Aeration Tank Rehab Reduces Power Demand

The largest wastewater treatment plant in Minnesota reduced its power demand by almost 20 percent in 1997 by retrofitting 11 of 16 secondary tanks with a fine bubble aeration system.

The largest wastewater treatment plant in Minnesota reduced its power demand by almost 20 percent in 1997 by retrofitting 11 of 16 secondary tanks with a fine bubble aeration system.

The last five tanks will be done this year, resulting in 25 percent less power demand compared to 1996 and savings of $1.9 million in annual energy costs at the Metropolitan Wastewater Treatment Plant in St. Paul. About $12 million in capital improvements for the project will be paid back within 5 1/2 years, by 2002.

To acknowledge the work, the plant’s electric supplier, Northern States Power (NSP), presented the plant with a rebate check for $1.3 million — the utility’s largest rebate ever — through its “Conservation Improvement Program.”

According to NSP, Metropolitan Council of Environmental Services was one of the utility’s five largest customers in 1996. In 1997, MCES dropped to 11th and may drop further when the fine bubble project conclude. Power demand will drop by 7,500 kW this year.

Energy Savings

MCES owns and operates nine regional treatment plants and 550 miles of interceptor sewers in the Twin Cities area, treating 300 mgd of wastewater from 104 communities. The largest facility, the Metro Plant in St. Paul (250 mgd), handles about 80 percent of the region’s flow, with average annual power costs of $8 million.

Energy Savings

To calculate the Metro Plant’s potential for energy savings, MCES engineers analyzed the plant’s power data, including on-peak and off-peak electric demand and usage, improved the plant’s power factor, and created new operating procedures for major equipment.

Energy Savings

These in-house analysis programs have been used for 10 years to maximize energy efficiency. The effort has resulted in energy reductions through the installation of efficient lighting systems, high-efficiency electric motors, and adjustable frequency drives for pumps and fans.

Energy Savings

For the fine bubble project, MCES conducted large-scale pilot studies on fine bubble diffusers and redesigned the aeration tanks for testing and optimizing the aeration process. To conserve energy, MCES engineers specified high-efficiency equipment for most of the construction projects and received many rebates from the plant’s electric supplier.

Energy Savings

In the new system, compressed air is delivered into the bottom of the treatment tanks and diffused through tiny holes into fine bubbles, effectively doubling the oxygen transfer efficiency to 14 percent from 7 percent offered by the old system. This results in less power for the plant’s seven large compressors (four at 5500 hp and three at 3000 hp).

Energy Savings

With nine fine bubble tanks on line, power demand is down about 4,700 kW, according to recent data. When all 16 tanks are on-line, power demand will drop about 7,500 kW, from an average on-peak demand of 26,000 kW (energy consumption is about 16 million kWh per month).

Energy Savings

Final savings will be $1.9 million per year, a drop of 24 percent from current average power costs of $8 million per year. Capital improvements worth $12 million will be paid back within 5 1/2 years.

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