Comparing Thermal Hydrolysis Processes (CAMBI™ and EXELYS™) For Solids Pretreatmet Prior To Anaerobic Digestion

Thermal hydrolysis is a sludge conditioning process applied to dewatered cake upstream of anaerobic digestion. The advantages of thermal hydrolysis are producing a Class A biosolids product, making residuals more digestible, and improving the dewatering characteristics of the resulting biosolids. Applying thermal hydrolysis to dewatered cake significantly reduces the viscosity allowing digesters to operate at much higher solids contents than conventional digesters significantly reducing volume requirements. There are currently two commercially available thermal hydrolysis processes, Cambi and Exelys (which is the second generation of Biothelys). They both apply pressure (120–130 psi) and temperature (330°F) for a set retention time of about 30 minutes to hydrolyze the solids prior to digestion. The major difference between the two processes is that Cambi consists of a series of batch tanks while Exelys utilizes a continuous plug flow reactor. The Cambi process is a more established technology with more than 25 installations worldwide while the Exelys process is still new with only one demonstration plant. The thermal hydrolysis process is normally applied as a pretreatment to anaerobic digesters in the Lysis – Digestion (LD) mode, however, the Exelys process can also be placed between two digestion steps which is known as the Digestion – Lysis – Digestion (DLD) mode of operation. Results from a desktop study for a “green field” 40 dtpd plant indicate that both thermal hydrolysis processes in the LD mode are economically favorable when compared to conventional mesophilic digestion; however, Exelys LD provides a thermal hydrolysis solution at a lower cost yielding a more favorable life cycle cost then the more traditional Cambi process. The Exelys system in the DLD mode was not found to be economically favorable for a “green field” system.