Improved Biogas Production Versus Increased Ash Content During Anaerobic Digestion with Digested Sludge-Derived Biochar Dosing

This study investigated possibilities to increase the efficiency of anaerobic digestion of sewage sludge using biochar produced by pyrolysis of digested sludge (sludgechar). Experiments were conducted in continuous laboratory bioreactors operated at the same loading rate, gradually increased from 3.2 to 4.5 g/(L·d) (COD) under mesophilic conditions (40 °C). Sludgechar (SCH) was dosed into the experimental bioreactor at a rate of 0.4–1.3 g/(L·d), corresponding to 12–28% of the added COD. Biogas production in the experimental bioreactor increased by 6.9–33% compared with the control bioreactor, while the CH4 concentration remained comparable, averaging 62.8%. The COD removal efficiency remained high in the sludgechar-supplemented bioreactor as the loading rate increased, whereas it decreased in the control bioreactor, corresponding to lower biogas production. The adsorption capacity, alkalinity, and mineral buffering properties of sludgechar prevented pH decline and the accumulation of volatile fatty acids (VFAs) at higher substrate loading. The pH values were less affected by increasing organic loading in the experimental than in the control bioreactor and remained within 6.9–7.0. Continuous experiments confirmed that sludgechar can facilitate stable operation at loading rates that would otherwise cause process failure. However, the low carbon-to-inorganic ratio of sludgechar is its significant disadvantage.