The role of NiFe2O4 nanoparticle in the anaerobic digestion (AD) of waste activated sludge (WAS)

Abstract Anaerobic digestion (AD) is a promising technology for the treatment of waste activated sludge (WAS) with energy recovery. However, the low methane yield and slow methanogenesis limit its broad application. In this study, the NiFe2O4 nanoparticles (NPs) were fabricated and applied as a conductive material to enhance the AD via promoting the direct interspecies electron transfer (DIET). The crystal structure, specific surface area, morphology and elemental composition of the as-prepared NiFe2O4 NPs were characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The biochemical methane potential (BMP) test was performed (lasting for 35 days) to evaluate the energy recovery in AD with the addition of the NiFe2O4 NPs. The results illustrate that NiFe2O4 NPs could accelerate both the hydrolysis, acidogenesis and methanogenesis, i.e., the cumulative methane production and daily methane yield increased from 96.76 ± 1.70 mL/gVS and 8.24±1.26 mL•gVS−1•d−1 in the absence of NiFe2O4 NPs (Group A) to 123.69±3.20 mL/gVS and 9.71±0.77 mL•gVS−1•d−1 in the presence of NiFe2O4 NPs (Group B). The model simulation results showed that both the first-order kinetic model and the modified Gompertz model can well simulate the experimental results. The hydrolysis rate constant k increased from 0.04 ± 0.01 d−1 in Group A to 0.06 ± 0.01 d−1 in Group B. And the maximum methane production potential and activity were both improved after adding NiFe2O4. The microbial community analysis revealed that the microorganisms associated with hydrolysis and acidogenesis were more abundant in the presence of NiFe2O4. And the methanogenic archaea were enriched to a larger extent, resulted in the higher methanogenesis activities via dosing NiFe2O4.