Application of magnetic multi-wall carbon nanotube composite into fermentative treatment process of ultrasonicated waste activated sludge

This study investigated the effect of supplementing nano-sized magnetite (Fe₃O₄ NPs), multi-wall carbon nanotubes (MWCNTs) and Fe₃O₄-MWCNTs composite on bioconversion of waste activated sludge to hydrogen, in batch systems. Substrate degradation efficiency (SDE) increased from 28 ± 3.8 (control) to 49 ± 5.9, 46 ± 4.8 and 52 ± 6.3% at optimal doses of 200 (Fe₃O₄ NPs), 300 (MWCNTs) and 200 mg/L (Fe₃O₄-MWCNTs), respectively. Based on dissolved iron and sludge conductivity measurements, superior SDE in Fe₃O₄ and MWCNTs batches have been assigned to enhanced dissimilatory iron reduction (DIR) and high sludge conductivity, respectively. Combined impacts for sludge conductivity and DIR were revealed in Fe₃O₄-MWCNTs system. In 200 mg/L (Fe₃O₄-MWCNTs) batch, catalytic activities of hydrogenase, protease and α-amylase peaked to 596, 146 and 131% (relative to control), respectively; as well as, highest volumetric H₂ production of 607 ± 59 mL/L was acquired. Performance deteriorations at high concentrations of nanoparticles were caused by cellular oxidative stress induced by generated reactive oxygen species.