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 (FeO NPs), multi-wall carbon nanotubes (MWCNTs) and FeO-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 (FeO NPs), 300 (MWCNTs) and 200 mg/L (FeO-MWCNTs), respectively. Based on dissolved iron and sludge conductivity measurements, superior SDE in FeO 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 FeO-MWCNTs system. In 200 mg/L (FeO-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 H 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.