Conserving energy in the wastewater system

Council makes progress on four-year energy reduction goal

After just the first year of an ambitious four-year energy-reduction effort, managers in the Metropolitan Council Environmental Services Division (MCES) are confident they will meet their goal of reducing energy consumption by 15 percent by the end of 2010.

“We made some process and equipment improvements in 2007 that helped us achieve about 15% of our overall goal,” said Bill Moore, MCES general manager. “We’ve known that most of our energy savings would come in the third and fourth years of our efforts, and we’ve identified the additional improvements that will get us there.”

Installing new low-wattage fluorescent lights and occupancy sensors in the five miles of tunnels at the Metro Plant will cut energy demand from lighting dramatically.

MCES collects and treats wastewater for the urbanized portion of the seven-county region, operating 8 treatment plants, approximately 600 miles of regional sanitary sewers, 70 pumping stations, and other operation and maintenance facilities. This round-the-clock service requires staggering amounts of energy, including nearly 200 million kilowatt-hours of electricity per year – at a cost of nearly $14 million. Natural gas, fuel oil, and motor vehicle fuel account for another $3 million annually.

The electricity numbers put MCES among Xcel Energy’s five largest customers in the state. But that status also qualifies MCES for energy conservation study funding and technical assistance from Xcel, as well as substantial energy rebates.

MCES identifies projects to achieve 2010 goal

Studies completed in 2007 identified many ways MCES can further reduce its energy consumption. Following is a description of some of the anticipated projects.

Cleaning air diffusers. More than half of MCES’s electrical demand is for powering large compressors that pump air into the wastewater, which fosters pollutant removal. The air goes through ceramic diffuser disks in the bottom of the treatment tanks and comes out in very tiny bubbles for maximum efficiency.

At the Metro Plant in St. Paul – MCES’s largest plant – motors ranging from 3,000 to 5,000 horsepower pump the air through more than 190,000 diffusers spread throughout 16 tanks – each about the size of a football field. A recent study has shown that after 13 years of use, the air diffusers have begun to plug, requiring the blowers to work harder and use more electricity. Cleaning the diffusers already is under way; when completed in 2009, electrical use should drop by about 25%.

Additional diffuser projects are on tap at the Metro Plant and MCES’s next largest plants – the Seneca Plant in Eagan and the Blue Lake Plant in Shakopee. They include finer control of dissolved oxygen levels to reduce energy used while maintaining the quality of the treatment process, replacing or altering diffusers and valves as necessary, and repairing air leaks.

Recovering steam. Incineration of the wastewater solids at the Metro Plant generates much heat, which is recovered to produce steam to heat buildings and run electricity-producing turbines. Work will be done to optimize the generation of steam and to ensure it is used efficiently through such measures as improving steam traps and eliminating energy-wasting leaks in high-pressure pipes.

Other process improvements. Other industrial process improvements include capturing heat from an odorous air scrubbing process, altering pumps so they operate more efficiently, and making sure new pumps have premium-efficiency motors and variable frequency drives to cycle up or down as situations require.

Lighting redesign. Many of the pumps and pipes taking wastewater through the treatment process are housed in tunnels beneath the tanks – about 5 miles of tunnels at the Metro Plant. More than 1,400 lights are in use; most of them are mercury-vapor or high-pressure-sodium lights. Using occupancy sensors with these kinds of lights isn’t the best option because the lights would take too long to cycle on and off.

A better solution will be installing about 1,000 low-wattage fluorescent lights and occupancy sensors, which is planned for late 2008 and early 2009. The move will cut the annual energy demand from approximately 2.1 million kilowatt-hours to just over 200,000 kilowatt-hours.

The Seneca and Blue Lake Plants each have about a half-mile of tunnels. They have been studied for lighting retrofits, and also could see energy reduction percentages similar to the Metro Plant. An MCES wastewater pumping station and maintenance base in Mounds View is installing energy-efficient fluorescent lighting this spring, and additional treatment plant and collection system buildings will be studied soon.

Recommissioning studies. When new systems are installed – for example, air handling units, water or air pumps, and wastewater solids processing facilities – they go a through an extensive commissioning process to ensure they are designed, installed and functioning properly. As time goes by or as buildings and processes are modified, the systems should be periodically recommissioned to find ways they can be brought back up to the original standards or improved to meet newer, more energy-efficient standards.

Recommissioning studies were completed last year on two buildings at the Metro Plant, and studies on four more buildings are in the works. Early estimates show the potential to save approximately 1.9 million kilowatt-hours. Similar studies also will be done at other MCES facilities.