This study examine the effect of an increased organic loading rate on the efficiency of the stirred tank reactor treating sewage sludge and sugar beet pellets and to report on its steady-state performance. The digester was subjected to a program of steady-state operation over a range of hydraulic retention times (HRTs) of 30 to 6 days and organic loading rates (OLRs) of up to 1.7 kgCOD/m(3) d to evaluate its treatment capacity. The COD removal efficiency was found to be 84.23% COD in the digester when treating mixture sewage sludge/lixiviation of sugar beet pulp at 1.27 kgCOD/m(3) d (10-days SRT). The volumetric methane level produced in the digester reached 0.7 m(3)CH(4)/m(3) d and the methane yield was 0.64 m(3)CH(4)/kgCOD(removal). Therefore, anaerobic co-digestion of sewage sludge and lixiviation of sugar beet pulp improve the biogas productivity and the organic matter removal in addition to lowering solids retention times in the system. (C) 2013 Elsevier Ltd. All rights reserved.