Phase-separated anaerobic digestion (AD) system was suggested to recover energy in the form of CH(4) from different effluent fractions of fermentative hydrogen production. In the present study, a two-phase AD system consisting of an H(2)-producing fixed-bed reactor (RH) and a CH(4)-producing upflow anaerobic sludge bed reactor (RM) was employed to investigate the responses of RM to three types of pretreated RH effluents: membrane-filtered, centrifuged, and poorly settled. The results showed that RM easily converted soluble byproducts in the membrane-filtered effluent, but it had difficulties in degradation of the mixture of soluble and colloidal organic matters in the centrifuged effluent under high organic loadings. The colloidal matters originated from extracellular and intracellular macro-polymers were believed to have been adsorbed onto the surface of granular sludge and formed a film of increasing thickness which retarded the soluble substrate supply to the inner acetogens and methanogens. Due to the fact that the degradation of H(2) biomass residue in the slightly settled RH effluent was inefficient under a tested hydraulic retention time (HRT) of 20 h, they were either trapped in RM causing the expansion of sludge bed or washed out with the effluent. This study recommended the suspended solids in the RH effluent be removed from the feed to RM; be treated elsewhere or recycled. Extra care should be taken to fine tune RM to accommodate the degradation of colloidal solids. (C) 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.