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Civil Engineering Magazine THE MAGAZINE OF THE AMERICAN SOCIETY OF CIVIL ENGINEERS

By JAY LANDERS

Water reuse has long been a staple of water providers in arid regions of the United States, especially for nonpotable uses. But facing demands to ensure greater resiliency and sustainability of future supplies, many providers are turning to reuse for potable applications, both indirect and direct. Amid increasingly successful efforts to address public perception questions regarding water reuse, the practice is beginning to take hold even in areas with more abundant water resources.

For decades, water reuse has been conducted successfully in drought-prone areas of the United States. Until fairly recently, those efforts targeted mainly nonpotable uses, primarily the irrigation of agricultural lands, golf courses, and other landscapes. With population growth continuing, water supplies growing scarcer, and treatment technologies becoming more sophisticated, water reuse continues to grow in popularity. Although still relegated largely to populated areas in such water-challenged states as California, Arizona, Texas, and Florida, water reuse is gaining ground in other areas. At the same time, the focus of water reuse increasingly is shifting to potable applications, making the practice an even more critical element in the unrelenting search to secure resilient, sustainable drinking water supplies.

In the United States today, the outlook for water reuse is "pretty rosy," says Melanie Holmer, P.E., the national water reuse leader for Brown and Caldwell, which has its headquarters in Walnut Creek, California. With the growing recognition of the finite nature of water supplies, "a lot of utilities are looking at their effluent and saying, 'This is worth something. We should reuse it,'" Holmer says.

Traditionally, a major impediment to water reuse programs has been "obtaining public support," says Larry Schimmoller, P.E., the global technology leader for water reuse at Jacobs, which has its headquarters in Dallas. "Not engaging and educating the public can really lead to negative attitudes about water reuse," Schimmoller says. "That has begun to change significantly in the last five years." Fortunately, he notes, the reuse industry has earned greater public acceptance of the practice by learning the lessons of past mistakes and beginning to work more proactively to educate the public about the water cycle and the benefits to be gained from water reuse.

For example, not long ago, the idea of brewing beer with recycled water and making it available to the public likely would have seemed far-fetched, even to the most ardent champions of water reuse. But today that scenario is a common one in the Portland, Oregon, metropolitan area, thanks to the efforts of Clean Water Services, of Washington County, Oregon. In 2015, the utility first provided to local brewers high-purity water that had undergone ultrafiltration, reverse osmosis (RO), advanced oxidation, and ultraviolet (UV) disinfection. "We had to get a change to our reuse permit to allow us to do this," says Mark Jockers, the government and public affairs director for Clean Water Services.

The resulting event, the Pure Water Brew Competition, can only be described as a massive public relations success. "It was like wildfire," Jockers says. "It just went completely viral on us." Of the approximately 400 media stories that ensued, most were "quite positive" and focused on the quality of the water and the advanced technology used to achieve it, he notes. Inspired by the success of Clean Water Services, water utilities and their partners in Wisconsin, Florida, Arizona, and Colorado have held similar competitions to promote water reuse (see "Brewers Make Beer Using Recycled Water to Promote Potable Reuse in Arizona, Colorado," Civil Engineering , December 2017, pages 32-34).

In early 2019, Clean Water Services commissioned its Pure Water Wagon, a mobile advanced treatment system that has a capacity of 5 gal./min. Developed with the support of Jacobs and Black & Veatch, which has its headquarters in Overland Park, Kansas, the Pure Water Wagon is intended to be taken to public events to showcase the technology that makes water reuse possible. "If people can see the technology and understand what the processes do, they're more likely to be accepting," Jockers says.

As populations continue to increase in arid areas and with climate change expected to bring hotter, drier conditions, potable reuse, in particular, is "going to become a normal part of the water cycle in cities, particularly in the West," says Troy Walker, the water reuse practice leader for Hazen and Sawyer, which has its headquarters in New York City.

Although nonpotable reuse will remain a common practice in many areas, Walker says that he expects to see a shift away from the historical trend of constructing and operating separate nonpotable distribution networks, commonly known as "purple pipe systems" for the pipe color used to indicate nonpotable water. "From a management standpoint, potable water reuse ultimately is going to be simpler," Walker says. "You don't have another distribution system of piped water. You don't have the risks of cross contamination from poor plumbing practices. You don't have the risk of building a nonpotable system that was relying on some industrial customers that never came to fruition."

Funding limitations could also help drive the trend away from nonpotable to potable reuse, says Jennifer Duffy, P.E., M.ASCE, the national water reuse practice lead for HDR, of Omaha, Nebraska. In the short term, the costs associated with installing separate treatment and distribution infrastructure for nonpotable reuse "can be very expensive," Duffy says, though such systems obviously can confer long-term benefits. By comparison, "potable reuse is really a more efficient way of reusing the water," even though it costs more to treat, she says. "Not having to distribute the water separately is a big offset."

At the state level, the regulatory approach to reuse varies significantly. For example, a handful of states have developed or are developing regulatory frameworks to address direct potable reuse (DPR), Holmer says. Although such efforts take time, these regulatory developments help foster reuse by alleviating the uncertainty that can prevent water utilities from acting. "If you don't know what the requirements are going to be, it's really hard to plan for that and justify that expense," Holmer explains. "Think about just trying to estimate the capital cost for something and not knowing what processes are going to be required."

Although it does not regulate water reuse, the U.S. Environmental Protection Agency (EPA) wants to foster greater implementation of the practice. To this end, the EPA in September released a draft National Water Reuse Action Plan, which "incorporates federal, state, tribal and local water perspectives and highlights key actions that support consideration and implementation of water reuse," the agency stated in its September 10 news release announcing the draft plan. "[The] EPA's goal is to issue a final plan that will include clear commitments and milestones for actions that will further water reuse to bolster the sustainability, security, and resilience of the nation's water resources," according to the news release.

Among its nearly 50 proposed actions, the draft action plan calls for a compilation of state policies and approaches to implementing water reuse programs, the development of materials to facilitate water reuse in National Pollutant Discharge Elimination System permits authorized under the Clean Water Act, the incorporation of water reuse considerations in projects developed by the U.S. Army Corps of Engineers as part of its Civil Works program, and examples of how water reuse strategies can support efforts to abate sewer overflows. Other proposed actions include compiling funding sources, developing and maintaining a searchable inventory of research, developing a coordinated national research strategy, and establishing goals for the extent and types of water reuse in the United States.

ASCE backs the draft action plan. "At first glance, we're very supportive of it," says Emily Feenstra, the Society's managing director of government relations and infrastructure initiatives. The draft plan reflects "a lot of our policy priorities," Feenstra says, including promoting technology delivery, improving water data, and coordinating research. "The spirit of this plan is really about integrated water management and recognizing that water is an essential resource and in very short supply, especially in the West," she says.

The EPA set a 90-day deadline for submitting comments, and ASCE is planning to respond. "We are identifying areas where ASCE can provide comments regarding various issues that relate to current and future needs of the profession from education, training, and practice perspectives," said Berrin Tansel, Ph.D., P.E., D.WRE, F.EWRI, F.ASCE, a professor of civil and environmental engineering at Florida International University. Tansel, who provided written responses to questions from Civil Engineering, is one of the Society members preparing comments on the draft action plan.

Duffy highlights the draft action plan's commitment to compile various sources of reuse information and make them publicly available. The "EPA's role in becoming a central resource for technical and operational information is critical to the advancement of the reuse industry and promotion of safe and cost-effective water resource solutions," Duffy says.

Schimmoller agrees. Having a "central repository of water quality information and treatment that's required to meet certain reuse applications" would be "very helpful to the industry," he says.

Using recycled water to replenish groundwater supplies is becoming an increasingly common practice, perhaps nowhere more so than in Southern California. And no other agency has done as much to promote this form of indirect potable reuse as the Orange County Water District (OCWD), of Fountain Valley, California. Since 2008, the district has operated the world's largest advanced purification system for potable reuse, the Groundwater Replenishment System (GWRS).

With its three-step process involving microfiltration, RO, and UV disinfection with hydrogen peroxide, the advanced facility purifies effluent provided by an adjacent wastewater treatment plant (WWTP) operated by the Orange County Sanitation District (OCSD), which is a partner of the OCWD on the GWRS. A portion of the highly purified water is injected into the ground to prevent seawater intrusion, while the remainder is sent to recharge basins to replenish the Orange County Groundwater Basin.

In 2015, the treatment capacity of the GWRS was expanded to 100 mgd. Today, the OCWD is moving forward with the final expansion of the GWRS, increasing the renowned facility's treatment capacity to 130 mgd. In September, the district awarded the contract to construct the expansion to Shimmick Construction, a subsidiary of AECOM, of Los Angeles. The project was designed by Black & Veatch.

To be completed by early 2024, the $310-million project will enable the GWRS to produce 134,000 acre-ft/yr, or enough for 1 million people. To achieve this goal, the final expansion of the facility requires supplementing the project with flows from the OCSD's other WWTP, which is on the coast about 5 mi away. To convey those treated flows, a 40 mgd pump station will be built and a pipeline between the wastewater plant and the GWRS will be rehabilitated.

The treatment process will remain the same for the final expansion of the GWRS, says Michael Markus, P.E., D.WRE, F.ASCE, the general manager of the OCWD. Upon completion of the project, the GWRS will accept 170 million of the 185 million gal. of treated effluent generated daily by the OCSD's two wastewater plants, Markus says. Approximately 23 mgd of brine left over from the GWRS process will be returned to the OCSD, which will discharge the concentrate by means of its ocean outfall.

Nineteen water agencies rely significantly on water from the Orange County Groundwater Basin to meet the needs of their customers. "The Groundwater Replenishment System has allowed us to keep the basin relatively full," Markus says. Currently, local water agencies are able to meet 77 percent of their needs from groundwater extracted from the basin, with the remainder taking the form of more expensive water imported either from Northern California or the Colorado River.

In addition to providing heavily populated Orange County with a reliable source of water, the OCWD has demonstrated the feasibility of using advanced purification processes to cleanse treated wastewater for reuse. "Hopefully, we've proven to the world that the system works," Markus says. "We've never had a permit violation."

Among those following in the footsteps of the OCWD is the Water Replenishment District (WRD) of Southern California, the entity responsible for managing and protecting groundwater within a 420 sq mi region of southern Los Angeles County. The WRD's service area comprises 43 cities, including part of the city of Los Angeles, that are home to 4 million people. Approximately half of residents' annual water supply, about 250,000 acre-ft, comes from local groundwater sources overseen by the WRD.

In August, the WRD held the grand opening of its 14.8 mgd Albert Robles Center (ARC) for Water Recycling & Environmental Learning. Located in the city of Pico Rivera, the facility uses ultrafiltration and RO membranes followed by disinfection with advanced oxidation to purify tertiary treated water from the San Jose Creek Water Reclamation Plant, which is operated by the Sanitation Districts of Los Angeles County. Completed at a cost of approximately $130 million, the ARC was designed and built by a team led by J.F. Shea Co. Inc., of Walnut, California, and Tetra Tech, of Pasadena, California.

All treated effluent from the ARC will be used to replenish the Central Basin, one of two groundwater basins within the WRD's service area. The purified water is sent to the existing San Gabriel River spreading grounds, where it infiltrates into the underlying basin, said Robb Whitaker, P.E., the WRD's general manager, who provided written responses to questions from Civil Engineering . The groundwater eventually is extracted by retail water agencies that provide it to their customers.

The ARC is a central component of the WRD's Water Independence Now program, an effort to end the district's use of imported water. Until recently, nearly 20 percent of the water used by the WRD to replenish groundwater supplies was imported. "With the completion of the ARC facility, [the] WRD will no longer be dependent on the California State Water Project or the Colorado River for replenishing the basins," Whitaker said.

The WRD's prospects for obtaining additional water to recharge its groundwater basins appear bright. In February, the district entered into an agreement with the Los Angeles Department of Water and Power to evaluate the potential to accept some of the city's recycled water. In particular, the two entities will analyze the possibility of the WRD replenishing recycled water from Los Angeles's massive Hyperion Water Reclamation Plant. In February, the city announced that it intended to reuse all effluent from the facility by 2035 (see "To Improve Resiliency, Los Angeles Aims to Recycle All Wastewater by 2035," Civil Engineering , April 2019, pages 24-26).

If a rallying cry can be said to exist among Southern California water providers, it would have to be water independence. Much like the OCWD and the WRD, the Eastern Municipal Water District (EMWD) aims to develop an advanced purification facility to cleanse treated wastewater for the purposes of groundwater replenishment and subsequent extraction and reuse. The largest water agency in Riverside County, the EMWD has a long history of providing recycled water, mainly for agricultural purposes. With its service area, located about 90 mi east of Los Angeles, expected to undergo a population boom in the coming decades, the district is positioning itself to take full advantage of the expected increase in wastewater flows to its water reclamation facilities.

"Recycled water has been a key component of our local water resources," says Joe Mouawad, P.E., the EMWD's assistant general manager of planning, engineering, and construction. The district, which also conducts desalination of brackish groundwater, has decreased its dependence on imported water by 30 percent since the 1980s. But it wants to do more. "If we can increase usage of recycled water, we can reduce our dependence further" on supplies from Northern California and the Colorado River, Mouawad notes.

By 2023, the EMWD aims to complete the construction of an advanced purification process facility adjacent to its existing San Jacinto Valley Regional Water Reclamation Facility, Mouawad says. Consisting of RO technology and advanced oxidation, the facility will accept tertiary treated water from the San Jacinto Valley facility, purify it further, and blend it with other tertiary treated water. The blended flows then will be conveyed to recharge basins located several miles away, adjacent to the San Jacinto River.

For the initial phase, the EMWD expects to recharge roughly 4,000 acre-ft/yr, Mouawad says. After spending at least six months belowground, the water will be extracted, disinfected, and delivered to a potable drinking water distribution system. CDM Smith, of Boston, is conducting groundwater modeling and ensuring regulatory compliance for the project.

In addition to the purification facilities, the estimated $70-million project will require the construction of a pump station and pipelines to convey treated flows to the recharge basins, extraction wells to remove the groundwater, and a process for managing the brine generated by the RO process. A significant portion of the project cost can be attributed to brine management, Mouawad says, because ocean disposal is not an option.

So the district is evaluating the use of brine evaporation ponds and plans to pilot-test a type of continuous recirculation RO technology that is designed to achieve higher water recoveries, thereby decreasing the volume of brine to be disposed of. If successful, the technology could enable the EMWD to reduce the footprint of the ponds. Moreover, the technology "could have industry-wide benefits for recycled water advanced treatment application," Mouawad says.

Further south, the city of San Diego is developing its indirect potable reuse program, but its will send purified effluent to existing raw water reservoirs rather than groundwater basins. Known as Pure Water San Diego, the program entails the phased design and construction of three advanced wastewater purification facilities. Ultimately, the effort, for which Stantec, of Edmonton, Alberta, Canada, is serving as program consultant, is intended to enable the city to obtain one-third of its water supply locally by 2035. Currently, San Diego imports roughly 85 percent of its water, and the city recycles only 8 percent of its municipal wastewater. The remainder is treated and discharged to the Pacific Ocean.

To be completed by 2023 at an estimated cost of $1.4 billion, phase 1 consists of the 30 mgd North City Pure Water Facility (PWF), which will accept recycled water from the city's existing North City Water Reclamation Plant (WRP). Designed by Carollo Engineers, of Walnut Creek, California, the PWF will use a five-step treatment process consisting of ozonation, biological activated carbon, membrane filtration, RO, and UV disinfection with advanced oxidation. A 30 mgd pump station will convey the treated water 8.4 mi, via a pipeline designed by HDR, to the city's Miramar Reservoir, where it will be blended with other raw water sources. The final 0.9 mi long segment of the pipeline, to be installed along the bottom of the reservoir itself, will disperse the treated water throughout the reservoir. After undergoing final treatment in the city's Miramar Water Treatment Plant, the finished water will enter the city's drinking water distribution system. (See the illustration above.)

The treatment train for the North City PWF was selected on the basis of pilot-testing conducted by the city at its 1 mgd Pure Water Demonstration Facility, which began operating in 2011. The state of California's Division of Drinking Water (DDW) requires the use of membrane filtration, RO, and UV and advanced oxidation for indirect potable reuse. "The facility also includes ozone and biologically active carbon filters as additional treatment steps required by the DDW to offset a reduced retention time in the receiving reservoir that serves as a drinking water source for the Miramar Water Treatment Plant," said Amy Dorman, P.E., the deputy director for Pure Water operations for the city of San Diego's Public Utilities Department, who provided written responses to questions from Civil Engineering .

To provide the additional wastewater for the PWF, San Diego will construct a new pump station to capture up to 32 mgd from four sanitary sewers and convey those flows to the existing North City WRP. The capacity of the reclamation plant will be increased from 30 mgd to 52 mgd, and a new pump station will be added to send up to 42 mgd to the nearby North City PWF. Two 10.7 mi long, 48 in. diameter pipelines will be constructed parallel to each other. One will convey the wastewater from the pump station to the WRP, while the other will send brine removed during the treatment process at the new PWF to San Diego's Point Loma WWTP. AECOM designed the pump station and pipelines, while Jacobs designed the expansion of the North City WRP.

Another benefit of the Pure Water San Diego program is that it will enable the city to avoid having to upgrade the 240 mgd Point Loma facility to secondary treatment, an effort that would cost an estimated $1.8 billion. Rather unique among U.S. WWTPs, Point Loma is a chemically enhanced primary treatment facility that discharges to the ocean through a 4.5 mi long, 310 ft deep outfall. "One advantage of the Pure Water program is that it has allowed the city to maintain the current treatment facilities at Point Loma because a large volume of flow will be diverted for treatment and reused to supplement the drinking water supply," Dorman said.

Construction of phase 1 began in June, Dorman said. Phase 2 will include the construction of the Central Area PWF, which will send purified water to Lake Murray and the San Vicente Reservoir. If needed, phase 3 would include the construction of the South Bay PWF, which would convey its treated flows to the Lower Otay Reservoir.

Perhaps no other project better exemplifies the trend toward water reuse in traditionally water-rich areas than the Sustainable Water Initiative for Tomorrow (SWIFT) program of the Hampton Roads Sanitation District (HRSD), in Virginia Beach, Virginia. The HRSD provides wastewater treatment services for 1.7 million people in 18 counties and cities within eastern Virginia. Although home to the Elizabeth, James, and York Rivers and situated at the southern end of the Chesapeake Bay, the region served by the HRSD relies mainly on surface water from reservoirs west of the region for its drinking water supplies. Outside of the Hampton Roads area, however, groundwater is the primary source of drinking water and industrial process water for all of eastern Virginia. And over time, withdrawals have exceeded the rate at which the aquifer is recharged, leading to groundwater depletion, land subsidence, and increased potential for seawater intrusion.

Seeking to combat these trends, the HRSD began a pilot test in 2016 of the advanced treatment processes that it plans to use for its SWIFT program, which will purify treated wastewater from several of the HRSD's existing WWTPs and return the cleansed water to the Potomac Aquifer (see "Virginia Utility Aims to Eliminate Most Discharges to Surface Waters," Civil Engineering , November 2016, pages 20-22.) Conducted at the district's York River Treatment Plant, the pilot test evaluated and compared the performance of membrane- and carbon-based methods of advanced water purification processes.

Based on the pilot-test results, the HRSD opted to construct a 1 mgd demonstration facility that uses the carbon-based approach, says Ted Henifin, P.E., the district's general manager. The process consists of traditional flocculation and sedimentation, ozonation, biologically active filtration, granular activated carbon filtration, and UV disinfection (see the illustration below). Chemicals then are added to adjust the pH of the disinfected water to ensure that it matches the chemistry of the existing groundwater in the Potomac Aquifer followed by additional disinfection by means of chlorine.

Opened in May 2018, the $25-million demonstration facility was designed and built by a team led by Crowder Constructors Inc., of Charlotte, North Carolina, and Hazen and Sawyer. In addition to housing the treatment processes of the demonstration project, the 27,500 sq ft structure-which is located at the HRSD's 30 mgd Nansemond Treatment Plant in Suffolk, Virginia-includes a public education facility and research space. One mgd of wastewater is removed from the Nansemond plant following the secondary treatment process and sent to the demonstration facility.

The HRSD opted to go with the carbon-based system (using granular activated carbon [GAC]) for two main reasons, Henifin says. It is as protective as a membrane-based system, but, unlike membranes, it does not remove salt. Having a total dissolved solids level of around 600 ppm, the finished water from the carbon-based system compares well, chemically speaking, with the groundwater to which it is added. "If we used the RO process, we would have to include a process to add salt" back into the treated water, Henifin notes. "That cost is eliminated." So, too, is the need to dispose of the highly concentrated brine that would result from the RO process.

However, it remains to be seen whether the carbon-based approach will prove less expensive than its membrane-based counterpart. The "big variable" concerns the frequency with which the GAC will need to be replaced within the treatment train's filtration unit, Henifin says. "Operating costs may be close to those of an RO facility, but the jury is still out," he notes.

The demonstration project also includes the use of a single recharge well to replenish the aquifer with the highly treated water. The approximately 1,400 ft deep well contains multiple screens at various elevations that have been determined to be the best for infiltrating the water. To date, this aspect of the demonstration program has been quite successful. "Water is flowing easily into the well," Henifin says. "The aquifer is big and thirsty."

Ultimately, the HRSD intends to construct five full-scale SWIFT facilities that will enable the district to recharge the Potomac Aquifer with 100 mgd. To be completed by 2032, the SWIFT program will cost an estimated $1 billion.

While certainly costly, the SWIFT program will do more than boost groundwater levels in the Potomac Aquifer and counter land subsidence. Once in place, the five new SWIFT facilities will enable the HRSD to discontinue more than 90 percent of its discharges of treated wastewater to surface waters, a move that is expected to have significant water quality benefits for local waterways and the Chesapeake Bay. The practice also will help the district comply with what are expected to become increasingly stringent discharge limits for nutrients and other pollutants in wastewater. "We'll be well below the numbers that they're looking for from the state perspective," Henifin says.

In Texas, El Paso Water (EPWater) has long been a pioneer in the field of water reuse. EPWater began providing reclaimed water for irrigation in 1963, and the utility has conducted indirect potable reuse since 1985. That year, EPWater began using wastewater treated to drinking water standards to recharge the Hueco Bolson, one of two aquifers that together provide 55 percent of the desert city's drinking water in an average nondrought year. Surface water, in the form of flows from the Rio Grande, provides 40 percent of El Paso's water supplies, while brackish groundwater treated by the Kay Bailey Hutchison Desalination Plant provides the remaining 5 percent.

EPWater is now pursuing DPR as a means of adding a drought-proof water source to its existing supplies by developing a 10 mgd Advanced Water Purification Facility (AWPF). Estimated to begin operations in 2025, the facility will be the first in the United States to send advanced purified water directly to a drinking water distribution system.

EPWater opted to pursue DPR more out of necessity than by choice. The poor quality of the groundwater near the site of the AWPF ruled out the possibility of aquifer storage and recovery. "We would have to use advanced treatment processes again to treat the blended groundwater," says Gilbert Trejo, P.E., the chief technical officer for EPWater. "That made no sense."

Reservoir augmentation was also not feasible, because El Paso's surface water originates from the Elephant Butte Reservoir, more than 150 mi away in New Mexico. Blending the purified effluent with water in the Rio Grande would not work either. In El Paso, the river flows only during irrigation season, from March through October, when water is released from Elephant Butte. "We wouldn't have water to blend it with all year-round," Trejo says. "All arrows were pointing toward direct-to-distribution potable reuse."

In 2016, EPWater completed a nine-month test of a pilot version of the AWPF. The utility has retained Carollo Engineers to design the full-scale facility and provide bidding and construction services. Currently at the 30 percent level, the design is estimated be completed in 2021, Trejo says.

First, EPWater must upgrade and expand its 39 mgd Roberto Bustamante WWTP, the effluent from which will be conveyed to the AWPF for additional treatment. Process improvements to be made to the WWTP as part of the upgrade "will provide better source water for the Advanced Water Purification Facility," Trejo says.

Expected to cost approximately $75 million, the AWPF will treat 10 mgd. The new facility will use microfiltration and RO membranes, followed by UV and advanced oxidation for disinfection and removal of contaminants of concern. The treated flows will then undergo GAC filtration to reduce the amount of residual peroxide left over from the disinfection process. Along with providing a "very effective, inexpensive way" to address peroxide, the GAC offers an "added treatment barrier that we're very familiar with," Trejo says.

Salts and minerals will be returned to the treated water, and chlorine will be added to flows before they enter an on-site storage tank. Finished water from the AWPF will be blended with EPWater's other drinking water sources.

One key factor working in El Paso's favor is its ability to dispose of the concentrated brine generated by the RO process in a simple, inexpensive fashion, by discharging it to a nearby irrigation canal. The agricultural users of the canal agreed to this arrangement after EPWater proved to their satisfaction that the concentrate would blend enough with the canal water that their crops would experience no adverse effects. "We're very fortunate to have this option," Trejo says, noting the difficulties that brine disposal typically poses to inland RO facilities. "That's the project killer right there," he says.

All told, the 10 mgd of finished water from the AWPF will amount to about 6 percent of the overall water produced by El Paso during the summer months, Trejo says. "It's not a lot of water," he notes. "But it's important because it will help us meet demand at peak times during the summer."

To be produced at an estimated cost of $1,500 per acre-ft, treated water from the AWPF will be El Paso's costliest water source to date. Per acre-foot of treated water, the city's groundwater sources cost $150, surface water totals $300, desalinated water costs $500, and aquifer storage and recovery costs $1,000. "It is expensive water," Trejo says of the product of the AWPF. "But it's a water supply that's needed." That said, EPWater is currently evaluating future efforts to import groundwater to meet demands in 2050 and 2060. "Those sources are expected to cost upward of three thousand dollars per acre-foot," he says.

For El Paso, failing to act is not an alternative, Trejo says. The arid city's economic growth depends on reliable, high-quality water supplies. As a result, DPR, though costly, makes long-term sense. "Hoping it rains and the reservoir stays full is not much of a plan," he notes.

Another approach to reuse, one that only recently has begun to gain significant attention in certain areas, is on-site treatment and nonpotable reuse within individual buildings or at a district scale. This decentralized approach to reuse has gained ground in recent years in San Francisco, in particular, because since November 2016 new development projects having 250,000 sq ft or more of gross floor area are required to implement on-site systems to treat and reuse available alternate water sources for nonpotable applications, including toilet flushing and irrigation. Alternate water sources include rainwater, stormwater, gray water, black water, and foundation drainage.

The on-site reuse ordinance helps facilitate the concept of the "right water for the right use," says Paula Kehoe, the director of water resources for the San Francisco Public Utilities Commission (SFPUC), which provides technical and financial assistance for on-site reuse projects that are undergoing the city's permitting process. Among the benefits of the reuse ordinance, Kehoe says, is that it can help the city conserve its drinking water supplies. "There's an opportunity to reduce potable water consumption by up to seventy-five percent in commercial buildings," she says.

The SFPUC's headquarters building in downtown San Francisco features its own on-site reuse system that collects and treats black water generated on the site. Known as the Living Machine, the system treats up to 5,000 gal./day by means of a process that includes an initial tank for trash removal and settling, a second tank for equalization and recirculation, screen filters, and an engineered wetland treatment system located in large planter boxes along the sidewalk in front of the building. Upon leaving the wetland treatment system, flows proceed through cartridge filters, UV disinfection, and a chlorine feeder, before being reused on-site for toilet and urinal flushing. "We've been able to decrease our potable water consumption by sixty percent with this on-site treatment system," Kehoe says.

Recently, the SFPUC has begun investigating the possibility of further treating the effluent from the Living Machine, to the point that it can be tested against standards used for drinking water systems. Dubbed PureWaterSF, the approach uses ultrafiltration, RO, and UV disinfection and advanced oxidation to treat about 80 percent of the flows from the Living Machine before sending the treated water back to the toilets and urinals for flushing.

Currently, the SFPUC is reviewing more than 90 on-site reuse projects that are in different stages of permitting, Kehoe says. Because San Francisco also has a stormwater management ordinance that requires new development projects to control runoff, approximately 70 percent of the projects feature rainwater harvesting to comply with the stormwater management ordinance.

However, "we're starting to see many more gray water and district-scale systems designed to comply with the nonpotable ordinance," Kehoe notes. Based on the various on-site reuse projects that are in the planning process today, the SFPUC estimates "we'll be able to save about two million gallons a day of potable water by 2040," she says. "That number could be higher if we see more development in San Francisco."

But the SFPUC is not relying entirely on developers to implement water reuse in the city's denser, more highly developed east side. For its part, the SFPUC is studying the feasibility of constructing a smaller "satellite" wastewater recycling facility that would serve several existing buildings plumbed to receive both potable and nonpotable water, Kehoe says. More broadly, the SFPUC is constructing a centralized recycled water facility to irrigate Golden Gate Park, Lincoln Park, and other green spaces on the west side of San Francisco. The utility is also building on its partnerships with neighboring utilities to develop additional opportunities for new supplies that can augment or offset drinking water supplies and is investigating the feasibility of treating wastewater to drinking water standards to supplement surface water and groundwater supplies. "We're continuing our research and investigating the potential for a centralized facility on the east side of the city," Kehoe says.

In 2016, the SFPUC and the U.S. Water Alliance formed the National Blue Ribbon Commission for Onsite Non-Potable Water Systems, which was later joined by the Water Research Foundation and WateReuse Foundation. Chaired by Kehoe, the commission has sought to support on-site reuse for nonpotable purposes, advance research, and develop guidebooks, model state regulations, model local ordinances, and model program rules for on-site nonpotable water systems.

This effort will surely help the entire water industry as it continues its evolution toward water recycling and reuse-a change that has been a long time in the making.

Jay Landers is a contributing editor of Civil Engineering .

Civil Engineering, November, 2019, © American Society of Civil Engineers. All Rights Reserved

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