According to Hansen, vendors seek partners that pay at least $250,000 a year in energy bills; and a city hall, library, and police station may not add up that much. But add a water or waste-water treatment plant, which consumes far more energy than any other infrastructure operation, and suddenly the proposal's a win-win proposal for client and contractor.
Jerry Collins Jr. was wastewater treatment administrator in the early 1980s when he installed an aeration optimization (AEROPT) system that's still in service at the 135-mgd Maynard C. Stiles Wastewater Treatment Plant in Memphis. According to Marvin Pate, the consultant who designed and implemented the system, the plant accounted for 40% of the city's energy bills. The drinking water facility represented another 20%, and all other public works operations combined accounted for the rest.
The wastewater treatment contact stabilization activated sludge facility has four 5,000-hp/120,000-cfm blowers, two of which ran constantly and consumed 80% of the energy the plant used. After analyzing the blower system's control logic, Pate realized, as others have since, that the plant could satisfy permit regulations without running the fans 24/7 at full speed.
Pate, who's now overseeing physical plant services for the University of the South in Sewanee, Tenn., installed sensors in the bottom of aeration tanks to monitor dissolved oxygen levels, and wrote a software program (in Fortran) to control the fans to maintain minimum air pressure.
“We were comfortable enough that we lowered the aeration tank dissolved oxygen levels to 0.8 mg/l; previously they were kept at 3.0 mg/l,” says Collins, who was promoted to public works director before moving to his present position as president of Memphis Light Gas and Water Utilities. “Any wastewater plant that uses aeration tanks would benefit from this type of system. It's also especially well-suited for systems using centrifugal blowers — the bigger, the better.”
While buildings and exterior lighting remain the primary target for public clients, consulting firm Burns & Mc-Donnell is seeing more interest from the wastewater sector because of the need to move forward with necessary capital improvements despite budget constraints
“Any system that uses some form of energy to operate has potential for energy-savings performance contracting,” says Walt Barnes, PE, technology director for the firm's Energy Services Group. “Using the operating cost savings to fund capital improvements can often be the answer to fund needed improvements.”
The firm has developed projects for treatment plants that include using methane from anaerobic digestion systems to produce heat or fuel generators to support the process. Integrated control systems offer potential savings by avoiding overtreatment and properly handling varying loads. “These control systems also make it possible to manage peak electrical loads that often earn better utility rates,” he says.
In Lynnwood, Wash., a 10-year contract with Siemens Building Technologies increased the efficiency of three 200-hp centrifugal blowers for the wastewater treatment plant's secondary aeration basins. Every eight hours, employees were averaging three dissolved-oxygen sensor readings and adjusting inlet valve positions to maintain the 3.0 mg/l set point — more efficient than using an outlet valve to throttle air flows or just over-aerating the basins. But since variable-speed drives were added to the blowers, the fans adjust their speed automatically. By lowering wear and tear on the blowers, the drives eliminated the plant's practice of firing up a different blower, which draws a tremendous spike in power, every month.
The city is so pleased with the financial results that it's looking for partners to explore installing a microturbine on an outfall to help power the plant, reducing energy costs that over time will pay for the microturbine, says Deputy Public Works Director Jeff Elekes. While the Washington State Department of General Administration pre-qualifies performance contractors every other year, Elekes says the city's going to pre-qualify its own for this project.