Comparison of modification methods for the preparation of coarse organic flocculant with macromolecular products from the hydrothermal liquid phase of sludge

Implementing appropriate measures to recycle and convert the organic matter in sewage into valuable products can effectively contribute to resource recovery. The macromolecular products derived from the hydrothermal liquid phase of sewage sludge (HL-MMP) were utilized as raw materials for preparing coarse organic flocculant (Co-flocculant), which can be re-used to enhance organic matter removal from sewage via flocculent settling. Two distinct chemical modification methods, namely etherification and grafting copolymerization, were compared and analyzed in terms of their suitability for the preparation. Through coagulation experiments, it was found that the Co-flocculant obtained from both methods yield satisfactory turbidity and COD removal. However, to achieve a higher turbidity and COD removal, the required dosage of the Co-flocculant (M-GTA) obtained by etherification is significantly larger than that of the Co-flocculant (M-DMC) obtained by graft copolymerization. Meanwhile the HL-MMP and Co-flocculant were characterized using FTIR, XPS, SEM and Charge Density, Intrinsic Viscosity of the latter were measured. The results of FT-IR and XPS showed that HL-MMP possesses numerous functional groups, which can serve as active sites for both modifying reactions. The charge density of the Co-flocculant obtained through the two methods is similar, however, the intrinsic viscosity of M-GTA is lower than that of M-DMC, indicating that the graft copolymerization seems to form products with larger molecular weight. Additionally, the analysis of the reaction mechanism revealed that the HL-MMP exhibits self-polymerization and bound monomer, DMC, facilitates polymerization in the graft copolymerization. Conversely, in the etherification, polymerization mainly depend on the addition of etherized monomer, GTA. The difference of reaction mechanisms can explain the relative quantitative requirements of modifying monomers and different preparation conditions. Therefore, graft copolymerization is deemed more suitable for modifying HL-MMP to synthetic Co-flocculant and deserves further comprehensive and detailed investigation.