Reduced graphene oxide-nano zerovalent iron assisted anaerobic digestion of dairy wastewater: A potential strategy for CH4 enrichment

Abstract

Efficient waste management and alternatives for waste reduction have been the prime factors in achieving sustainable development goals. Large quantities of waste biomass can be transformed to energy by anaerobic fermentation. This study focused on improving the electron transport and subsequent CH4 enrichment in the anaerobic digestion of dairy wastewater by adding reduced graphene oxide-nano zerovalent iron (RGO-NZVI) composite. Different ratios (wt/wt) of RGO and NZVI were added to the anaerobic digesters and monitored their effects on biogas volume, composition, COD removal and VFA accumulation. There was 86.27 ± 2.8% high CH4 and 47.37 ± 1.3% improved COD removal in the digester containing the conductive additive compared to control. Comparing different ratios of RGO-NZVI revealed that a proportion of 2:1 was beneficial for maximum CH4 generation. Further, it was found that higher concentrations of the conductive additives could be fatal for microbial metabolism. A ratio of 0.5:1 had minimal support on degradation and the composition of CH4 in this digester was 34.14 ± 1.5% less than control. The metagenomic analysis showed that the diversification of microbial community and switching of major mechanism to direct interspecies electron transfer caused higher CH4 generation.