A review on the phosphorus bioavailability of thermal treated sewage sludge

Due to the shortage of phosphorus (P) resources, thermal treatment serves as an efficient sludge management method, both enabling sludge reduction and P recovery. Although thermal treatment retains P in chars and ashes, the bioavailability of P decreases due to the presence of heavy metals and minerals in sludge, making P uptake by plants challenging. This review analyzed the bioavailability of P present in chars and ashes, and explored methods for enhancing P bioavailability for plants uptake. Starting with describing test methods to enhance P bioavailability, this study compared the impacts of various thermal treatment methods on P bioavailability and factors affecting P bioavailability. The common extractants of bioavailable P from chars and ashes were water, citric acid, neutral ammonium citrate, and formic acid. Several methods to enhance P bioavailability in chars and ashes were summarized, including the addition of low molecular weight organic acids, fulvic and humic acids, lignin, enzymes, phosphate solubilizing bacteria, and co-treatment with biomass or chemicals. The transformation of P into hydroxyapatite and the extractable P were the critical factors for enhancing P bioavailability in chars and ashes. In future research, focus can be directed towards: 1) the trade-off of using raw sludge or sludge derived chars and ashes as P fertilizer in terms of P bioavailability; 2) the correlation between physicochemical tests and P fertilization performance in the greenhouse pot experiments to evaluate P bioavailability; 3) the concurrent fate of heavy metals and other pollutants in soil when evaluating P bioavailability in chars and ashes.