Biogas is becoming an increasingly popular product from the treatment of wastewater, agriculture, food, and municipal solid waste. The process of anaerobic digestion (AD) allows organic waste streams such as sewage sludge, manure, and landfill organics to be converted into usable products such as biogas, fertilizer, and soil amendments. The benefits of resource recovery from waste streams depend on the current economic context and establishing a defined market for value-added products. However, there is a significant challenge in evaluating these opportunities without first understanding the environmental impact associated with various AD resource recovery systems. Applying life cycle assessment (LCA) to commercial biogas production provides a valuable tool for evaluating the environmental impact of waste management processes and assists in economic decision-making.
Using life cycle assessment as a basis for evaluating the biogas production at the Swedish Biogas International, LLC (SBI) Facility in Flint, Michigan this study quantifies the environmental benefits of implementing AD at the Flint Water Pollution Control Facility. The study compares the emissions associated with the incineration of biosolids to emissions from Class B land application on a local brownfield site and the use of biogas in an electrical generator to that of upgrading biogas to biomethane. Several other options for the use of AD byproducts are investigated including kiln drying of biosolids, phosphorus recovery, and the growth of energy crops (maize) for use as an AD feedstock.
The results are quantified using a dynamic Excel-based model, which incorporates primary data collected at the Flint SBI facility and previous research data from the U.S. Environmental Protection Agency and private sources. The intent of the model is to provide the management of SBI with a quantitative analysis of the environmental impacts of the facility compared to previous operations. The knowledge can be used to optimize the biogas management process and select the best opportunity for biosolids management within the context of the City of Flint, Michigan.
The primary environmental impacts investigated were Global Warming Potential, Acidification, and Smog Formation. All scenarios showed a substantial improvement over incineration. Upon termination of incineration, Global Warming Potential is greatly reduced due to avoided N2O emissions.
Electricity generation is preferable to biogas upgrading due the credit from avoided emissions from Michigan’s coal intensive energy mix. The alternative and supplemental benefits incur high initial investment costs but could provide additional revenue for SBI while making significant improvements in environmental impacts. Energy crops provide a benefit in the form of carbon sequestration, but maize has a poor biomass to biogas conversion, and so is not an optimal feedstock for AD.