Ca(OH)(2) treatment was applied to enhance methane yield. Different alkali concentration, incubation temperature and duration were evaluated for their effect on methane production and COD conversion efficiency from (non-) extruded biomass during mesophilic anaerobic digestion at lab-scale. An optimum Ca(OH)(2) pretreatment for grass is found at 7.5% lime loading at 10 degrees C for 20 h (37.3% surplus), while mild (50 degrees C) and high temperatures perform sub-optimal. Ca(OH)(2) post-treatment after fast extrusion gives an additional surplus compared to extruded material of 15.2% (grass), 11.2% (maize straw) and 8.2% (sprout stem) regarding methane production. COD conversion improves accordingly, with additional improvements of 10.3% (grass), 9.0% (maize straw) and 6.8% (sprout stem) by Ca(OH)(2) post-treatment. Therefore, Ca(OH)(2) pretreatment and post-treatment at low temperature generate an additional effect regarding methane production and COD conversion efficiency. Fast extrusion gives a higher energy efficiency ratio compared to slow extrusion. (C) 2014 Published by Elsevier Ltd.