Organic fraction of municipal solid waste (OFMSW) was used to feed two pilot plants of 10001 working in anaerobic conditions. The OFMSW had previously been treated using a system of extrusion which, due to exerted pressure, separates the undesired fractions of waste from organic waste and reduces the organic fraction in a kind of homogeneous jam. Pilot tests were performed in semi-continuous conditions with a stepwise progressive increase of the total solids content (TS) of the input material from 3% TS w/w (1.5 g VS l(-1) d(-1) organic loading rate) to 10% TS w/w (4.3 g VS l(-1) d(-1) organic loading rate) using activated sludge as diluting agent. The average specific biogas production obtained was 600 l kg(-1) VS. When the input IS content was increased to 10% w/w, the biogas average specific production went up to 8001 kg(-1) VS. The methane content in the biogas was always higher than 60% measured by volume. (C) 2011 Elsevier Ltd. All rights reserved.