For the past 26 years, the 42,000-m3/d (11-mgd) Neely Wastewater Reclamation Facility has beneficially used all of its effluent. Some is directly reused to irrigate municipal parks, golf courses, and common areas of some homeowner association neighborhoods; the rest is sent to groundwater recharge facility basins. The Town of Gilbert, Ariz., earns aquifer storage credits that can be exchanged for future potable water supplies through groundwater recharge operations at its three facilities.
Now, a new project is enabling Gilbert to capitalize on the land used for groundwater recharge efforts again by installing a 2-MW solar power system through a public–private partnership at its Neely Recharge Facility. The solar power system, installed above five groundwater recharge basins, is expected to provide approximately $2 million in energy cost savings during the next 20 years.
With 8000 photovoltaic tracking solar panels, this solar power system will offset more than 39 million kg (86 million lb) of carbon dioxide over the life of the system and generate more than 4 million kWh of electricity annually.
No upfront capital costs to the town
The Gilbert Solar Project was made possible through a land-license agreement and a solar Power Purchase Agreement (PPA) between Gilbert and solar energy contractor SPG Solar Inc. (Novato, Calif.). Through the PPA, the solar contractor paid all of the capital costs to design and construct the $10.4 million project. In addition, the contractor will operate and maintain the solar power system for the 20-year term of the agreement.
The project was facilitated, in part, by an incentive from the local power utility, Arizona Public Service (APS; Phoenix). The incentive program provides as much as 60% of the project capital costs to be paid by APS to the solar power system owner over a 10-year time period in exchange for the system’s renewable-energy credits. The owner of the Gilbert Solar System is Integrys Inc. (De Pere, Wis.), a renewable-energy investment group.
A hedge against rising electricity rates
While the solar power system itself is not owned by Gilbert, all power generated by the system is dedicated to help offset approximately 40% of the power needs at the Neely Wastewater Reclamation Facility. The PPA outlines a fixed rate that Gilbert will pay for the power generated by the system. The solar rate schedule is anticipated to be less than the rates that Gilbert otherwise would pay the local power utility during the agreement term.
The PPA includes an annual 2.5% escalator over the course of the agreement term, while the local electric utility’s rates are predicted to increase by 4% per year. The project’s forecast savings for the 20-year term is projected to become more significant as time goes by. While the cost savings during the first 5 years of the agreement is anticipated to be only an average of about $15,000 per year, the average cost savings during the final 5 years of the agreement is forecast to be more than $200,000 per year.
The solar energy project was sized carefully to ensure that the peak power production from the system would not exceed the demand required by the wastewater facility. This effort involved town personnel, the solar contractor, and Severn Trent Services (Fort Washington, Pa.), with which the town contracts to operate the Neely Wastewater Reclamation Facility.
At the Neely facility, four 38,000-m3/d (10-mgd) variable-speed centrifugal pumps deliver influent to the facility’s headworks for bar screening and grit removal. From there, the flow passes through two parallel denitrification basins into four oxidation ditches. Each ditch contains five 3.7-kW (5-hp) rotors for mixing, and together they share three blowers, each with a capacity of 113 m3/min (4000 ft3/min). Next, five clarifiers — three that are 23 m (75 ft) in diameter and two that are 30 m (100 ft) in diameter — allow solids to settle.
Clarified effluent either is delivered to the Neely Recharge Facility site located adjacent to the plant or directed to six traveling-bridge sand filters. The filtered effluent is disinfected with bleach as it passes through the three chlorine contact basins. The disinfected effluent then is sent to a 7.6 million-L (2 million-gal) onsite effluent storage reservoir and pumped out through a dedicated distribution system to direct-reuse sites for irrigation or to other recharge facility sites located throughout Gilbert.
The Neely Facility does not have onsite solids handling. Solids are pumped through a large sewer interceptor to a regional treatment plant located in Phoenix for treatment and disposal.
The Neely Recharge Facility is one of three groundwater recharge facilities owned and operated by the Town of Gilbert. The 30-ha (75-ac) site has 11 recharge basins that are each approximately 1 m (3 ft) deep and 1.2 to 1.6 ha (3 to 4 ac) in size. The solar arrays were constructed above five basins in a manner that will enable future water recharge efforts to continue. The 8000 photovoltaic solar panels associated with this project are installed on steel beams that have been driven 6 m (20 ft) into the ground. Underneath each row of panels is a shock-absorber system designed to enable the system to withstand winds up to 145 km/h (90 mi/h).
The steel supports are spaced more than 4.6 m (15 ft) apart, and the system also has a height clearance of 2 m (6.5 ft) beneath the arrays to allow for water recharge operations, as well as maintenance of the basins with small equipment.
Fifteen months from initial concept to producing energy
The solar energy project timeline began in June 2010 with discussion among a team composed of town staff, the solar contractor, and the treatment plant’s operations and maintenance provider. In August 2010, the team discovered and applied for the APS rebate incentive.
In September 2010, the team received word that its proposed 2-MW solar project had been selected to receive an incentive. In November 2010, the Gilbert Town Council approved a land-license agreement and the solar PPA.
Engineering design and permitting for the solar power system began in late November 2010 and was completed in February 2011. Construction began in late February 2011 and continued through early November 2011. The system began producing power on Sept. 24, 2011, and was dedicated officially on Nov. 16, 2011.
Mark Horn is the wastewater manager for the Town of Gilbert, Ariz.
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