Carbon Footprinting for Biosolids Processing and Management Alternatives at DC WASA's Blue Plains AWTP

Abstract

Carbon footprinting was used to evaluate several biosolids processing alternatives considered for the Blue Plains Advanced Water Treatment Plant AWTP biosolids management plan update. These alternatives include a combination of a thermal hydrolysis process followed by anaerobic digestion; anaerobic digestion followed by thermal drying; and lime stabilization of dewatered solids. Energy and mass balance was conducted for the different alternatives where biogas was used for energy recovery in a combined heat and power facility to produce electricity and necessary heat for the thermal hydrolysis and thermal drying processes. CO2 emission factors for the different processing were obtained from published literature and were used to estimate CO2 emission. The analysis showed that the Blue Plains facility has the potential of producing about 11 MW of electricity, at an annual equivalent savings of $9.6 M, from biogas when processing annualized solids production through anaerobic digestion. Carbon footprinting benefits from land application of biosolids and dried pellets were considered in this analysis. Results showed that of the various digestion options, thermal hydrolysis offered the most benefit in terms of reduced CO2 emissions. This benefit was further enhanced when the offsets associated with land application of the Class A biosolids produced by this process were considered. Beneficial reuse of class B, lime stabilized biosolids, also offsets CO2 emissions, however the benefit is reduced somewhat by CO2 emissions associated with lime production. When the dried pellets are used as a fuel source offsetting use of fossil fuel, the carbon footprinting for processing combining thermal hydrolysis and thermal drying are the most beneficial.