The practice of replenishing reservoirs or groundwater basins with highly treated wastewater is not new. Orange County, Calif.; El Paso, Texas; Payson, Ariz.; and Fairfax County, Va., are a few communities that inject advanced treated wastewater (purified water) into aquifers or reservoirs.
The City of San Diego, with the recent success of its Water Purification Demonstration Project, is one step closer to using purified water for reservoir augmentation. Started in 2011 and completed a year later, the project showed that using advanced water purification technology could provide safe and reliable drinking water for the city.
Adjacent to the city’s 30 mgd (design) North City Water Reclamation Plant, the 1 mgd Advanced Water Purification (AWP) facility uses a multi-barrier treatment approach: microfiltration/ultrafiltration (MF/UF), reverse osmosis (RO), and ultraviolet (UV) light with hydrogen peroxide.
Although this technology has proven effective in purifying recycled water to potable water quality, the California Department of Public Health (CDPH) requires that purified water be retained in an environmental buffer, such as a groundwater basin or surface water reservoir before it is used in a drinking water system. Cycling water through this buffer protects public health through dilution with other water sources and a holding time for natural treatment.
The San Diego Demonstration Project is the second phase of a process for evaluating ways the city can increase the use of recycled water. It began with a 2005 water reuse study identifying reservoir augmentation as the preferred option for developing recycled water sources. In 2007, the city council directed the mayor and city staff to conduct the Demonstration Project to evaluate whether advanced water purification technology could safely and reliably produce purified water.
Limited water supply
The driving force for the water reuse study is the city’s high dependence on imported water and the need for a buffer against droughts.
“We import water from over 400 miles away, and the demand and costs for this water keep rising,” says Alma Rife, senior public information officer for the city’s public utilities department. “Making more use of recycled water will enhance our current water supply.”
The city’s drinking water supplies come from the Colorado River, the Bay Delta, and rainfall runoff to local reservoirs. Approximately 90 percent of the city’s water supplies are imported.
By building a full-scale indirect potable reuse (IPR) system for reservoir augmentation, the city would have approximately 15,000 acre-feet a year of high-quality drinking water from purified water. This would supply around 8 percent of the city’s water.
“We would still need to import water, but an alternative water source from IPR could be added as part of our portfolio approach to meeting our water needs,” says Marsi Steirer, deputy director and project director for the Demonstration Project.
Unlike Orange County, which injects purified water directly into aquifers for groundwater recharge, San Diego would send the purified water to the San Vicente Reservoir. “We don’t have the option of groundwater recharge, since our basins are small and brackish, and not as large as Orange County’s,” Steirer says.
San Diego has been reclaiming water for use in irrigation and manufacturing since 1997 at the North City plant. In 2006, the city built the 15 mgd (design) South Bay Reclamation Plant.
Tertiary water from the North City plant is sent to the AWP facility and filtered through membranes that remove constituents of emerging concern (CECs). The water is further treated with hydrogen peroxide and ultraviolet light disinfection (a step referred to as advanced oxidation). This process produces water with lower concentrations of constituents than imported water supplies that have not been treated with advanced technologies.
The AWP facility consists of an MF skid and membranes (Pall Corporation), UF skid (H2O Innovation) and membranes (Toray), RO skid (ENAQUA) and membranes (Hydranautics and Toray) and UV system (Toray).
During the yearlong demonstration project, a team of laboratory experts monitored the system’s water quality for all constituents and parameters regulated in drinking water, and other constituents commonly found in water supplies that are not regulated. The results of 9,000 laboratory tests showed that the final effluent was similar in quality to distilled water.
The next step
Should a full-scale IPR system for reservoir augmentation be approved by CDPH, the city would have to secure funding and build a 22-mile pipeline to carry the water to the reservoir. The treated water would blend with imported untreated water in the reservoir before being sent to potable water plants for further treatment and distribution to customers.
Such a facility could cost around $370 million and take eight to 10 years to place into operation. The plant would be staffed 24/7. “One operator has been running the demonstration plant, but it is not presently a 24-hour operation,” Steirer says. “The full-scale plant would be.”
In some ways, the hardest work has already been done. “A research study conducted in 2004 to explore public perception about indirect potable reuse for drinking water found that the acceptance rate was only 26 percent. Today, it’s 73 percent,” Rife says.
This came about after extensive public education including presentations and tours of the AWP facility, information on the utility’s website and newspaper articles. Rife says the city will continue its public education efforts.
“There has been phenomenal public interest in this project and in the demonstration plant tours,” Steirer says. “People really get it, especially the students.”
Communities like Orange County helped pave the way. “The people at Orange County were very helpful and supportive of us,” Steirer says. “It’s a great example of one utility helping another.”