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The District of Columbia Water and Sewer Authority (DC WASA) has decided to incorporate thermal-hydrolysis pretreatment in conjunction with mesophillic anaerobic digestion as the backbone of a long-term biosolids treatment plan to produce Class A biosolids. The Authority has been lime stabilizing their raw solids from the Blue Plains Plant for many years, transporting them to the State of Virginia for land application. DC WASA began the process of identifying an alternative approach to lime stabilization. Selection criteria for the new process included a compact footprint, cost effectiveness, ease of operation by local staff, production of a Class A product, minimization of odor potential, elimination of indicator organism reactivation or regrowth in addition to maximizing energy recovery from biogas. A broad number of alternatives were evaluated and discarded, with thermal hydrolysis and thermal drying as the two remaining alternatives. After evaluating both remaining alternatives, including hybrid of both and sub-alternatives to reduce project cost and eliminate the risk from using only one alternative, thermal hydrolysis in conjunction with mesophillic anaerobic digestion was ultimately chosen as the two processes that best meet the needs of the Authority. This paper summarizes the decision making process for selecting thermal hydrolysis along with the conceptual design for the thermal hydrolysis process.

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.