A novel biological process for recycling phosphorus from wastewater as fertilizer

Abstract

Background For water purification, Fe-oxide sludge is frequently used as a P sink, generating P-enriched Fe sludge. Microorganisms can transform the P-enriched sludge into products with higher P concentration or can directly promote the precipitation of P-rich compounds from water. However, there is no evidence of these products' efficiency as fertilizers. This study aimed to assess the effectiveness of microbially-mediated vivianite (biovivianite) production as Fe and P fertilizer for white lupin and durum wheat respectively. Results To this end, two completely randomized block experiments were conducted with white lupin (Fe experiment) and wheat (P experiment). The Fe and P sources used included biovivianite produced by microbial reduction of P-containing ferrihydrite (simulating saturated Fe-oxide sludge) at pH 6.5 (VivInsol6.5) and pH 7.0 (VivInsol7.0), biovivianite produced with soluble Fe(III) citrate (C6H5FeO7) in the presence of soluble phosphate at pH 7 (VivSol) (simulating the direct removal from water) and vivianite from a commercial company (ComViv). Fe-EDDHA and Fe(II)-sulfate (FeSO4.7H2O) were used as reference fertilizers in the Fe experiment and potassium dihydrogen phosphate (KH2PO4) was used for the P experiment. Overall, products dominated by vivianite and metavivianite (Fe2+Fe3 + 2(PO4)2(OH)2·6H2O), was the most effective Fe source for white lupin followed by Fe-EDDHA, ComViv, and VivSol with VivInsol6.5 as the least effective but without significant differences with Fe(II)-sulfate. Total P uptake by wheat plants from VivSol (dominated by vivianite and phosphate-green rust) was not significantly different from KH2PO4 in supplying P to the wheat plant. The particle sizes of the biovivianites were 16 µm, 18 µm, and 28 µm for VivInsol7.0, VivSol and VivInsol6.5 respectively. Conclusions The mineral constituents of the biovivianite coupled with the smaller particle sizes contributed to its effective uptake by the plants. The results reveal that biovivianite production is a novel way of producing efficient P and Fe fertilizers from water purification, providing new tools for a circular economy approach in the use of a non-renewable resource.