Understanding the effect of biochar characteristics on anaerobic digestion with focus in food processing industry

Biochar has been considered an additive for improving the performance of biogas plants. The direct interspecies electron transfer (DIET) mechanism has been widely used to explain the increase in methane yield. However, this increase may be due to other factors that are still not fully clarified. Thus, the present research aims to unveil the effect of biochar on anaerobic digestion (AD) with a focus on the circular bioeconomy. For this purpose, coffee silver skin from the coffee roasting industry was used as biomass for biochar production. Different biochar samples were produced at 200, 400, 600 and 800 °C and applied in AD divided into two parts (part 1: effect of biochar on increasing methane yield and part 2: mechanism of biochar's effect as a substrate using negative controls). Analyzing the effect of biochar (part 1), the addition of biochar as an additive led to an increase in methane yield of 35.87 % and 45.72 % for the batch reactors containing biochar prepared at 200 °C, exfoliated and native, respectively. At 400 °C, the methane yield increases were relatively modest, reaching 5.71 % and 0.46 % for exfoliated and native, respectively. Likewise, analyzing the biochar's effect as a substrate (part 2) demonstrated that the increase in methane production was mainly due to the effect of biochar as a substrate, ruling out DIET as a mechanism. Therefore, the use of biochar in the context of circular bioeconomy in the food industries should be considered in more in-depth studies with application options, in addition to AD.