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Wastewater project will recover methane, reduce fossil fuel use

A project to recover methane from wastewater solids and use it to replace natural gas, which fuels a later stage of the solids management process, is under construction at the Metropolitan Council’s Blue Lake Wastewater Treatment Plant.

Once completed in 2011, the new facilities are expected to reduce the plant’s energy bill by $800,000 to $900,000 annually.

"This is a major step in our ongoing efforts to recover or produce energy in our wastewater treatment facilities, allowing us to reduce our purchase of fossil-fuel energy,” said Bill Moore, general manager of the Council’s Environmental Services Division.

Adding anaerobic digestion to the process

The Blue Lake Plant currently sends solids removed from wastewater through a large, natural gas-fueled heat dryer operated by its private-sector partner, New England Fertilizer Co. (NEFCO). The dryer produces fertilizer pellets that NEFCO applies to farmland. The new facilities under construction will add an anaerobic digestion step to the process, breaking down organic matter into solids, liquids and gas before the solids go through further dewatering en route to the heat dryer.

The benefits from anaerobic digestion will be twofold:

• The recovered methane gas will replace upward of 70% of the natural gas currently required to fuel the heat dryer, saving the plant $800,000 to $900,000 annually in energy costs.
• The process will reduce the volume and mass of the solids going to the heat dryer by approximately 30%. This will reduce the load to the heat dryer, meaning the current capacity will be sufficient to handle increasing wastewater flows to the plant through 2020 or beyond.

Construction on the $28 million project began in August, and is scheduled for completion in the first half of 2011. The new facilities include four 1.5-million-gallon digester tanks and covers; a two-story, 21,000-square-foot digester operations building; a 5,000-square-foot chemical handling building; and a 500-foot service tunnel to connect to the plant’s existing 5,000-foot tunnel network. Modifications will be made for the heat dryer to utilize methane from the digesters, including equipment to reduce moisture in the methane and increase its pressure.

Excess methane will heat building

During times when methane production exceeds the heat dryer's needs, the excess gas will be used to heat the digester operations building and preheat the solids before they are digested. In addition, scrubber water that heats up while controlling odors from the dryer exhaust stack also will be used to preheat the solids.

Project is part of larger plant upgrade

The Blue Lake Plant, located in Shakopee, serves approximately 300,000 people in 28 southwest-metro-area communities. Construction of the anaerobic digesters is part of a six-year, $200 million upgrade begun in 2007 to meet the increasing needs of the plant’s service area, which is projected to grow to some 420,000 people by 2030. The upgrades will:

• Enhance pollutant removal
• Improve safety and reliability
• Replace aging equipment and support systems
• Reduce energy usage
• Increase capacity to serve area growth