In this paper, anaerobic co-digestion of spent metalworking fluids (SMWF) and pig manure (PM) was evaluated. Three SMWF:PM ratios were tested in order to find the highest process efficiency. The best results (COD removal efficiencies of 74%) were achieved co-digesting a mixture with a SMWF:PM ratio of 1:99, w/w(1) (corresponding to 3.75 mL SMWF/L-reactor week), which indicates that SMWF did not affect negatively PM degradation. Furthermore, two different weekly SMWF pulse-frequencies were performed (one reactor received 1 pulse of 3.75 mL/L-reactor and the other 3 pulses of 1.25 mL/L-reactor) and no differences in COD removal efficiency were observed. Microbiology analysis confirmed that Pseudomonas was the predominant genus when treating anaerobically SMWF and the presence of a higher fraction of Archaea was indicative of good digester performance. This study confirms the feasibility of anaerobic co-digestion as an appropriate technology for treating and valorising SMWF. (C) 2013 Elsevier Ltd. All rights reserved.