Resource Recovery Efficiency and Microbial Community Response in Anaerobic Chain Elongation of Discharging Wastewater from Spent Lithium-Ion Batteries

• • Anaerobic chain elongation tolerated up to 40% (v/v) actual discharge wastewater in feed, achieving a caproate titer of 6.38 g/L; at 60% loading, butyrate and caproate synthesis ceased entirely, defining a hard operational threshold for reactor design. • • Fluoride (F-) was identified as the dominant inhibitory factor, with a threshold between 600 and 900 mg/L; below 600 mg/L butyrate production was unaffected, at 900 mg/L severe substrate inhibition occurred with only marginal late-stage recovery, and at 1200 mg/L chain elongation was completely blocked—guiding pretreatment targets for fluoride removal. • • High salinity (TDS ≈ 12 g/L) and metal ions (Li+, Ni2+, Co2+, Mn2+) were experimentally ruled out as primary inhibitors, redirecting engineering focus toward fluoride-specific mitigation rather than costly desalination or metal precipitation. • • Microbial community succession was stage-dependent: Clostridium kluyveri dominated during start-up, shifted to Caproicibacterium and Thermocaproicibacter in stable operation, and recovered via a synergistic Oscillibacter valericigenes and Caproicibacterium sp. consortium; enrichment of Brevundimonas diminuta and Clostridium ljungdahlii supported complex organic degradation, underpinning substrate supply for chain elongation.