Magnetic sludge byproducts for adsorptive phosphorus removal: Resource recovery from iron-based anaerobic sewage...

This study evaluated feasibility of resource recovery from iron-based sewage sludge from a novel Fe(III)-dosed anaerobic bioreactor used for wastewater treatment. Sludge samples were calcined at five different temperatures (300, 350, 400, 450, and 500 °C) to investigate the transformation of the sludge into different magnetic phases of iron oxide particles. The material phase analysis revealed the presence of 14 to 39 wt% magnetite and 8 to 19 wt% maghemite for different temperature treatments, which indicate the successful conversion of sludge materials into magnetic particles. This magnetic conversion was further confirmed by magnetization measurements of the sludge byproducts that found a 6.3 to 10.9 emu/g saturation magnetization and a 0.7 to 2.0 emu/g remanent magnetization. Due to surface effects phenomenon of nanocrystals, the magnetization values were observed to increase with calcination temperature along with the crystallinity and crystallite size of the thermally-treated sludge materials. This indicates the crystallinity of the samples played a significant role in determining the magnetization properties of the sludge byproducts. Phosphate adsorption capacity and kinetics of the sludge byproducts were evaluated for the samples calcined at 350 and 500 °C. Both samples showed a high phosphate adsorption capacity, but the sample treated at 350 °C showed relatively higher capacity presumably due to smaller crystallite size and reduced crystallinity of the particles in the sample. This study demonstrated that a simple thermal treatment of the sludge can render dual benefits of recovering magnetic particles and further utilizing them for beneficial applications.