Q&A: Ask the Report Chair

Dr. Trussell, the chair of the committee that authored this report, answered your questions about the report and its findings from January 10 to January 24, 2012. The question and answer period is now closed, but you can find the answers to all the questions submitted and answered here.

How much could water reuse projects augment the nation’s water supply?

Of the 32 billion gallons of municipal wastewater discharged nationwide each day, approximately 12 billion gallons are discharged to an ocean or estuary—an amount equivalent to six percent of total water use in the United States. Reusing this water would directly augment the nation’s total water supply.  When reclaimed water is used for non-consumptive applications, the water supply benefit of water reuse could be even greater if the water can again be captured and reused.  Inland effluent discharges may also be available for water reuse, although extensive reuse has the potential to affect the water supply of downstream users and ecosystems in water-limited settings.

What is the extent of water reuse in the United States?

Overall, data on water reuse projects is sparse, but the report’s authoring committee was able to provide some estimates on the extent of water reuse.

In the 2004 Environmental Protection Agency Guidelines for Water Reuse, water reuse facilities in the United States generated an estimated 1690 million gallons per day, and it was estimated that water reuse was growing at a rate of 15 percent per year.  As of 2002, the Environmental Protection Agency estimated that Florida reused the largest quantities of reclaimed water, followed by California, Texas, and Arizona. At that time, these four states accounted for the majority of the nation’s water reuse, although the Environmental Protection Agency reported that at least 27 states had water reuse facilities in 2004, with growing programs in Nevada, Colorado, Washington, Virginia, and Georgia.  According to the most recent data (from 2009-2010), Florida and California reuse 659 and 646 million gallons per day of reclaimed water respectively.  Three of the four states with the largest water reuse capacity are located in the arid southwest where population growth and climate variability have created recent supply challenges. Water reuse in these states has become commonplace as a means to expand the water supply portfolio and provide an additional drought-resistant supply.

Potable water reuse projects account for a small fraction of the volume of water currently being reused.  In contrast, de facto reuse (sometimes called “unplanned” potable reuse) is common.  De facto reuse occurs when drinking water supplies contain a significant fraction of wastewater effluent, typically from upstream wastewater discharges, even though the water supply is not formally recognized or permitted as a water reuse project.  The last assessment of the extent of de facto reuse occurred over 30 years ago, so limited information is available on its current extent.  The report recommends an updated analysis of the extent of de facto reuse in the United States.

What are the health risks associated with drinking reclaimed water?

The very nature of water reuse suggests that any substance used or excreted by humans has the potential to be present at some concentration in the treated product water. However, there is now a portfolio of modern treatment options available to mitigate water quality issues. Additionally, the occurrence of a contaminant at a detectable level does not necessarily pose a significant risk.  Instead, only by using dose-response assessments, can a determination be made of the significance of a detectable concentration.

In this report, the committee compared the estimated risks of a conventional drinking water source that contains a small percentage of treated wastewater (i.e., de facto potable reuse) against the estimated risks of two different potable reuse scenarios. The analysis suggests that the risk of exposure to 24 selected chemical contaminants in the two planned potable reuse scenarios does not exceed the risk encountered from many existing water supplies. There is a great degree of uncertainty about pathogen contaminants, but the risk from potable reuse does not appear to be any higher than currently experienced in at least some current drinking water treatment systems, and may be much lower.

This helps demonstrate that state-of-the-art water reclamation systems can provide a comparable level of protection from contaminants to that experienced in many drinking water supplies today, assuming that quality assurance strategies ensure the reliability of the treatment processes.  Nevertheless, it should be emphasized that the committee presents these calculations as an exemplar.  This should not be used to endorse certain treatment schemes or determine the risk at any particular site without site-specific analyses.

The report brief contains a chart illustrating one part of our analysis, and the full report contains an entire chapter on this topic, supplemented by an appendix detailing the methods and assumptions of the analysis.

Updated: 1/17/12

What should be the quality of treated wastewater used for irrigation, and for watering lawns and golfcourses (I’m referring to the maximum permissible values of parameters, like SAR, TDS, TSS, BOD, Boron, heavy metals, etc)?

The report generally discuses water quality parameters that affect different water uses (mainly in chapter 2, click here to download the report). Additionally, Chapter 10 of the report provides many tables excerpted from the EPA Guidelines for Water Reuse and various state regulations for water quality for a number of reuse applications.  Currently, there are no national regulations that set maximum permissible values for water quality parameters for nonpotable reuse, and regulations vary by state. The committee did not make recommendations with regard to maximum values of water quality parameters.

What precautions should be taken, while using treated wastewater?

State-of-the-art water reclamation systems can provide a comparable level of protection from contaminants to that experienced in many drinking water supplies today, assuming that quality assurance strategies ensure the reliability of the treatment processes. However, the potential for failures in the system should be assessed and mitigated. Redundancy and quality reliability assessments, including process control, water quality monitoring, and the capability to divert water that does not meet predetermined standards, are essential components of reuse systems.

Treated wastewater intended for nonpotable uses, such as landscape irrigation, is treated to different standards than water intended for drinking, and therefore, the potential for unintended uses should be assessed. For example, the inadvertent cross-connection of potable and nonpotable water lines could allow people to drink water intended only for irrigation, presenting human health risks from exposure to pathogens. For high-level nonpotable uses of reclaimed water where human contact or inadvertent ingestion of the water may occur, such as the irrigation of parks and playgrounds, water managers should take the precaution of disinfecting nonpotable reclaimed water to reduce microbial pathogens to low or undetectable levels. Rigorous installation procedures for nonpotable plumbing systems, including tests for cross connections and use of backflow prevention devices, combined with periodic cross-connections inspections thereafter can substantially reduce potential health risks.

Does the report consider water reuse from shale gas well sites? If so, what are the conclusions? How much can be recovered? Can it be recycled safely for drinking water, agriculture use, and other industries?

Water Reuse: Potential for Expanding the Nation’s Water Supply through Reuse of Municipal Wastewater considers only the reuse of municipal wastewater, and so I can’t comment on water reuse from shale gas well sites. However, other reports from the National Academies’ Division of Earth and Life Studies may be of interest to you, for example Management and Effects of Coalbed Methane Produced Water in the United States (a brief version of the report is also available).