This work analysed the effects of Biochar (BC) addition to the Anaerobic digestion (AD) of wastewater Mixed sludge (MS) in semi-continuous mode. A 3 L digester was operated at 37 °C for 100 days, feeding MS collected every three weeks in the same wastewater treatment plant, and 10 g L−1 of BC. The average performance of MS digestion (biogas 188 NmL d−1, 68% methane) improved in presence of BC (biogas 244 NmL d−1, 69% methane). According to the results of the multiple linear regression analysis performed on the experimental data, the 79% variation of the soluble COD in the MS was the driving factor for the 38% increase of biogas and methane yields. In conclusion, in the considered experimental conditions, the variability of the substrate’s composition was the key factor driving the performances of the AD of MS, independently of the addition of BC.