Phosphorus recovery from pig manure: Elucidating the competition between vivianite and siderite formation

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.