Ever sunny and warm, Arizona continues to be one of the nation's fastest-growing states. But while the influx of aging baby boomers and new business is good for the economy, it requires water providers to be increasingly innovative to ensure supply meets demand. In a state that's mostly desert, this means going beyond asking residents to use brooms to sweep sidewalks and driveways, water early in the morning or evening, and make sure that sprinklers aren't spilling water into the street.
Located about 30 miles outside of Phoenix, Surprise, Ariz., is the second fastest-growing suburb of the state's largest city. Like much of the West, what was once a small farming town now encompasses almost 70 square miles of residential and commercial development. Anew home was built in Surprise every three hours in 2005, and, at that rate, population is expected to increase 72% by 2020.
In Arizona, water managers tasked with ensuring supplies meet future demand face a plethora of challenges beyond financing. The state Department of Water Resources, for example, places strict limits on how much groundwater utilities can draw, while the Department of Environmental Quality requires any water that's discharged back into aquifers to be Class B quality or better.
After assessing existing and possible future water sources in light of these regulations, as well as growth projections, the city's Water Services Department identified treated wastewater effluent as its primary source of renewable water. The department projects that, by 2020, almost half of the city's effluent—a total of 36,787 acre-feet/year—can be used for irrigation or to replenish groundwater supplies.
To generate that much effluent, the department is more than doubling its waste-water treatment plant's capacity, from 7.2 to 16.3 mgd, and building a network of wells, pipes, booster pumps, and filtration stations to treat and recharge the additional effluent. In anticipation of this $9.5 million upgrade, managers have been shoring up their capital-improvement budget with permitting and other revenues generated by the city's booming economy.
Making High Quality Affordable
Until now, the department has stored much of the plant's excess effluent in recharge infiltration basins. Given the price of land around Surprise and the basins' relatively low infiltration rates, the result of soil plugging due to total suspended solids composed mainly of organic materials, building more basins was not the most cost-effective option for storing and treating the expanded plant's excess effluent.
Local consultants proposed using vadose zone recharge wells instead. These wells treat effluent to higher quality levels than surface infiltration basins (for an explanation, see the sidebar below) and are so small that an entire cluster of the wells uses less space than the average recharge basin.
Since its development by the Scottsdale Water Resources Department more than a decade ago, other Arizona cities—including Glendale, Mesa, and Gilbert—are deploying vadose zone recharge well technology.
“By increasing the amount of high-quality reclaimed water that they deposit back into the aquifer, city utilities accrue ‘water credits' they can ‘spend,' to draw ground-water during system emergencies or supply shortages or as demand rises with population, much like a bank account for water,” says Floyd Marsh, water resources manager and practice leader for the Phoenix office of Lockwood, Andrews & Newnam Inc.
“This is the most efficient and cost-effective way to recharge water into the underground system,” says Kinga Stanek, senior project manager at Surprise's Water Services Department.
Connecting Treatment to Filtration
Surprise is installing the wells in phases in conjunction with the treatment plant's expansion so the amount of additional treated water matches the water department's discharge capacity. Between now and 2011, a cluster of 32 wells—along with pipelines, filtration systems, booster pump station modifications, and an electrical instrumentation control system that's integrated with the plant's existing SCADA system—will provide an estimated recharge capacity of 10 mgd.
Each well will be 48 inches in diameter and drilled to 180 feet. Since ground-water lies 300 to 500 feet below the surface, water from the wells will trickle through more than 120 feet of sand, clay, gravel, and silt—a process known as water polishing—before it reaches the aquifer.
“Permitting agencies see this as a real advantage because of the additional treatment that the water undergoes,” says Stanek.
To maximize space, the wells will be placed in a rectangular formation with each located 100 feet from the other, a configuration that allows replacement wells to be constructed in between when an individual well fails to perform over time.
The closed-loop design will recharge reclaimed wastewater by:
- Diverting it into two temporary storage basins with a combined capacity of 15 million gallons
- Sending the reclaimed water into a set of pipelines using booster pumps
- Filtering out sediments, suspended solids, and organic matter through a filter system connected to the pipelines
- Injecting the filtered water through vadose zone recharge wells
- Discharging the material backflushed from the filter system into a nearby sewer line, where it will be returned to the plant for recycling.
The department's long-term goal is to build a second recharge facility at the city's recreation campus, located four miles from the wastewater treatment plant.
If all goes according to plan, operators working at the 5-mgd facility may rub shoulders with members of the Texas Rangers Major League Baseball team, who travel to Surprise each year for spring training at the half-acre park.
— Srinivasan is a technical writer for Houston-based engineering consulting firm Lockwood, Andrews & Newnam Inc.