Energy Recovery at Blue Lake WWTP – an award Winning Experience

The Metropolitan Council Environmental Services (MCES) Blue Lake WWTP (Shakopee, MN) added two-stage mesophilic digestion in 2012 to its existing solids process treatment. Project objectives were: to reduce the solids load to the downstream rotary drum drying process; reduce the volume of resulting pellets and to generate biogas to fuel the dryer. Design estimates indicated that biogas generated through digestion would meet most of the dryer gas needs. Actual operation of the digesters has eliminated the need for natural gas for thermal drying, with an annual gas cost savings of approximately $500,000 in 2013. The digester commissioning process has had some initial challenges that led to a longer startup window due to overloading and foaming; however, digester feed control successfully stabilized the process. The Blue Lake experience provides a compelling story of the real world benefits of coupling digestion with thermal drying, the value of renewable energy production, and considerations for digester start-up. While there are significant benefits associated with addition of anaerobic digestion, there are also additional operations, maintenance, and start up issues to be considered when implementing a digestion process.