Phosphorus runoff from agricultural land is a major driver of eutrophication, with manure serving as a significant source of phosphorus input. In regions such as the Netherlands, high livestock densities and limited land availability pose challenges for manure management, particularly in pig farming. Recovering phosphorus from manure and redistributing it to phosphorus-deficient areas offers a sustainable solution. This study explores phosphate recovery via vivianite (Fe₃(PO₄)₂·8H₂O) precipitation—a method previously demonstrated in municipal wastewater treatment plant sludge—and evaluates its applicability to pig manure. Vivianite formation was investigated in fresh, 4-month-aged, and digested pig manure, as well as in Thermal Hydrolysis Plant (THP) derived digested sewage sludge. A key finding is that high dissolved inorganic carbon (DIC) concentrations inhibit vivianite formation by promoting siderite (FeCO₃) precipitation. In digested manure, a DIC threshold of approximately 3 g/L HCO₃⁻ was identified, below which vivianite formation is favored. THP sludge, characterized by elevated DIC, exhibited similar inhibitory effects. More generally, vivianite was shown to form without significant competition with siderite if the DIC concentration is <2.5 times the iron concentration. Experimental results were compared with thermodynamic predictions using Visual MINTEQ and experiments in ultrapure water, revealing discrepancies which may be attributed to the ionic composition in environmental matrices. Strategies such as combining ammonia and DIC stripping or targeting fresh manure were shown to enhance vivianite formation. These findings can be used to propose the integration of vivianite-based phosphorus recovery into broader resource recovery frameworks, including biomethane production, ammonium recovery, and carbon capture.

Vivianite (Fe₃(PO₄)₂·8 H₂O) has emerged as a promising mineral for phosphorus (P) recovery from digested sludge, and it may also contribute to phosphate management in lake sediments and manure, given the similar anaerobic conditions across these environments. However, organic ligands in these matrices have been proposed to complex with iron (Fe), thereby reducing the efficiency of vivianite formation. This study aims to elucidate the impact of organic ligands on vivianite formation, particularly focusing on their binding strength with iron and the subsequent effects on vivianite formation in pig manure. Organic ligands not only form complexes with iron but also influence the crystal growth process. We investigated how different organic ligands affect the formation and dissolution of vivianite, assuming that ligands with higher iron-binding strength would enhance phosphate solubilization. Our findings revealed that citrate nearly completely inhibited vivianite formation (up to 100 %) and caused a 50 % dissolution of existing vivianite, while humate hindered vivianite formation by 40 % but only led to a 10 % dissolution. Interestingly, pig-derived dissolved organic matter had minimal effects on the precipitation of iron and phosphorus but significantly altered the morphology of the resulting products, which varied depending on the age of the manure filtrate. While the iron binding strength of organic ligands does influence vivianite formation, it does not solely account for the reduced vivianite formation observed in complex matrices like manure. Therefore, a more nuanced assessment of the role of organic matter in vivianite formation is warranted.