MCES Energy Conservation

Fine Bubble Aeration System

Metropolitan Council Environmental Services (MCES), a division of the Metropolitan Council, provides water resources management services for the Twin Cities metropolitan area. Among its responsibilities is providing wastewater collection and treatment services in a cost- and quality-competitive manner. A challenge in recent years has been finding ways to improve wastewater treatment processes to use energy more efficiently and, subsequently, reduce costs to ratepayers.

MCES looked at the secondary treatment process at its Metropolitan Wastewater Treatment Plant (Metro Plant) as having the greatest potential for large-scale energy reductions. The Metro Plant treats over 200 million gallons of wastewater each day (approximately 80 percent of the metro area's wastewater). The aeration process in this phase of treatment accounts for well over half of the entire plant's energy requirement. In this process, massive volumes of compressed air are pumped into large treatment tanks filled with wastewater to stimulate the growth of microorganisms, which in turn consume solid and dissolved pollutants suspended in the wastewater. In the late 1990s, MCES invested $12 million to retrofit all 16 of the Metro Plant's secondary treatment tanks with a Fine Bubble Aeration System. This new system delivers the air more efficiently and in lesser amounts - thereby reducing the energy required to produce it -- while still maintaining high-quality wastewater treatment.

The Fine Bubble Aeration System has helped the Metro Plant reduce its power demand from mid-1990s levels of approximately 194 million kilowatt hours per year to current levels of approximately 144 million kilowatt hours per year. This is a 24 percent decrease in the plant's energy requirement and a significant reduction in what the power company must generate. Reducing the plant's annual power demand by 50 kilowatt hours correlates to 8,130 less tons of coal being burned to produce electricity. Based on published conversion factors (America's Energy Choices: Technical Appendixes, Union of Concerned Scientists, 1992), this annual reduction in coal burning prevents air emissions of 173 tons of nitrous oxides, 512 tons of sulfur oxides, and 58,500 tons of carbon dioxide each year.

Aerating wastewater to improve treatment results has been in wide use for more than 40 years, but improving this technology from a coarse bubble to a fine bubble process has been developed only in the past 10 years. In the innovative new process, air is pumped through 275,000 porous ceramic diffuser disks that have been installed in the 16 treatment tanks, delivering fine bubbles that double the oxygen transfer rate. This requires less air and less energy to power the air compressors. Several years before the retrofit, MCES's research and development group was among the first in the nation to develop a large-scale pilot project to test the new technology in a portion of the tanks to determine whether the concept was feasible and could be environmentally beneficial. MCES is committed to making wise use of environmental as well as financial resources. The 24 percent reduction in energy usage made possible by this new process saves MCES approximately $1.9 million in annual energy costs compared with mid-1990s levels. That savings, combined with approximately $1.7 million in rebates from the former Northern States Power Company, will cover the project's $12 million capital investment in about 5½ years. These and other energy savings, along with labor and other non-labor cost reductions are helping MCES attain its groundbreaking, three-year goal of reducing its annual budget by $20 million from the 1998 level. This will decrease wholesale wastewater rates that MCES charges to cities by 12.6 percent from 1998 to 2001, allowing cities to pass the cost savings on to their citizens or invest in their own wastewater infrastructure projects.

The former Northern States Power Company (now Excel Energy) was a major partner in this project. The company rewarded MCES with approximately $1.7 million in rebates through its Conservation Improvement Program, which encourages energy-reducing improvements by its highest-demand customers. Upon presenting the first of two rebates to MCES, James Ashley, NSP's General Manager of Marketing and Sales, said: "NSP has been pleased to work in collaboration with MCES on this very large energy conservation project. This represents the largest single conservation effort undertaken by an NSP customer since we began our program to help customers save energy through conservation improvements." This project alone served to lower MCES from 5th place to 11th place on the power company's list of highest-demand customers, with the likelihood of dropping further in coming years. NSP collaborated with MCES to establish a baseline year for comparing future energy cost savings and to identify savings that could be attributed specifically to the new Fine Bubble Aeration System. The ultimate stakeholders in this project are the ratepayers - the residents and business in 104 of the metro area's communities who are served by MCES's wastewater collection and treatment system. They benefit when MCES passes cost savings along to their communities, and by MCES continuing to protect the public health and environment.

MCES's Fine Bubble Aeration System Project can be replicated by similar wastewater treatment facilities across the country and around the world. This particular environmentally beneficial process improvement was among the first - and largest - to be done in the United States. Since then, other facilities have begun similar aeration system upgrades, or are learning from MCES's experience and are planning their own projects. Staff who were instrumental in developing MCES's new system have shared their expertise with the wastewater industry on numerous occasions via technical conferences and technical journals, and continue to be available to offer assistance. MCES's model of a process improvement that can result in significant environmental and financial benefits can be achieved by other like-minded facilities that can commit the necessary capital resources - in effect an investment for which the payback period is relatively short for a project of this size.