The effect of coupling biochar addition and hydrothermal pretreatment in anaerobic mono-digestion and co-digestion of hydrothermally pretreated thickened waste activated sludge (PTWAS) with food waste (FW) was investigated in six semi-continuous lab-scale digesters at organic loading rates (OLR of 2, 3, 4 and 8 kgCOD/m 3/day) with biochar (BC) addition to the test reactors in the first and final phases. Coupling biochar addition with co-digestion of 30% FW+ 70% PTWAS, improved methane yield (MY) by 87.5% to 0.15 LCH 4/gCOD added when the systems experienced high ammonia of 2.4 g/L at OLR of 8 kgCOD/m 3/d. A Non-competitive ammonia inhibition constant (Ki) of 2.83±0.085 was obtained. The maximum COD-to-BC ratio to overcome inhibition was 16.5 g COD substrate/g BC corresponding to TN-to-BC ratio of 0.84 g TN substrate/g BC.