> News+Events > Features > Water - June 2009

Energy reductions achieved faster than expected

Wastewater treatment projects yield big savings

When the Metropolitan Council Environmental Services Division (MCES) set out on a four-year, 15% energy-reduction goal at the beginning of 2007, most of the progress was expected to come in the third and fourth years. But at the end of 2008, just halfway into the energy-saving effort, the regional wastewater collection and treatment agency already had met two-thirds of its goal with still more progress in store.

“In establishing our goal, we felt a 15% reduction in fossil-fuel energy use was ambitious but attainable,” said Bill Moore, MCES general manager. “Now we’re using ‘ambitious’ to describe the work of our staff and the collaborations we have with Xcel Energy and our engineering consultants. Not only have they studied and identified many energy-saving opportunities, but they’ve implemented changes in our facilities that are delivering real results.”

To achieve the 15% goal by the end of 2010, MCES would have to reduce its energy usage by 35 million kilowatt-hours per year, compared with 2006 numbers. By the end of 2008, MCES had completed 26 energy-related studies that identified 40 energy-saving opportunities. These opportunities are estimated to save 46 million kilowatt-hours of energy annually, which easily would surpass the goal.

Sixteen of the identified energy-saving measures were carried out over the last two years, delivering annual savings of 23 million kilowatt-hours. That’s 66% of the goal, and equates to about $1.4 million in savings per year. Some of the improvements have earned energy conservation rebates from Xcel Energy, which amounted to $225,000 in 2008 alone. Additional energy-saving measures will be implemented in this year and 2010, and 24 more energy-related studies are on tap this year.

Projects yield great savings

Here are some of the energy-savings measures that have been implemented:

• Reducing air pumping during warm months. The large Metro Plant in St. Paul, which treats about 185 million of the 255 million gallons of wastewater generated in the region daily, found that it could reduce the amount of air pumped into the wastewater without inhibiting the biological treatment process. This can be done during the five warmest months of the year and save 6.7 million kilowatt-hours a year.
  
• Cleaning ceramic air diffusers. Also at the Metro Plant, fine-pore ceramic diffusers that deliver air to the wastewater had begun to plug with solids after more than a dozen years of use. This makes air compressors work harder and, consequently, use more energy. High-pressure washing was done in 2008 on about half of the 190,000 diffusers, restoring them to like-new condition and saving 4 million kilowatt-hours annually. Additional diffuser cleaning is planned for this year, and a new pressure monitoring system detects when diffusers are getting plugged.
  
• Installing new air diffusers. At the Blue Lake Plant in Shakopee, new air diffusers were installed, air leaks were fixed, and other modifications improved the oxygen-transfer efficiency, saving 3 million kilowatt-hours a year. Similar improvements were made at the Seneca Plant in Eagan; however, they were offset in part by increased energy use needed to meet stepped-up clean water discharge permit requirements. The net savings, however, amount to about 900,000 kilowatt-hours a year.
  
• Closing a small treatment plant. In late 2008, MCES implemented changes to two treatment plants located in central Dakota County. MCES closed the Rosemount Plant, which was the smallest of eight plants in its system, and diverted the wastewater to the Empire Plant. The energy consumption for treating an identical amount of wastewater was almost twice as high at Rosemount as it is at Empire. This change will save 1.5 million kilowatt-hours annually.
  

 

On tap in 2009: Replacing tunnel lighting

The major energy-saving project scheduled for 2009 involves replacing most of the 1,400 lights in the five miles of tunnels beneath the treatment tanks at the Metro Plant. About 1,000 low-wattage and efficient lights will replace mercury-vapor and high-pressure-sodium lights. The work will begin in June and be completed by the end of the year. The measure will cut 1.9 million kilowatt-hours out of the current 2.1 million kilowatt-hour usage.

At the Blue Lake Plant, construction is scheduled to begin this year on anaerobic digesters to augment the existing wastewater solids processing system. Biogas recovered from the digesters annually will replace about $750,000 of natural gas that is now being used to dry the solids so they can be applied to farmland by MCES’s private partner, New England Fertilizer Co.

Study focuses on recovering steam energy

Energy studies scheduled for this year will continue to focus on improving wastewater collection and treatment process efficiency, redesigning lighting systems, and closing buildings no longer used for their original purpose, such as those in the former solids processing system at the Metro Plant.

One of the studies, which began in 2008, is more closely examining a specific aspect of the Metro Plant’s new solids processing system that went online in 2004. A significant amount of heat is recovered from the solids incinerators and produces steam for heating buildings and for running turbine generators to produce electricity. The steam is produced at a very high pressure (450 pounds per square inch), but needs to be converted to low pressure (15 psi) for building heating. This summer MCES will finish studying the effectiveness of an additional turbine generator that will use this steam energy that otherwise is lost in the conversion. Installation of the equipment could be completed by late 2009 or early 2010.