Phosphorus recovery from synthetic biosolid digestion supernatant through lignin-induced struvite precipitation

Phosphorus recovery from wastewater is essential to resolve the problems of the fast depletion of phosphate rocks due to the increasing demand of phosphorus fertilizer and eutrophication caused by excessive phosphorus discharging from wastewater. In this study, the feasibility of adding lignin as seed materials to promote phosphorus recovery through struvite (NH4MgPO4·6H2O) crystallization from synthetic biosolid digestion supernatant was evaluated via batch experiments. Influencing factors including lignin dosage, Mg/P molar ratio, and pH were tested. Lignin addition enhanced phosphorus recovery by 44.6% at a relatively low pH of 7.9 with Mg/P molar ratio of 1.5 and lignin dosage of 6 g/L, improved the quality of recovered struvite crystals by reducing the potential of co-precipitation. The phosphorus recovery improvement became less noticeable and the extent of reducing the potential of co-precipitation became more evident as pH increases from 7.9 to 9.5. The characterizations by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy and zeta potential analysis, combined with precipitation modeling by a thermodynamic model software (Visual MINTEQ) were done to understand the process. Struvite-lignin clusters were identified, which likely provide nucleation sites on lignin to enhance struvite crystallization. It can be concluded that the inclusion of Kraft lignin as a seeding material might be a sustainable strategy to enhance struvite formation, improve phosphorus recovery, and yield high-quality struvite. •Lignin can be used as seed material to promote struvite crystallization.•Lignin addition enhanced PO4–P recovery efficiency.•Lignin addition improved struvite quality by reducing co-precipitation.•New approach of using a waste material (Kraft lignin) was developed.