California power company partners with wastewater utility for energy-saving results
Pacific Gas & Electric (PG&E) achieved win-win results by offering the California Wastewater Process Optimization Program (CalPOP). The city of American Canyon, located in the Napa Valley of Northern California, was a promising candidate for this utility-sponsored program that helps wastewater utilities reduce their energy usage and, in turn, helps PG&E meet growing demand with customer energy efficiency improvements. Of particular interest at the American Canyon treatment plant were the energy savings achieved at the plant’s ultraviolet (UV) treatment system.
Located about 35 miles northeast of San Francisco, American Canyon was incorporated in 1992 along the Napa Valley Wine Trail, more formally known as Highway 29, at the southern end of Napa County. Development followed World War II with the McKnight Acres subdivision in the 1940s and Rancho Del Mar in the 1950s. By 2010, the population had risen to about 19,500 in 5,973 residential units within the city’s 5.5 sq miles. A permanent “green belt” surrounds much of the city, with the Napa River and a 500-acre wetlands preserve to the west, the 640-acre Jack and Bernice Newell Wilderness Preserve to the east, and the vineyard-covered foothills of the Sulphur Springs Mountains to the northeast.
The city’s wastewater treatment plant (WWTP) entered service in October 2002, with secondary treatment that featured one of the nation’s first uses of tandem membrane bioreactor/UV disinfection in the process chain. The facility’s inaugural capacity was rated at 2.5-million-gal-per-day (mgd) average dry weather flow and 5-mgd peak wet weather flow.
These systems combine a suspended growth biological reactor with solids removal via filtration. The membranes can be designed for and operated in small spaces and with high removal efficiency of contaminants such as nitrogen, bacteria, biochemical oxygen demand and total suspended solids. However, high energy consumption is an undesirable common feature of this type of treatment plant because of the need for air scouring to control bacterial growth on the membranes.
Disinfected effluent was discharged into either of two receiving waters, the North Slough of the Napa River or constructed wetlands. The quality of the treated effluent led local vineyards to recycle some of it for irrigation use.
Like many other early membrane bioreactor treatment plants, this plant had a number of design deficiencies, which began to emerge with the advancing age of the process equipment, the ever more stringent environmental regulations, and the expansion of the customer base. The plant operations staff had identified several priority upgrades; these and other issues were cited as reasons for cost-effective upgrades in a 2010 Wastewater Treatment Plant Phase I Upgrades and Expansion Preliminary Design Report.
Fortunately, the upgrades that would result in energy savings were eligible for CalPOP, which helps wastewater treatment facilities optimize their processes, thereby cutting energy costs and saving money. Quantum Energy Services & Technologies Inc. (QuEST), the program’s authorized implementer and a nationally known energy efficiency consultant, works with plant operators to perform free process audits, qualifying them for zero interest on-bill financing (OBF) loans. Actual installation of energy efficiency measures is the responsibility of the customer, with some technical assistance available from QuEST.
The energy efficiency CalPOP program was launched in 2001 and supported by the financial resources of PG&E. All of the state’s investor-owned utilities are required by the Public Utilities Commission (PUC) to fund and administer energy efficiency programs that result in significant reductions in overall power demand. To that end, the PUC structured the utilities’ rates so they benefit financially from selling fewer kilowatt-hours than they would have without the programs.
American Canyon and PG&E partnered in a relationship capable of meeting mutual goals. Although power consumption at the plant did not increase prior to this energy efficiency project, rates did, making the American Canyon plant eligible for the CalPOP program. Monthly bills had reached $27,000 ($324,000 per year) and imposed a strain on the municipality’s $5 million wastewater utility fund budget, derived primarily from a residential tax base.
Teamwork Pays Off
The municipal utility’s staff worked closely with the local Carollo Engineers office and Berkeley-based QuEST. QuEST selected Lescure Engineers, a subcontractor based in Santa Rosa, to perform the CalPOP engineering audit at the American Canyon WWTP.
Initially, the city had hoped for a potential 31% reduction in energy use after implementing three energy efficiency measures (EEMs) at the American Canyon WWTP. The EEMs consisted of an air supply optimization measure; a return activated sludge optimization measure; and improvement to the 144-lamp/single channel Wedeco UV system, its dose control, and other elements of the system that were reaching the end of their useful lives, noted Jay Atkinson, plant operations manager.
The audit team worked closely with the plant staff to identify and execute three energy efficiency retrofits recommended by Carollo Engineers and a Xylem Wedeco field representative.
The scope of upgrades to the Wedeco system was identified through a free inspection and recommendation program Xylem offers called TotalCare. TotalCare is a comprehensive, integrated portfolio of services that ensures Xylem’s equipment—Flygt, Godwin, Leopold, Sanitaire and Wedeco brands—are running at their best. Standardized service packages range from basic to advanced.
Energy Efficiency Retrofit 1. At the time of the TotalCare inspection, the UV system was operating all three banks of UV lamps in the channel at full power instead of modulating the arrays. Recommendations included a complete relamping with Wedeco’s recently introduced and more energy-efficient Eco-Ray UV lamps and ballasts, a programmable logic controller processor that enabled monitoring by the plant’s SCADA, improved instrumentation and a transmissivity monitor in the influent channel, along with UV intensity sensors in the dosing zones. Energy savings were predicated to be a conservative 50%. However, with the upgrades, added modulation and power use on balanced demand, the Wedeco TAK 55 system now achieves the desired disinfection with only one bank operating at half power.
The result has been an 80% reduction in energy use. Data collection for measurement and verification occurred between Oct. 28 and Nov. 13, 2013. The yearly savings of $26,511 (314,624 kWh) versus the UV upgrade cost of $124,586 will achieve a simple payback period within 7.1 years, or 6.2 years when the UV upgrade cash incentive of $31,915 is deducted from the capital cost.
Energy Efficiency Retrofit 2. To optimize air supply, staff developed and installed an dissolved oxygen and ammonia control system to ensure proper level of treatment to meet the discharge requirements without wasting energy.
Energy Efficiency Retrofit 3. As a return activated sludge optimization measure, the staff also installed a patent-pending distributed feed system in the aeration basins to improve the treatment efficiency by eliminating a short-circuiting condition.
Altogether, these improvements cost $393,493. PG&E estimates an annual energy saving of $127,011. Through the CalPOP program, the upgrades were underwritten by a $146,576 cash incentive (rebate) and a $246,915 zero-interest OBF loan—recoverable in 17 months through OBF. The OBF loan is structured so the power bill savings equal the loan principal repayments, and the power bill remains constant until the loan is paid off. With this financial package, the city was able to install the improvements without impacting its capital and operating budgets.