Resource Recovery from Spent Lithium–Ion Battery Discharging Wastewater via Anaerobic Conversion to Medium-Chain Fatty Acids and Butanol

The rapid expansion of lithium–ion battery applications has led to the generation of substantial discharging wastewater (DW) during recycling processes, presenting both an environmental challenge and a potential resource opportunity. This study demonstrates, for the first time, the anaerobic bioconversion of DW into medium-chain fatty acids (MCFAs) and butanol. We conducted the process in batch serum bottles with open-culture inocula at 32 °C and pH 6.5–7.0, employing a staged ethanol addition strategy over 40 days. Under these conditions, approximately 70% of the organic carbon in DW was microbially processed, yielding 7.88 g/L butyrate, 2.11 g/L caproate, and 3.79 g/L butanol. Notably, this butanol concentration represents the highest value reported to date in chain elongation (CE) systems, likely attributable to elevated hydrogen partial pressure and high ethanol availability. Microbial community analysis revealed that Lachnoclostridium and Anaerostignum were the dominant hydrolytic genera, while Clostridium kluyveri and g_Haloimpatiens emerged as key species during the chain elongation phase. Co-occurrence network and correlation analyses indicated a highly cooperative microbial community, with significant overlap between microbial taxa capable of producing MCFAs and those producing butanol. This work presents a sustainable strategy for resource recovery from high-salinity lithium–ion battery DW and provides insights into metabolic regulation for steering product spectra in CE-based bioprocesses.