/09/2017
OMB Category:Other
Record ID: 336253