The effect of adding vegetable waste as a co-substrate in the anaerobic digestion of swine manure was investigated. The study was carried out at laboratory scale using semi-continuous stirred tank reactors working at 37 degrees C. Organic loading rates (OLRs) of 0.4 and 0.6 g VS L(-1) d(-1) were evaluated, corresponding to hydraulic retention times (HRTs) of 25 and 15 d, respectively. The addition of vegetable wastes (50% dw/dw) resulted in an improvement of 3 and 1.4-fold in methane yields at HRTs of 25 and 15 d, respectively. Changes on microbial morphotypes were studied by Scanning Electron Microscopy (SEM). Samples analyzed were sludge used as inoculum and digestate obtained from swine manure anaerobic reactors. SEM pictures demonstrated that lignocellulosic material was not completely degraded. Additionally, microbial composition was found to change to cocci and rods morphotypes after 120 d of anaerobic digestion. (C) 2011 Elsevier Ltd. All rights reserved.