None of the wastewater treated at the Ocotillo Water Reclamation Facility in Chandler, Ariz., is sent to a receiving river or lake. All of the plant’s 8.6 mgd of effluent is reused — for aquifer recharge, urban and farm irrigation, or industrial process water.
It’s not surprising that a community in the Sonoran Desert would place a high value on water. Chandler, a Phoenix suburb, gets about nine inches of rain a year. Even in March, the wettest month, it only gets about an inch. All new developments are required to use reclaimed water whenever possible. Reclamation is also cost-effective, as it would cost millions to build an effluent pipeline to the nearest river more than 20 miles away.
The Ocotillo facility, one of three wastewater plants in the city, has been operated by Severn Trent Services since 1999. In 2007-08, the plant used the company’s Site Energy Management Plan (SEMP) to improve its energy efficiency, leading to the 2010 Large Wastewater Treatment Plant of the Year award from the Arizona Water Association.
“SEMP evaluates the treatment process as a whole to see where efficiencies can be achieved,” explains Severn Trent senior area manager Keith Greenberg.
Continued effort toward efficiency at the Ocotillo biological nutrient removal plant was a key factor in winning the award. Of special note was an aeration blower replacement in 2010 that earned an $86,300 rebate from Salt River Project, the local electrical utility. Three existing aeration blowers were replaced with high-efficiency blowers from HSI of Houston.
Using air bearings and high-efficiency motors, the blowers are 30 to 35 percent more efficient than the plant’s original 25-year-old blowers, according to Greenberg. Replacing the 300 hp conventional centrifugal blowers with the same size high-speed turbo blowers is projected to reduce overall plant energy use by 1 million kWh or more. With a total cost of $720,000, the project has an expected payback of 7.7 years. That doesn’t include the $30,000 annual savings from decreased maintenance on the blowers.
“The original plant blowers were very mechanical with a lot of moving parts, and they required ongoing service from an outside vendor for routine maintenance,” says Greenberg. The bearings on the new blowers are cushioned by air, which reduces friction. The motors are also made of rare earth metals, whose magnetic characteristics and light weight reduce energy use and wear.
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“Determining the exact effect the blowers had on reducing the overall use of electricity is very complex,” notes Greenberg. “However, there is no doubt that we have reduced energy costs through the implementation of the SEMP and by installing high-efficiency blowers. The total plant electric use is substantially less than it would have been had the original blowers not been replaced.”
Another aeration improvement was switching to LDO probes from Hach Company in place of older membrane probes. “They’re much more efficient and much more accurate,” says Greenberg. “They allow better control and better utilization of the air that is available, and you don’t have to replace the membranes.”
The Ocotillo facility also takes part in a utility program called PowerPartner, designed to reduce energy use during times of high demand. By agreeing to curtail its energy use, the plant receives an incentive payment from Salt River Project both for participating and for its actual performance during curtailment events.
“It brings in $10,000 to $11,000 per year, and the savings are shared with the city,” says Greenberg. “If called upon, we have to reduce our power use by 300 kW.”
That is achieved in several ways, including shutting off pumps, reducing dissolved oxygen to the minimum needed for treatment, and cutting demand throughout the plant by minimizing the use of lights and other electrical equipment.
The agreement with the electric utility restricts the number of curtailment events to one a day, no more than three times in seven days, up to four hours at a time, and no more than 60 hours per year. There is no penalty if Ocotillo can’t cut its use by 300 kW, but that would reduce its incentive payment. Greenberg says anything more than two hours becomes difficult for the plant to meet, but most curtailment events haven’t lasted that long. In 2011, the plant was asked to reduce its power consumption only once.
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The details of such curtailment programs vary with the utility. One limiting factor for a wastewater plant is the time of peak flows compared to the peak demand for the electric utility. “Our flow starts picking up about 10 a.m. and we peak from about noon to 3 p.m.,” says Greenberg. “We usually get called for curtailment early in the morning.”
The electric utility also offers a wide variety of energy rebates. “All of our lighting projects are upgraded to the Salt River Project rebate standards for high efficiency,” says Greenberg. That includes a recent project that relit an entire building with LED lights. The city also takes part in the utility’s premium efficiency motor rebate program and looks for applications for variable-frequency drives.
In the planning stages is a total upgrade of the plant’s HVAC system to switch to high-efficiency air conditioners and control valves. “We use as much of the utility program as we possibly can to achieve lower electrical costs with high-efficiency equipment and lighting,” says Greenberg.
The plant’s odor control system has also been improved to reduce impact on the neighborhood. “We optimized our system with hydrogen sulfide detectors for direct gas sampling that will trigger sodium hypochlorite use instantaneously, so that we use the right amount and use it only when we have to,” says Greenberg.
The plant also uses a “green” bioremediating parts washing system, the ChemFree SmartWasher, which combines a biodegradable solvent with microbes to eliminate parts washer waste. Greenberg says it all shows that energy efficiency is just one aspect of green operation. A holistic approach has helped the plant conserve critical water resources in the desert while reducing its impact on the planet.