Pig waste (PW) and paper sludge (PS) possess complementary properties that can be combined for successful anaerobic digestion. Biochemical methane potential (BMP) tests revealed that a PW:PS 3:1 (v/v) ratio had the highest normalized CH4-COD removal (54%), while PS had the lowest value (11%) and PW had 44%. Batch BMP tests revealed a significant decrease in lag times for methane production in the order of PW:PS 1:3 (14 days) < PW:PS 1:1 (17 days) < PW:PS 3:1(20 days) < PW (23 days). Hydrolysis constants (k(hyd)) were higher for all PW:PS combinations than for either of the individual waste streams: 0.004 d(-1) (PS) < 0.02 d(-1) (PW) < 0.024 d(-1) (PW:PS 3:1) < 0.03 d(-1) (PW:PS 1:1) <0.05 d(-1) (PW:PS 1:3). Semi-continuous reactors performing co-digestion of PW and PS at a 2:1 ratio showed 1.5 times higher methane production than baseline PW-only reactors, confirming the BMP results. (C) 2012 Elsevier Ltd. All rights reserved.