Calcium phosphate granules recovered from black water treatment: A sustainable substitute for mined phosphorus in soil fertilization

Phosphate is essential for food production. However, phosphate rock, which is the main natural source, is becoming worse in quality due to the depletion of the reserves and contamination with heavy metals and radioactive elements. Marketable phosphate contains more than 13 wt% of phosphorus (P), but run-of-the-mine phosphate rock is of lower grade (8 to 11 wt% of P). In this study, calcium phosphate granules (CaP granules) produced during anaerobic treatment of vacuum collected black water (feces and urine, BW) were assessed for their potential as a substitute for phosphate rock. The quality assessment was based on elemental composition (macro nutrients and heavy metals), crystallography and morphology analyses, dissolution tests, and quantification of micropollutants. CaP granules contained 10 wt% of P, from which 35% was dissolved within 5 min in citric acid and 85% in H2SO4. The incineration of the CaP granules increased the P content to 15 wt%, by eliminating the remaining organics (29%), pathogens, and organic micropollutants. Heavy metals (Cu, Zn, Cr, Ni, Cd, As, and Pb) and organic micropollutants in CaP granules were below the Dutch and European regulatory limits for direct fertilizer application. Moreover, incinerated CaP granules complied with the requirements for use in the fertilizer industry. At the current process conditions, CaP granules produced from BW can potentially replace 12% of the phosphate rock used in agriculture as fertilizer.