Coal thermal power plant magnetic fly ash as an accelerant for anaerobic digestion of cow dung and aloe peel residue: Optimizing the particle size under static magnetic field

Abstract

Anaerobic digestion (AD) is a promising technology for converting organic waste into bioenergy (biogas); however, the substrates and accelerants can significantly affect its efficiency. Moreover, the AD performance can be influenced by the particle size of the accelerants. In this study, we investigated the effect of magnetic fly ash (MFA) as an accelerant in the co-digestion (AcoD) of cow dung (CD) and aloe vera leaf residues (ALR) under a weak magnetic field, focusing on the effect of different MFA particle size (354, 149, 74, 37, and 25 μm). Our findings revealed that particles with the size of 37 μm produced the highest biogas yield (617.7 mL/g VS), methane content (66.5 %), and biodegradability (83.3 %), achieved the maximum reduction for chemical oxygen demand (55.9 %), total solid (46.4 %), and volatile solid (54.3 %). This particle size also enhanced methane recovery (64.2 %), digestate stability (49.9 %), and fertility (38.4 g/Kg). Moreover, 37 μm particles positively influenced bacterial (e.g., Bacteroidetes_vadinHA17, Synergistaceae, etc.) and archaeal (e.g., Methanospirillaceae, Methanobacteriaceae, etc.) communities, boosting biodegradation and methane production. In particular, using 37 μm particles improved direct interspecies electron transfer (DIET), as demonstrated by superior electrochemical properties such as a higher redox peak current (1.60 mA), greater limiting current values (0.63 mA, absolute value), and lower internal resistance (1.7 Ω). Thus, utilizing 37 μm particles as accelerants for AD offer a cost-effective strategy for optimizing biogas production while contributing to sustainable waste management and energy solutions.