Long-term evaluation of the anaerobic co-digestion of food waste and landfill leachate to alleviate ammonia inhibition

•The co-digestion of FW and LL was feasible.•Replacing 40% of the dilution liquid with LL yielded the optimum performance.•LL addition alleviated the ammonia inhibition that the FW-based AD system suffered.•The LL addition improved propionate, butyrate, and methane metabolism.•Long-term use of leachate posed no ecological risk for digestate utilization.