Direct Potable Reuse Vs. Indirect: Weighing The Pros And Cons
By Laura Martin
Dwindling water supply is an issue in many areas of the world. Water use has been growing significantly in recent years, with water withdrawal predicted to rise by 50 percent in developing countries and 18 percent in developed countries by 2025, according to the United Nations Environment Programme.
This increase in water use makes water scarcity a serious issue for many communities, especially those that frequently experience periods of drought and other environmental challenges. For those communities water reuse is sometimes the only practical solution.
“Treated wastewater is increasingly being seen as a resource rather than simply waste,” reports the EPA in their 2012 Guidelines for Water Reuse.
There are two potable water reuse options currently gaining prevalence: direct potable reuse (DPR) and indirect potable reuse (IPR).While all water is eventually reused in some sense in a conventional water treatment system, DPR and IPR both involve a proactive decision to transform treated wastewater into drinking water.
For IPR, this is done by releasing treated wastewater into groundwater or surface water sources with the intent of using it for drinking water supplies, and then reclaiming it and treating it to meet drinking water standards. For DPR, purified water, created from treated wastewater, is introduced directly into a municipal water supply system without an environmental “buffer” of any kind.
Experts are still weighing in on which, if either, is the solution to water scarcity. Water Online shares pros and cons of each.
Advantages Of DPR
Cost savings: Drinking water treatment and wastewater treatment typically occur in the same or nearby locations in DPR systems, requiring a short pumping distance for product delivery. The close proximity of both waste and drinking water treatment may present considerable cost savings for municipalities when compared to IPR methods, according to “Drinking Water Through Recycling”, a report by the Australian Academy of Technological Science and Engineering (ATSE). The study compared several hypothetical options for alternative water supplies for a coastal Australian city.
The study determined that a hypothetical DPR system would cost $616 million in total calculated indicative capital costs, compared to $1,287 million for an IPR system. Operating costs for the DPR system were estimated at $53 million per year, compared to $72 million per year for the IPR system.
Reduced carbon footprint: A smaller bottom line isn’t the only advantage to the shorter pipelines used in DRP systems. The energy required to pump water over short distances results in less greenhouse gas emissions than the amount needed to pump water over long distances, according to the ATSE study.
Water security: The short distance water has to travel in DPR systems also leaves little opportunity for outside factors to affect it. According to the EPA Guidelines for Water Reuse, DPR systems are less vulnerable to damage from earthquakes, floods, and other natural and human-made disasters. Long-distance water transmission systems, such as those used in IPR, are more vulnerable to damage, leading to additional costs and maintenance.
Disadvantages Of DPR
Additional setup: Some DPR systems may require additional water quality or process performance monitoring and/or an engineered storage buffer, according to the ATSE report. These items may increase costs and would require additional time, planning, and labor to implement. The purposes of a storage buffer include balancing variability between water production and water use, balancing water quality variability, and providing time to detect and respond to any contaminants in the water, according to the ATSE report
Safety concerns: There are only four municipal DPR projects currently operating, located in the Namibia, South Africa, and the United States (Texas and New Mexico), according to the ATSE report. Because DPR has not been widely implemented, there is a lack of consensus in the scientific community about its safety. There are concerns about the effectiveness of DPR systems to fully remove pharmaceuticals, personal care products, and endocrine distributing chemicals from the water. The ATSE also reports that there is a lack of faith in government agencies’ capacity to safely operate and monitor DPR projects.
Public perception: The “yuck factor” associated with DPR is one of the biggest reasons it has not been more widely implemented, reports the ATSE. Because much of the public is misinformed or has negative feelings regarding DPR, utilizing such systems may require municipalities to employ dedicated public relations managers. Phase 2 of the City of San Diego Water Purification Demonstration Project, which began in 2005, used nearly half of the project’s funding for education and outreach purposes — to make the public feel more comfortable with DPR.
Advantages Of IPR
Environmental Purification: In IPR, treated wastewater is discharged into an environmental system such as a river, lake, or aquifer. Some wastewater industry stakeholders believe that the time treated water remains in this environment system allows any remaining contaminants to be degraded by physical or biological processes, according to the ATSE report. Dilution of the water in the environment also may minimize any potential risk by decreasing the concentration of contaminants that may be present, according to the article “Indirect Potable Reuse: A Sustainable Water Supply Alternative,” which appeared in the International Journal of Environmental Research and Public Health (IJERPH).
Well-established method: IPR has been successfully implemented in the United States, Europe Singapore, and other areas, according to the IJERPH article. California has the most IPR systems, which are also being utilized in Arizona, Colorado, Texas, Florida, and Virginia. No adverse health impacts have been reported as a result of IPR systems in these communities, according to the article.
Positive public perception: Currently, more consumers are comfortable with IPR than DPR, according to the ATSE report. In a survey of Australians with known interests in wastewater management, drinking water management, and/or water reuse, the ATSE found that many of the respondents believed the incorporation of some form of environmental buffer to be a necessary factor for achieving public confidence and support.
Disadvantages Of IPR
More expensive: IPR systems typically have higher capital and operational costs, according to the ATSE report. The construction and maintenance of transfer systems to move the water from the treatment facilities to a suitable environmental buffer can also be costly. Many IPR systems rely on extensive enclosed pipelines and associated pump stations to deliver water from the water treatment facility to the environmental buffer. Additional costs come from pumping water long distances, especially to higher elevations.
Process inefficiencies: IPR systems essentially treat the same water twice, which some experts feel is a waste of time and resources. During the wastewater treatment process, many IPR schemes use reverse osmosis and advanced oxidation to thoroughly remove all contaminants from the water, according to the ATSE report. The highly treated water is then returned to the environmental buffer, where it mixes with source water. In some cases, this raises total organic carbon and total dissolved solids in the treated water, which can impact the costs and requirements of retreating the water during the drinking water treatment process, according to the ATSE report.
Still not 100% backed by public: Although IPR is more accepted than DPR, it is still controversial. According to the EPA Guidelines for Water Reuse, the primary opposition to IPR is related to the perceived health risks and the perception of the quality of the natural water source being degraded by the addition of reclaimed water.
What other pros and cons are there for IPR or DPR? Is either method the solution to water scarcity? Please share your thoughts in the Comments section below.
For more on potable reuse: