Nanomaterial-amended anaerobic sludge digestion: Effect of pH as a game changer

Using nanomaterials as supplements in batch-fed anaerobic digestion (AD) has led to conflicting results in the literature, warranting the need for a standardized approach. Here, we investigate the role of pH in AD by performing batch biochemical methane potential (BMP) assays utilizing municipal sludge under two conditions: optimal initial pH (≈ 7.5) and elevated initial pH (≈ 9). We also examine the effects of synthesized nanomaterials, e.g., graphene oxide (GO), magnetite, magnetic GO, and magnetic reduced GO (MrGO), with different surface functionalities on BMP performance under these pH conditions. Our results show that the AD system is sensitive to pH, with the ultimate BMP reached much earlier at the neutral pH condition (20 days (d)) than at the elevated pH condition (45 d). Furthermore, the effects of nanomaterials on BMPs are pH-dependent, with MrGO improving the BMP rate by 56% on the onset of the plateau in the methane production graph at the neutral pH, while the BMP rate decreased by 14% at the same time scale at the elevated pH. Our findings demonstrate the need for standardized methods and highlight the importance of closely monitoring pH in future studies on nanomaterials-amended AD systems.