Influences of reaction parameters and complexation pretreatments on the distribution of phosphorus during hydrothermal carbonization of dewatered sewage sludge

Abstract

This study investigated the effects of reaction parameters and pretreatments with complexing agents, namely ethylenediaminetetraacetic acid (EDTA), glutamate diacetic acid (GLDA), and citric acid (CA), on the phosphorus contents and forms in the sludge hydrothermal carbonization products. The results showed that reaction temperature had the greatest effect on the distribution and forms of phosphorus in liquid and solid products. High reaction temperatures inhibited the transfer of phosphorus from the solid phase to the liquid phase. Indeed, the proportions of phosphorus in the liquid phases only accounted for 3.56 % at reaction temperature and time of 160 °C and 60 min, respectively. However, this proportion increased to 16.32, 12.62, and 13.15 % following complexation pretreatments with EDTA, GLDA, and CA at 0.1 mol/L, respectively. In addition, the complexation pretreatments enhanced the transfer of phosphorus from the solid phase to the liquid phase. The total phosphorus contents in the liquid phase (TPL) reached 896.46 mg/L. The finding can be explained by the binding of the complexing agents with metal ions, such as Mg, Ca, Al, and Fe, transferring phosphorus originally bound to these metal ions to the liquid phase. In addition, the results demonstrated the effectiveness of the magnesium ammonium phosphate (MAP) crystallization method for recovering phosphorus from the EDTA-pretreated liquid product. X-ray diffraction (XRD) and scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) analyses showed high purity and crystallinity of the obtained struvite to some extent.