Phosphorus recovery from high solid content liquid fraction of digestate using seawater bittern as the magnesium source

Phosphorus recovery from digestate is considered a challenge because the possible discharge can lead to eutrophication. This study focuses on phosphorus recovery as struvite from the liquid fraction of swine manure digestate at a high total solids concentration, by using a lab-scale crystallizer operated in continuous mode (7 L⋅d). A by-product of salt production (seawater bittern, SWB) was assessed as Mg source for the formation of struvite instead of a chemical dosage (MgCl2) within a circular economy approach. Different Mg/P (1.8:1; 2:1; 3:1) ratios and different TS contents (TS 3.5 and 4.5 %) were studied. The maximum P recovery of 85 % and N recovery of 52 % was obtained at 4.5 % of TS and Mg/P ratio of 2:1, corresponding to an overall P and N recovery on the raw digestate of 70 % and 46 %, respectively. The presence of struvite was confirmed by X-ray diffraction (XRD) and scanning electron microscopy (SEM-EDS). Dried samples were then used as fertilizer in agronomic pot tests using Brassica rapa chinensis. Struvite obtained, showed comparable fertilizing properties in comparison with conventional fertilizers in terms of P (Mineral 5.6 ± 0.4; Poultry 5.7 ± 0.2; Struvite 5.9 ± 0.1 g kg