Characterization of Phosphorus Recovered from Sewage Sludge Ash: A Brazil Case Study

The lack of interest in the reuse of phosphorus in agriculture is mainly due to the high abundance of pathogens, organic pollutants, microplastics, and possibly toxic metals. Therefore, different forms of treatment are necessary to take advantage of phosphorus recovery potential, one of which is the use of ash from incinerated/calcined biological sludge. A high rate of conversion of the non-apatite inorganic phosphorus fraction into apatite phosphorus was obtained in this study because of the use of commercially pure CaO additive in the dry sludge calcination tests, which is more bioavailable to plants. The obtained phosphorus pentoxide content ranged from 12 to 17%, surpassing several phosphorus-based raw materials and fertilizers. In addition, the ashes have been shown to contain toxic metals far below those recommended by Brazilian and international environmental legislation, so they can be applied directly to the soil for crop fertilization, or be used in P extraction and separation technologies for fertilizer production.