Enhanced bioavailability of phosphorus in sewage sludge through pyrolysis aided by calcined clam shell powder

Phosphorus is an indispensable element, while the existing phosphate rock resources are not enough to maintain the long-term demand of modern agricultural development. To ensure the sustainability of economic and social, it is necessary and urgent to seek the alternative phosphorus source of phosphate rock. In this study, calcined clam shell powder (CSP) was firstly used as a modifier and mixed with sewage sludge (SS) for co-pyrolysis to obtain sludge-derived pyrochars containing a large amount of bioavailable phosphorus. The effect of different temperatures (500, 600, 700, 800 °C) and dosages (0%, 10%, 20%, 30%) on the change in the form of phosphorus during the pyrolysis process were investigated. With the increase of temperature and the addition of CSP, the orthophosphate (Ortho-P) increased from 63.69% in the original SS to nearly 100%. Furthermore, the main phase of inorganic phosphorus (IP) changed, mainly in the form of apatite phosphorus (hydroxyapatite increased from 8.88% to 35.97%), and the concentration of easy-to-lose phosphorus reduced to 0.11 mg/g. After co-pyrolysis of SS with CSP, the passivation situation of heavy metals (HMs), such as Zn, Mn, Cr, Cu, Cd and Pb was significantly improved, thus reducing the potential environmental risk. The planting experiment proved that the sludge-derived pyrochars obtained by adding 20% CSP at 800 °C promoted the germination of seeds and the growth of seedlings. Consequently, the product of SS co-pyrolysis with CSP could be used as an excellently bioavailable and environmentally friendly phosphate fertilizer for agricultural production.