Simultaneous Recovery of Vivianite and Humic Acids from Waste Activated Sludge via Ferric Trichloride Flocculation and Enzymatic Hydrolysis Co-Treatment

Synchronously recovering phosphorus as vivianite and humic acids (HAs) from waste activated sludge (WAS) is of great significance for the carbon neutralization of wastewater. In this study, flocculation, enzyme degradation (lysozyme/protease/amylase/cellulase in a 1:1:1:1 ratio), and pH adjustment were used to reclaim vivianite and HAs. After FeCl3 coagulation–precipitation and enzymatic hydrolysis of the sludge for 11 h, the supernatant was enriched with Fe2+ and PO43−, with the molar ratio of Fe2+:PO43− of 2.21. To improve the purity of the vivianite, the crude protein was separated at pI 6.0. The purity of the crystals reached a peak of 97.44 ± 0.04% at pH 7.5. HAs extracted from the residuals had a high affinity for metal adsorption, and the adsorption process was both endothermic and efficient. Overall, this study demonstrates the feasibility and effectiveness of the joint reclaiming of vivianite and HAs, providing new insights into multiple resource recovery from WAS.