The aim of this work was to evaluate the feasibility of a novel upflow anaerobic solid-state (UASS) reactor equipped with liquor recirculation. The performance characteristics were obtained from a thermophilic UASS reactor (26.5 L) operated for 68 days with a mixture of maize silage and straw. In order to prevent an accumulation of volatile fatty acids, the process liquor was continuously recirculated through anaerobic filters. By raising the volatile solids (VS) loading rate from 7.1 to 17 g(VS) L(-1) d(-1), the overall methane yield declined from 384 to 312 kg(VS)(-1), while the yield contribution of the anaerobic filters increased from 12% to 70%. The chemical oxygen demand (COD) of biodegradable particulate matter was removed by 86-93% with a maximum hydrolysis rate of 16.4 g(COD) L(-1) d(-1). Based on the results, we conclude that the UASS reactor is a promising solution for the digestion of various organic materials. (c) 2009 Elsevier Ltd. All rights reserved.