In this study the upflow anaerobic solid-state (UASS) reactor was operated at various conditions to optimize the process parameters for anaerobically digesting wheat straw in a continuous process. Additionally, particle size effects have been studied in the operation at 55 and 60 degrees C. Moreover, the incremental effect of the organic loading rate (OLR) to the system was examined from 2.5 to 8 g(VS) L-1 d(-1). It was found that the UASS operating at 60 degrees C with a small OLR yields highest methane production, but the advantage over thermophilic operation is negligible. The rise in OLR reduces the systems yields, as expected. From OLR = 8 g(VS) L-1 d(-1) a second stage is necessary to circumvent volatile fatty acids accumulation. (c) 2013 Elsevier Ltd. All rights reserved.