Analysis of the influence of activated biochar properties on methane production from anaerobic digestion of waste activated sludge

This work describes the impacts of biochar (BC) addition to the anaerobic digestion (AD) of waste activated sludge (WAS). Three BCs, produced by pyrolysis at 550 °C of different waste biomasses (soft wood, sewage sludge and rice husk), then physically activated at 900 °C with CO2, were investigated as additives. AD tests were performed in batch mode at 37 °C, feeding 2% total solids and 10 g L−1 BC. While none of the considered BCs increased biogas yield compared to control digesters, the activated BCs with higher surface area, porosity and hydrophobicity (RH550a from rice husk and SS550a from sewage sludge) boosted methane yield (up to 105% for RH550a). The experimental methane production were: 0.037 Nm3 kgVS−1 for SWP550a, 0.081 Nm3 kgVS−1 for SS550a, 0.142 Nm3 kgVS−1 for RH550a and 0.069 Nm3 kgVS−1 for control reactors. CO2 adsorption (3.14 mmol g−1 for RH550a, 0.97 mmol g−1 SS550a) calculated from experimental data was consistent with literature (0.4–2.3 mmol g−1 BC). The fitting of experimental methane productions through the modified Gompertz equation showed an acceleration of methane production for all BCs, with a reduction of the lag phase compared to control reactors (0.5 days vs 2.6 days). This work, although confirming literature data about CO2 adsorption, brings new insights on the influence of specific physico-chemical properties of BC as additive in AD of WAS. Surface area, porosity, hydrophobicity and alkali and alkaline metals content in ashes were the most important BC properties affecting AD of activated sewage sludge.