Biogas production from recycled paper mill wastewater by UASB digester: Optimal and mesophilic conditions.

•UASB was proposed for RPMW anaerobic treatement under mesophilic conditions.•Start-up up of the UASB reactor was completed in the 24 days.•80,76 % COD and 90 % TS were removed at an OLR of 7,27 g COD/L d.•At optimal OLR of 8.31 g COD/L d was required to produce 62.51 L/d biogas.•For HRT 15.14 h, after reactor start-up 80.63 % of COD was removed.•The average biogas composition was 72.98 % CH4, 19.76 % CO2, 0.9 % O2.