By Brooke K. Mayer, PhD, PE

Over the past several decades, water reuse has expanded dramat­ically, driven by three main pressures: 1) addressing urbanization and water supply scarcity, 2) achieving efficient resource use (i.e., efficiently managing the water-energy-nutrient nexus), and 3) environmental and public health protection.[1] Given the confluence of these factors, reuse is expected to continue to increase from current levels of approximately 7% reuse[2] to upwards of 37% reuse in the US.[1,3]

Highlighting the increasing urgency of water reuse, in 2021 the US Bureau of Reclamation declared the first-ever official water shortage on the Colorado River, which supplies water to 35 million people.[1] This triggered mandatory cuts in water delivery for areas of the Southwest, with approximately 18% reduction of Arizona’s annual apportionment, 7% of Nevada’s, and 5% of Mexico’s.[4] The associated drought contingency plans include water source diversification (e.g., water reuse), conservation efforts, underground water storage, and reductions in agriculture irrigation.[4] Accordingly, the need for improved understanding of water reuse strategies and performance metrics is increasingly critical.

There are no federal regulations for water reuse in the US. Instead, states maintain primacy in developing water resources, and some states have programs addressing water reuse.[1] As of 2012, 30 states and one US territory had adopted regulations and 15 states had established guidelines or design standards governing water reuse (Figure 1).[1] In states or nations without established standards, EPA’s 2012 Guidelines for Water Reuse1 can assist in developing integrated water management planning and programs.

As indicated in Figure 1, reclaimed water can be used for different purposes, ranging from groundwater recharge to agricultural irrigation to potable reuse. Potable reuse can be further classified as shown in Figure 2. This “fit for purpose” framework facilitates design and operation of cost-effective treatment meeting the quality proscribed by its intended use.[1] Accordingly, “a portfolio of treatment options, including engineered and managed natural treatment processes, exists to mitigate microbial and chemical contaminants in reclaimed water, facilitating a multitude of process combinations that can be tailored to meet specific water quality objectives.”[3] This is broadly illustrated in Figure 3, where water reuse may be appropriate for varying end users after secondary, tertiary, or more advanced wastewater treatment processes.

Regardless of the reclaimed water’s end use, pathogen mitigation is the most critical treatment objective.[1] Waterborne pathogens can include helminths, protozoa, bacteria, and viruses. As living microorganisms, the risks these pathogens pose to human health differ fundamentally from chemical toxins. Microorganisms can occur in high numbers in feces and may be difficult to treat.[7] In particular, enteric viruses (more than 200 of which may occur in wastewater, representing the greatest diversity of species relative to other pathogens)[8,9] generally occur in much greater concentra­tions and exhibit elevated infectivity compared to other pathogens. Thus, they are the most likely pathogens to be spread through water reuse, thereby posing the greatest risk.[7,8]

In water reuse, as in all forms of water treatment, the objective is to reduce human health risks to very low levels, most often by reducing the probability of yearly infections to less than 1 in 10,000 or less than 1 in 1,000,000, as completely eliminating risk is untenable. Figure 3 illustrates the risks associated with several important waterborne pathogens after different stages of de facto reuse or DPR, where risks decrease as more advanced treatment is implemented. Using quantitative microbial risk assessment (QMRA), Chaudhry et al. (2017)[6] found that the yearly 1 in 10,000 (10-4, or -4 reported on a log scale) risk of infection threshold was exceeded for norovirus and Cryptosporidium in the de facto reuse scenario (even with a small degree of wastewater impact on the source water), whereas the four different DPR configurations tested reduced risk to within acceptable levels.

Estimates of risks after treatment rely on accurate quantification of viral loads in the wastewater initially. A review of virus levels in untreated wastewater showed concentrations ranging from 158,000 to 9.8 billion per liter (assessed using molecular techniques).[9] This wide variability reflects differences due to types of virus, protocols used, testing location, and timing with respect to the number of infected individuals shedding viruses at any given time.

Unfortunately, it is impractical to monitor for every individual virus in wastewater; thus, indicator microorganisms are commonly used as signs of the presence of human enteric viruses (similarly, surrogate microorganisms are commonly used in process challenge testing as models of viral pathogen treatability). Among others, pepper mild mottle virus, crAssphage, and human polyomavirus have been suggested as indicator viruses; however, little is known about their removal through wastewater treatment processes.[11] Greater understanding of indicator fate during treatment, particu­larly compared to pathogenic viruses, is needed as susceptibility to treatment processes varies by type of virus, water quality, operational parameters, etc. For example, adenovirus is more resistant to UV light compared to other viruses, while reovirus is more resistant to chlorination.[8]

Based on best available data at the time, the treatment targets for IPR in California and DPR in Texas currently specify 12-log (99.9999999999%) reduction of enteric viruses.[1,10] When using reclaimed water to irrigate edible crops, a 6-log reduction of viruses has been suggested.[9] As of 2012, epidemiological studies of reuse did not identify any patterns relating adverse health effects with water reuse projects in the US.[3] However, given that concentrations of viruses in wastewater may exceed estimates used for these risk-based water reuse guidelines, Gerba et al. (2017)[9] suggested that 2 – 3 logs of additional reduction beyond current recommendations may be prudent to ensure safe use of reclaimed water.

As water reuse is increasingly implemented, quantitative reliabil­ity analyses are essential, and should incorporate not only variability in virus loading, but also expected variability of effluent quality, mechanical reliability, and the consequences of mechanical failure.[12] Attention should also be given to scaling water reuse to effectively serve a range of applications. For example, Fane et al. (2002)[7] demonstrated a diseconomy of scale wherein the risks of water­borne infections, particularly related to waterborne enteric viruses, increased with increasing population served by water reuse systems. This finding suggested that decentralized urban water reuse, which has historically received less attention compared to centralized systems, may play an important role in water reuse portfolios in the future.[7] Regardless of the installation’s scale, the success of potable reuse in any configuration ultimately depends on reliable pathogen removal.[6]


  1. USEPA. 2012 Guidelines for Water Reuse.; 2012.
  2. Rauch-Williams T, Marshall M, Davis D. Baseline data to estab­lish the current amount of resource recovery from WRRFs. In: Water Environment Federation. ; 2018:Available at: https:// www.wef. org/globalassets/as.
  3. National Research Council. Water Reuse: Potential for Expand­ing the Nation’s Water Supply through Reuse of Municipal Wastewater. National Academies Press; 2012. doi:10.17226/13303
  4. USBR. Reclamation announces 2022 operating conditions for Lake Powell and Lake Mead.
  5. EPA. Maps of States with Water Reuse Regulations or Guidelines. Published 2022. Accessed April 25, 2022.
  6. Chaudhry RM, Hamilton KA, Haas CN, Nelson KL. Drivers of microbial risk for direct potable reuse and de facto reuse treatment schemes: The impacts of source water quality and blending. Int J Environ Res Public Health. 2017;14(6):1-20. doi:10.3390/ijerph14060635
  7. Fane SA, Ashbolt NJ, White SB. Decentralised urban water reuse: The implications of system scale for cost and pathogen risk. Water Sci Technol. 2002;46(6-7):281-288. doi:10.2166/wst.2002.0690
  8. Gerba CP, Betancourt WQ, Kitajima M, Rock CM. Reducing un­certainty in estimating virus reduction by advanced water treatment processes. Water Res. 2018;133:282-288. doi:10.1016/j.watres.2018.01.044
  9. Gerba CP, Betancourt WQ, Kitajima M. How much reduction of virus is needed for recycled water: A continuous changing need for assessment? Water Res. 2017;108:25-31. doi:10.1016/j.watres.2016.11.020
  10. California State Water Resources Control Board. California Title 22. Published online 2018:1-99.
  11. Ahmed W, Kitajima M, Tandukar S, Haramoto E. Recycled water safety: Current status of traditional and emerging viral indicators. Curr Opin Environ Sci Heal. 2020;16:62-72. doi:10.1016/j.coesh.2020.02.009
  12. Eisenberg D, Soller J, Sakaji R, Olivieri A. A methodology to evaluate water and wastewater treatment plant reliability. Water Sci Technol. 2001;43(10):91-99. doi:10.2166/wst.2001.0589

About the author
Dr. Brooke K. Mayer is an Associate Professor in the Department of Civil, Construction and Environmental Engineering as part of the Opus College of Engineering at Marquette University. She holds Bachelors, Masters and Doctorate degrees in civil engineering with an emphasis in environmental engineering from Arizona State University. She is a registered Professional Engineer in the state of Arizona.



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