Sustainability Comparison of Class A and Class B Biosolids Technologies

Increasing numbers of municipalities are incorporating sustainability goals into project decision making and planning criteria. The primary criteria influencing solids processing and biosolids utilization program decisions include capital and operations cost, regulatory compliance, and public perception. There is currently an information gap when it comes to bringing sustainability criteria into the selection process for Class A and Class B biosolids program alternatives. Since biosolids treatment processes can heavily impact a utilities' carbon footprint, quantifying GHG emissions is one approach to provide a broader evaluation of technologies. Results can be incorporated into a “Triple Bottom Line” (TBL) evaluation that is customized to meet facility objectives and local conditions. The TBL provides the utility with a tool to compare economic, environmental, and social criteria.If managed appropriately, biosolids production and utilization is a way to offset emissions from wastewater treatment operations and accrue carbon credits. A carbon footprint analysis was conducted for both Class A and Class B biosolids production technologies. The greatest reduction in GHG emissions was achieved through thermophilic anaerobic digestion with biogas capture for scrub and sale. Co-generation, while beneficial, proved to be less desirable due to the low GHG intensity of the commercial power source in the region. Additional carbon credits were obtained through land application of the biosolids due to carbon sequestration potential and fertilizer offsets. The largest sources of debits included chemical addition, causing alkaline stabilization to have the highest carbon emissions. Polymer addition for thickening and dewatering also provided significant debits.Results of this analysis enable any utility to make an informed decision about how to maximize biosolids production benefits from a sustainability perspective. While these findings will be significant, the findings are site specific. Regional energy production profiles and varying biosolids characteristics may cause different results in different locations. For example, the emissions offsets associated with electricity production through cogeneration in this region where hydro and wind power are the primary sources of energy are much lower than the emissions offsets in the Midwest where the primary source of energy is coal. Therefore the benefits from offsetting natural gas through biogas capture are much more significant. The findings provide guidance for other agencies planning future solids handling process upgrades and a basis for considering non-cost criteria on a quantitative basis.