DC WASA Adopts Thermal Hydrolysis for Anaerobic Digestion Pretreatment: Conceptual Design Details for the Largest Cambi™ System

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.