Identical assumptions about anaerobic biochemical processes were applied to the simulations of UASB systems, which mainly differ from encapsulated systems in reactor hydrodynamic and mass transport properties. System design. All reactor vessels were assumed to be cylindrical and constructed using concrete with rockwool for insulation and thin carbon steel exterior facing. The UASB reactor also included stainless steel three-phase separators. Polyethylene glycol (PEG) was assumed to be the main encapsulant material. Hollow-fiber membrane contactors could be applied to remove dissolved CH4 from the effluent and/or to actively extract dissolved H2 from an externally recirculating sidestream of the first-stage reactor, depending on the system configuration. High density polyethylene (HDPE) pipes were used for liquid influent and effluent streams, whereas stainless steel pipes were assumed for biogas streams. Equipment like water pumps, vacuum pumps, air compressors, heat exchangers, and control systems were all included within the system boundary when applicable (Figure ). Detailed design and costing algorithms of all unit operations and equipment can be found in Section S2 of the SI.