The work reported concerns anaerobic thermophilic municipal sludge digestion (55 degrees C) in a completely mixed anaerobic digester on a pilot-plant scale (850 L). The experimental protocol was defined to examine the effect of increasing the organic loading rate (OLR) on the efficiency of this digester and to report on its steady-state performance. The reactor had previously been converted from mesophilic to thermophilic conditions following the protocol previously proposed by the authors: this was achieved by a modified method that combined systems investigated in Chicago and in Vancouver. The reactor was subsequently subjected to a program of steady-state operation over a range of sludge retention times (SRTs) of 27, 20, and 15 days. The digester was fed with raw sludge [containing approximately 34.8 g/L volatile solids (VS)] three times per day. Under thermophilic conditions and with a 27-day SRT, the reactor was operated with an OLR of 1.48 kg VS m(-3) day. The solids removal efficiency of the reactor was 42.9%, whereas the volumetric methane production rate in the digester reached 0.35 m(3) m(-3) day. Over an operating period of 150 days, an OLR of 2.63 kg VS m(-3) day(-1) was achieved with 41.8% VS removal efficiency in the pilot sludge digester (SRT: 15 days). During this period the volumetric methane production rate in the digester reached 0.20 m(3) m(-3) day(-1) and 0.20 m(3)/kg VSr (VS reduction). The greatest efficiency in terms of substrate removal was 54.3% for an OLR of 1.71 kg VS m(-3) day(-1) and SRT of 20 days. Under these conditions, the generation of biogas and methane was at levels of 0.86 and 0.58 m(3) m(-3) day(-1), respectively, with a methane yield of 0.70 m(3)/kg VSr. (c) 2005 American Institute of Chemical Engineers.