Magnetic bio-flocculation for cost-effective fast organic matter pre-concentration for sewage with enhanced capture and settling of sludge

Abstract

Sewage pre-concentration has been promising for carbon-neutral anaerobic wastewater treatment. Magnetic bio-flocculation, which combines the advantages of adsorption/bio-oxidation processes and magnetic separation for cost-effective, fast, and eco-friendly chemical-additive-free pre-concentration, is proposed in this study. The results show that 85.5–89.5% chemical oxygen demand (COD) removal within 30 min can be achieved. The maximum recovery rate of organic matter (OM) can be increased from 79.6% to 88.2% by replacing aeration with stirring coupled with a magnetic field (MF). This can be attributed to the increased production of loosely bound extracellular polymeric substances (LB-EPS) by 23.7–62.9 mg L⁻¹ and decreased production of the tightly bound EPS (TB-EPS) by 3.8–5.7 mg per g VSS in the stirring set, where the magnetic force alleviated the microbial surface negative charge, strengthened the attachment of magnetic seeds (MS) to sludge through MF-induced extra fragmented LB-EPS and compensated for the negative impact of LB-EPS by intensifying microbial OM capture. The decreased median diameter by 0.93–4.13 μm and the improved fractal dimension ranging from 2.15 to 2.18 further support the above effects. The combination of MS and the MF improved the sludge settleability, which is supported by the maximized hindered settling velocity accelerated by 1.6–7.2 m h⁻¹. The V₀/n values calculated from the Vesilind function, which mean the mass of sludge passing through a unit section per unit time, reached a maximum value of 52.71 after coupling the MS and MF. As such, magnetic bio-flocculation with stirring is found to be promising for achieving optimized practical sewage pre-concentration.