Impact of biochar on anaerobic digestion: Meta-analysis and economic evaluation

The growing global energy demand encourages the request for renewable sources, as biomethane from the anaerobic digestion (AD) of waste biomass. Biochar (BC) can effectively increase methane production when supplemented to AD, depending on BC physico-chemical properties. This study was developed in two phases. Firstly, a systematic meta-analysis of current literature was performed to correlate AD performance with BC properties, aiming to define their optimal range. The obtained results prove that BC enhances and accelerates biomethane production. Considering 408 experimental conditions of 76 studies in batch mode, biomethane yield and maximum production rate were significantly increased by BC addition. From the results of the subgroups meta-analysis, an optimal range of BC physico-chemical properties may be suggested as follows: high ash (≥ 20%) and low C contents (< 50%), high O/C molar ratios (≥ 0.3), high contents of O (≥ 20%) and N (≥ 0.6%), acidic pH (< 7.0), low surface area (< 10 m2 g-1). Secondly, an economic analysis aimed at assessing the economic profitability of BC addition to an existing AD plant suggest avoiding a dose above 0.45–0.76 gBC gVS -1, independently of the specific AD operating conditions. In conclusion, BC application in full-scale digesters is able to maximize biomethane production and economically feasible.