Arizona City Solves Water Challenges with Advanced Booster System

For the city of Cottonwood, Arizona, a rapidly growing population put significant strain on the community's aging and disjointed water delivery system. Serving a customer base of 30,000 people in the state's second-fastest growing county, the water system struggled to provide a consistent supply of water to its residential service area, which nearly doubled between 1990 and 2010.

The area's rapid growth caused the city's water supply to be managed through a patchwork of four separate and privately owned water systems, each controlling individual, and sometimes overlapping, service areas. The smaller water supply operations were not interconnected, eliminating the normal efficiencies of a single, integrated water utility. As a result, residents were plagued with frequent water outages that would last a day or two, as well as inconsistent pressure and continual water hammer noises. "If a municipality does not own the water system within its boundaries, it does not control its own destiny," said former Cottonwood Development Services General Manager Dan Lueder, who retired in May 2015. Another challenge was the variation in length of water lines and elevations of the rural community's water distribution network. For example, one booster pump had to pump water across nearly 6,300 linear feet and up a 200-foot elevation—a significant hill to climb for any pump. To complicate the problem, Cottonwood averages a mere 12 inches of rain annually, presenting the city with serious water supply challenges. Water conservation was crucial.

Devising a Solution

To address these challenges, the city created a water management strategy in 2005 that included modernizing its water system with effective monitoring, control and pumping technology. The strategy was developed after the city acquired private water companies in a move that presented several piping and pumping challenges. One of the first tasks Lueder tackled was integrating the separate water systems—representing roughly 10,000 primarily residential service connections—into a single municipal utility department. In 2004, the city began incorporating the private water systems and established a Water Division, which was responsible for supplying and distributing water through storage tanks, 28 wells, fire hydrants, pumps and water meters. "We did a lot of work identifying pressure zones," Lueder said. "It has been an interesting experience to basically take us from the 20th to the 21st century in water supply and water delivery." The city's plan also called for the Wastewater Division, which was operated as a separate entity, to be incorporated into the utility department. This division manages a 1.5-million-gallons-per-day wastewater treatment facility, five sewage lift stations, 48 miles of collection system and about 80 acres of effluent reuse area.

Dealing with Aging Pipes

The majority of pumps in the city's distribution system were constant-speed, across-the-line models that would pump well water into a holding reservoir and then to service stations and communities. In addition to providing pressure surges, the fixed-speed pumps could not provide incremental pressure; they were either off or running at top speed. If the pressure in the hydro-pneumatic holding tank dropped below 50 pounds per square inch (psi), a pump would activate to replenish the tank to about 75 psi. This pressure swing subjected the pipes to a 25-psi pressure change that stressed and prematurely aged the lines, many dating to 1930. Because of the constant pressure surges and water hammer, the city faced significant leakage and capital repair costs across the 100 miles of pipe in the system—as much as five to seven leaks per week in the main line and one or two leaks in the service line. Doug Ryan of Grand Canyon Pump and Supply's Phoenix location, which provides equipment sales and application support to the city, recalled when two 11-year-old booster pumps showed signs of failure in 2006. Instead of replacing like with like, Ryan presented a new water-boosting option for the reconfigured system. Ryan chose an integrated pressure boosting system that offers up to six vertical multistage pumps in parallel operation. The chosen technology is designed to optimize pumping efficiency over a range of flow rates. Ideal for water supply systems, as well as industrial and irrigation applications, these integrated pumping systems use an advanced controller that adjusts pump speed and stages additional pumps to meet fluctuating system demand. Project lead engineer Ken Knickerbocker of Pineview Consulting LLC said the decision to select the system was not rushed. "What I typically do, even with the systems that have a good history behind them, is go through and check the engineering, including on packaged systems. You have to check the hydraulics, power, drives, controls, etc.," he said. "I looked into the system specifically for the integration of the controller with the variable frequency drive (VFD), motor efficiency ratings and the ability for the system to manage water usage, which thereby ensures maximum system efficiency," he added, impressed by the pump system's vertical multistage in-line centrifugal pumps. A highlight of the packaged unit was that it could be easily moved from one location to another, an important feature for Cottonwood, which had reconfigured its pumping network. Ryan noted that one of the city's booster systems is now operating in its third location. "I'm still careful and recommend that engineers still be careful about specifying systems," Knickerbocker said. "Don't just take for granted that you're going to get a good package. You need to look into it and ... do your due diligence."

Addressing Pressure Problems

According to Ryan, one or more of the pumps was online all the time to keep the system pressurized. "Rather than running flat out at top speed and peak horsepower to reach the desired pressure, the more energy-efficient option is to design a system that ramps up to the desired speed to maintain a constant pressure and stages additional pumps as necessary to meet the specific flow demand," he said. "Since demand for municipal water delivery varies throughout the day, why not vary output?" The variable speed motors and advanced controllers help maintain high efficiency with speed control and pump staging. Another advantage of the new pump, Ryan said, is the ability to further reduce pipe fatigue and energy use by switching from constant to proportional pressure. At lower flow rates, such as during overnight hours, the pump controller will automatically lower the pressure set point because there is less friction head loss, he said. The result of eliminating these pressure surges in the system was a slash in the number of pipe breaks and leaks that require repair. According to Lueder, the city realized a 30 percent drop in the number of leaks between 2010 and 2014. "Based on an average $500 to repair each leak, we've saved more than $38,500, over and above the electrical cost savings," he said. The sizeable reduction in leaks was a major factor that allowed Cottonwood to draw less water from the ground in 2014 than the combined volume used by the private water companies in 2000. The level of unaccounted water—the difference in the amount of water extracted versus the amount of water billed to customers—has dropped from 40 percent to 11 percent. Lueder cited the new packaged booster systems for the reduction of water loss through leaks. "We're drawing less water from the aquifer today than five years ago, which is imperative if we are to bridge the water crisis facing the Southwest," Lueder said.

Aiming for Conservation

The city's energy savings were bolstered by a utility rebate from Arizona Public Service, which offers financial incentives for the use of energy-efficient motors over 15 horsepower. In 2012, with roughly half of the Cottonwood utility's pump stations retrofitted with some type of VFD pump technology, Arizona Public Service paid the city a rebate of $24,702. Although the investment in a packaged system is more than some alternatives, Lueder said reliability cannot be given a price tag. "Even for a small community like ours, the premium performance and energy savings are worth the investment," he said.