Industrial Utility Efficiency

Aeration Energy Offers Opportunities to Save

Aeration systems at wastewater treatment facilities present significant, cost-effective energy savings opportunities. Aeration—the introduction of air into the wastewater stream to support anaerobic bacteria and mixing—is a key function at the majority of wastewater treatment facilities in North America. Aeration accounts for 25-60 percent of total energy consumption at wastewater treatment facilities , and a significant piece of operating budgets sector-wide.

The aeration system may also be a rich source of cost-effective energy savings. Savings opportunities range from large and capital-intense to operational and low-cost. The two short case studies below illustrate each type of opportunity. Regardless of scale, state and local energy efficiency programs have resources to support aeration system improvements—and other cost-effective upgrades—that save energy at wastewater facilities. To learn more about efficiency program resources for water and wastewater facilities in your area contact your electricity and gas providers.

More For Less

In 2008, Snohomish Public Utility District, an energy efficiency program administrator in northeast Washington, assisted a wastewater treatment facility that needed additional aeration system capacity, and was interested in energy efficiency. The facility serves a community of about 20,000, with an average daily flow of approximately half a million gallons. At the time of the project, the facility used parallel 85 horsepower brush rotors to mix and aerate the wastewater in the aeration basins.

Engineers from the efficiency program helped the wastewater facility to analyze three upgrade scenarios for treatment capacity, energy performance, and maintenance costs, and to compare the initial and lifecycle costs of all three systems. The scenarios considered included (1) adding a third brush rotor, (2) replacing the existing brush rotors with a positive displacement blower, controls, and fine-bubble diffusers in the aeration basin, and (3) replacing the brush rotors with a high-speed, gearless “turbo” blower with integrated dissolved oxygen controls, and fine-bubble diffusers.

The first option—adding a third brush rotor—had the lowest initial cost, but was ruled out because of high energy and maintenance costs, which would have resulted in a higher total cost compared to options two and three. The wastewater facility ultimately selected option three, the high-speed blower and fine-bubble diffusers—which had the lowest annual energy and maintenance costs.

High-speed blowers can offer significant efficiency improvements, compared to mechanical aerators, or positive displacement and multi-stage centrifugal blowers. They achieve this through the use of highly-efficient permanent magnet motors and a frictionless magnetic or airfoil bearing design that improves the unit’s mechanical efficiency. The blower installed at the Washington wastewater facility also included integrated dissolved oxygen sensors and a variable speed drive that modulated the speed of the blower to maintain a specified oxygen concentration in the wastewater stream. The fine-bubble diffusers, installed in the bottom of the aeration basin, provided an additional efficiency improvement, by more evenly distributing air throughout the basin, and increasing the overall oxygen transfer efficiency, compared to mechanical mixing.

The completed project met the wastewater facility’s need for expanded aeration capacity, while at the same time reducing aeration system energy consumption by 61 percent, and saving the facility \$16,000 per year in energy costs, and an additional \$500 in reduced maintenance. The energy efficiency program provided a \\$39,000 project incentive, to offset the higher initial cost of the high efficiency equipment.

Tiny Bubbles

Wastewater treatment systems use biological processes to remove pollutants. The majority of these biological processes require oxygen, which is provided through a system of air compressors, distribution pipes, valves and diffusers. Over time the diffusers become fouled, reducing oxygen transfer efficiency, and increasing the amount of air needed to support the process.

Aeration energy is the largest non-labor operations cost for most wastewater treatment plants. The aeration oxygen is added simply in the form of bubbles to waste water to help keep the bacteria alive that’s cleaning the water. The smaller the bubble, the better the system.

At the Metropolitan Wastewater Treatment Plant in St. Paul, Minnesota, energy benchmarks indicated significant diffuser fouling had occurred. Xcel Energy, the electrical utility for the plant, funded a study to evaluate various cleaning techniques and the impact on diffuser performance. As a result of the study, a combination of chemical and high-pressure cleaning methods were recommended and employed to effectively return the diffusers to like-new condition.

The solution made for smaller profile bubbles that allowed for optimized oxygen transfer. That efficient transfer translates to a reduced airflow requirement. In other words, the blowers producing the bubbles don’t have to work as hard. In addition, the clean diffusers feature a lower pressure drop, which further reduces the blower power required.

The diffuser cleaning project reduced the required airflow by 48,000 scfm, resulting in 11 million kilowatt hours of energy savings, or approximately 22% of aeration energy use. In-house staff completed the cleaning to further reduce costs and the project paid for itself in energy savings in only three months.

Conclusion

The brief case studies above demonstrate that aeration systems within wastewater treatment plants can be a rich source of cost-effective energy savings, and that energy efficiency programs administered by local utilities can help identify, quantify and implement these energy-saving projects. The authors encourage water and wastewater facility managers to contact their electricity and natural gas providers to inquire about available efficiency program resources for water and wastewater facilities in your area.

CEE is a consortium of efficiency program administrators from across the U.S. and Canada who work together on common approaches to advancing efficiency for the public good. For more information visit www.cee1.org. CEE would like to thank the following people and organizations for their assistance in preparing this article.

Larry Rogacki and staff at Metropolitan Wastewater Treatment Plant, St. Paul, MN

Jim Conlan, Snohomish Public Utility District

Erin Mathe, Xcel Energy

Pete Vinck, Xcel Energy