Biosolids Master Plan and Digester Gas Utilization

Greeley and Hansen was retained to evaluate the South Bend’s current Class B biosolids management program and make recommendations for biosolids management for a 20-year planning horizon. This comprehensive sludge management and energy evaluation provided a prioritized long-term capital improvement plan to maximize use of a renewable energy source, resulting in a significant reduction in energy costs at the wastewater treatment plant and provided long-term operability of the City’s anaerobic digestion facilities. This effort placed the City of South Bend on the forefront of using digester gas to create compressed natural gas for use in the City’s Solid Waste Fleet. 

Key elements of the project included: 

• The Biosolids Master Plan focused on an evaluation of the current process and other sludge minimization alternatives focused on energy utilization and recovery of the existing digester gas. 

• The City’s anaerobic digestion facilities were upgraded to high-rate digesters and digester gas was conditioned to increase the City’s flexibility to utilize this beneficial resource.  

• During preliminary planning, multiple end-use options for the cleaned digester gas were evaluated.  

Based on the life cycle costs, as well as operational needs from the plant, there were four cost-effective end uses for the cleaned gas including natural gas process engines, compressed natural gas vehicle fuel station, domestic heating, and digester heating. 

With the digester improvements, plant operating efficiency improved while reducing plant air emissions and dependence on costly fossil fuels. With reductions of about 30% in annual energy costs, this project saves the City hundreds of thousands of dollars each year and provides environmental benefits by reducing the carbon footprint of the plant. 

Project Highlights: 

• Sludge management plan for 20-year planning horizon 

• Energy recovery led to 30% energy cost reduction 

• Annual operational cost savings 

• Reduction of carbon footprint 

• Maximized use of existing assets by enhancing anaerobic digestion