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

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.