Feasibility of spent metalworking fluids as co-substrate for anaerobic co-digestion

•Spent metalworking fluids (SMWF) were co-digested with pig manure (PM).•Biochemical methane potential of SMWF was very low (25%).•Anaerobic co-digestion of SMWF and PM enhanced process efficiency (70%).•Pseudomonas was corroborated as the main species during SMWF treatment.

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/w1 (corresponding to 3.75 mL SMWF/Lreactor 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/Lreactor and the other 3 pulses of 1.25 mL/Lreactor) 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.