The anaerobic digestion of olive mill wastewater is severely hindered by the microbial toxicity of the effluent. Successful biodegradation is usually attained only by dilution and mixing with other organic wastes. In this work the addition of biochar has been investigated as an alternative to co-digestion. In batch tests, raw olive mill wastewater selectively inhibited methanogenic bacteria even at 1:10 dilution. Acidogenesis was not affected and further biodegradation was hindered by acidification and high hydrogen partial pressure. Methanogenesis resumed after 20 days and ended after 40 days from inoculation. Overall soluble COD removal was 32.9% with a methane yield of 2.35 ± 0.27 L STP/L of digestion medium. Treatment of wastewater with 15% (w/v) poplar biochar removed 71% of inhibitory polyphenolic compounds and effectively accelerated the onset of methanogenesis, with a reduced duration of 26 days. Biochar treatment of olive mill wastewater followed by anaerobic digestion led to a higher soluble COD removal (61.6%) with a slightly decreased biogas yield of 1.62 ± 0.30 L STP/L of digestion medium
