The triple action of pinewood biochar-based materials to improve biogas yields: pH regulation, DIET, and colonization

Cheese whey wastewater (CWW) is an effluent stream with a high organic content produced by the dairy processing industry. Although anaerobic digestion (AD) is a promising process to treat CWW, it is prone to acidification caused by the accumulation of intermediate metabolites such as volatile fatty acids during AD because of CWW's high organic content. Our study explored how biochar (BC) and magnetite biochar (MBC) at concentrations of 2.5 and 5.0 g/L could play a role in relieving the acidification, and which mechanisms are at play. The results showed that at low anaerobic granular sludge (AGS) concentrations, biogas production in BC/MBC-assisted digestion could recover from acidification within 25∼44 days, while this issue pertained to the control reactor. As a result, BC and MBC-assisted systems led to a significantly higher biogas production of +119% ∼ +191%. The microbial community analysis showed that BC and MBC enriched the top 20 bacteria selected based on relative abundance and direct interspecies electron transfer (DIET)-related bacteria. Correlation plots of the material and AGS characterization showed that the main mechanism of action of BC and MBC at low AGS was pH regulation rather than DIET as reported in the literature. Alkaline earth metals such as Ca and Mg present in biochar-based materials increased alkalinity to provide buffering capacity and increase the pH. This study allows researchers to move away from the narrow perspective of overly focusing on DIET and expand to a more comprehensive look at the multiple mechanisms such as pH regulation, colonization, and DIET.