Precipitating phosphorus as struvite from anaerobically-digested dairy manure. (10th March 2022) - Wagner, Edward
Abstract: Midwestern US agroecosystems suffer greatly due to excess phosphorus (P) in runoff from land-applied manure. Precipitating P as struvite would remove it from the manure stream, allowing efficient storage, transportation, and use as slow-release fertilizer. To achieve struvite precipitation, organic solids and calcium (Ca) must first be removed from the manure. This study assessed struvite precipitation from dairy manure using a novel processing pipeline: first diluting the manure, then acidifying and treating it with a Ca chelator, followed by simultaneously flocculating Ca and organic solids, and finally raising the pH to precipitate struvite. The pH manipulations were determined through chemical equilibrium modeling in Visual MINTEQ. Oxalic acid was the most efficient chelator tested, reducing soluble Ca from 200 mg/L to below 40 mg/L while leaving magnesium concentrations unaffected. An anionic low-weight flocculant was most effective at removing solids, reducing it from 1.2% to 0.5% in diluted (4:1) manure. This process train comprising oxalic acid and the anionic flocculant, reduced dissolved P by 93% producing an estimated struvite yield equivalent to 72% of total P initially present in manure. Results from this study show that a simple one-step removal of Ca and organic solids can achieve the P removal efficiency comparable to multi-step methods. Graphical abstract: Image 1 Highlights: Novel single-step processing pipeline successfully precipitated manure P as struvite. Processing train involved manure dilution, chelation, flocculation, precipitation. Upper bound of struvite yield is 93% of dissolved P (or 72% total P) in manure. MINTEQ chemical equilibrium modeling used to formulate pH manipulations.