Phosphorus recovery from waste-activated sludge through vivianite crystallization enhanced by magnetic biochar

Recently, phosphorus recovery as vivianite (Fe3(PO4)2·8H2O) from anaerobic digestion of waste-activated sludge (WAS) has attracted increasing attention because of its high recovery efficiency and economic value. However, the relatively small crystal size restricts the separation and recovery of vivianite. In this study, magnetic biochar (MB) was prepared by the impregnation-pyrolysis method and selected as an additive to enhance vivianite crystallization during WAS anaerobic digestion. The effects of MB iron loading and dosage on the phosphorus recovery efficiency and methane yield were investigated through batch experiments, X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were used to analyze the recovered precipitates. The results showed that the phosphorus recovery efficiency reached 45% and the methane yield increased by 27% when the mass ratio of biomass to FeCl3·6H2O (mB:mFe) was 2:1 and the dosage of MB was 1 g/gVS. SEM images displayed that the vivianite produced by MB as the seed crystal (102 μm) was larger than those without addition of MB (32 μm), and vivianite synthesized with MB could be easily separated from the sludge by a magnet. The microbial analysis found the functional bacteria related to Fe3+ reduction and methane production were further enhanced and enriched by the MB. In summary, this research confirms the feasibility of recovering phosphorus in the form of vivianite from waste-activated sludge using magnetic biochar as the seed crystal, which is conducive to its subsequent separation and utilization.