Process safety and environmental protection: An optimized solid phase denitrification filter by using activated carbon fibers for secondary effluent treatment

•An optimized solid-phase denitrification filter by using ACF was established.•Nitrate removal from secondary effluent was investigated by the optimized system.•The system with PHBV 3020 and ACF obtained the maximum denitrification rate.•94∼100 % nitrate removal was achieved at HRT of 30 min.•The ACF worked as favorable habitat for denitrifying bacteria.