By Nathaniel Homan.
Reusing water is not a new concept to many Californians. Many municipalities across California have facilities that treat wastewater to high standards, which allows it to be reused for agricultural irrigation, landscape irrigation, and industrial use. Other municipalities, such as the Orange County Water District, treat wastewater even further using advanced technologies, and use the treated wastewater to supplement drinking water supplies by injecting it into underground aquifers. In this manner, they practice indirect potable reuse, or IPR.
However, there is a third method of reusing wastewater that is not currently practiced in California: direct potable reuse, or DPR. DPR is an emerging water supply option which can provide a significant amount of drought resistant, high quality water in arid regions such as California.
In direct potable reuse, advanced treated wastewater is directly introduced into the drinking water system either upstream or downstream of a drinking water treatment plant (see Figure 1). The distinction between indirect and direct potable reuse is that IPR systems include an environmental buffer between wastewater treatment and potable reuse, whereas DPR systems do not.
Environmental buffers are uncontrolled hydraulic systems such as a groundwater aquifers, rivers, lakes, and artificial reservoirs. Historically, environmental buffers were thought to provide additional treatment of recycled wastewater through natural environmental processes. While the concept of additional treatment may have been true in the past, as treatment technologies have matured, the quality of water produced by advanced wastewater treatment has improved. Now, treated wastewater is often of higher quality than the water in the environmental storage buffer. Rather than providing additional treatment, environmental storage buffers can actually degrade the quality of advanced treated wastewater.
Environmental buffers also increase the time between treatment of wastewater and the intake of the treated wastewater to the drinking water system. Use of a buffer allows wastewater treatment plant operators time to respond to monitoring results and prevent water that does not meet treatment standards from entering the potable water system. Some DPR projects include an engineered storage buffer to provide additional residence time for the treated wastewater.
In many situations DPR is a less costly and more efficient reuse scheme than IPR. Many communities may not have access to a large surface reservoir to use as an environmental buffer, or if there is one, the treated wastewater may have to be pumped long distances to reach the reservoir. In IPR systems that use a groundwater aquifer as an environmental buffer, the treated wastewater must be injected underground and later pumped back out. This two-stage process consumes energy and requires the construction of injection wells. For many communities, DPR is a more feasible and cost effective water management strategy.
One such community is Windhoek, Namibia, where DPR has been practiced for over 40 years. Windhoek is located in one of the most arid regions of the world, and it relies on the New Goreangab Water Reclamation Plant to treat wastewater and provide nearly a quarter of its 15 million gallon per day demand for water. More recently, several municipalities in the United States have implemented DPR. The city of Big Springs, Texas, Wichita Falls, Texas, and Cloudcroft, New Mexico, have all implemented DPR projects, while other cities, such as El Paso, Texas, have DPR projects planned in their future. So far, all of the DPR projects in the U.S blend the advanced treated water with raw water before passing the blended water through a conventional drinking water treatment plant. The New Goreangab plant is the only DPR project in the world which introduces the advanced treated water directly into the potable distribution system.
There are many benefits to implementing DPR in California and other arid regions:
- DPR can increase the amount of available water in California by reusing wastewater that would otherwise be discharged to the ocean. The amount of additional water recoverable in this manner is estimated at 1.2 million acre feet per year – more than the entire storage capacity of Folsom Lake.
- DPR can increase water supply reliability, as wastewater is not as subject to seasonal and annual variations as other water sources.
- DPR can increase the quality of drinking water. Because the effluent produced by DPR is of such high quality, if blended with the traditional source water ahead of a drinking water treatment plant, it can improve the quality of drinking water distributed to users.
- DPR can reduce energy consumption by providing a local source of water for municipalities. A large portion of the cost of water in arid regions comes from the energy required to transport it long distances. While the treatment technologies used for DPR are energy intensive, in areas such as Southern California, the energy required to produce water with DPR is less than that required to transport water from the State Water Project to users (Figure 2).
Many who are opposed to the concept of DPR label it as “toilet to tap,” or “drinking wastewater.” There are several reasons why the perception of DPR as “drinking wastewater” is misleading. First, the treatment technologies used to treat water for IPR and DPR projects produces an effluent which is typically of higher quality than that of the drinking water source (whether groundwater or surface water) for a given municipality. Second, many large municipal areas that would benefit most from DPR projects, such as Los Angeles County, have source water which has already been used by upstream users several times before it reaches the intake to their drinking water treatment plant. In essence, Los Angeles residents are unknowingly practicing “toilet to tap,” but without the careful engineering and safety measures that a real DPR project would incorporate.
While DPR is not yet legal in California, authorities in California have begun investigating DPR as a legitimate water reuse strategy. The California Water Code section 13563 mandates that the State Water Resources Control Board report on the feasibility of developing criteria for DPR by the end of 2016. To accomplish this goal, the SWRCB established an expert advisory committee to hold hearings and gather data in 2014. Progress of the committee can be found here. While legalization of DPR in California is still a ways off, many speculate that it is inevitable. The acceptance of DPR in Texas and New Mexico, California’s search for new water sources in the current drought, and California’s push towards sustainable technologies are all factors which indicate that DPR will have a future in California.
Nathaniel Homan is earning his Masters degree in Environmental Engineering at UC Davis. He is working with Dr. Peter Green and Dr. Thomas Young to minimize waste from strong base anion exchange systems used to remove hexavalent chromium from drinking water.