Sewage sludge is an inevitable byproduct from municipal wastewater treatment processes and has become a serious threat to the environment. Among various treatment techniques for sewage sludge, pyrolysis has been recognized as an effective alternative to largely reduce the volume of sewage sludge, destroy pathogens, and convert the sludge into high value-added carbonaceous materials (biochar) for multiple applications like soil amendment, carbon sequestration, and contaminants mitigation. Sewage sludge typically contains 2-3% of phosphorus (P) by dry mass, which makes its derived biochar as potential fertilizer during soil application. Pyrolysis of sewage sludge can affect both quantity and speciation of P in the solid residues (biochar), and it has been reported that the mobility and bioavailability of P is closely related to its speciation. An insight into P speciation is indispensable to understand the fate of P, which further helps to predict the improvement of soil conditions after recycling the sewage sludge-derived biochar as soil amendment. Therefore, in this study, the speciation of P was systematically interpreted in sewage sludge and its derived biochar obtained at a temperature range of 200-700 °C via sequential fractionation, X-ray diffraction, and 31P nuclear magnetic resonance (NMR). Furthermore, since heavy metals are always recognized as a big concern when utilizing the pyrolyzed biochar as soil amendment, MgCl2 was added as the Cl-donor to remove heavy metals during the pyrolysis processes. After that, the P plant availability of the modified char was analyzed for the potential application of the modified biochar as soil amendment. Results show that inorganic orthophosphates were detected as predominant molecular configurations of P from solution 31P NMR spectra. Poor-crystalline AlPO4 mineral was further evidenced by XRD patterns of the sewage sludge and biochar. Moreover, heavy metals like Cu, Co, Pb and Cd were effectively removed by the addition of MgCl2 during pyrolysis, together with an enhanced performance of P plant availability. Through the intensive study on the speciation and environmental behavior of P, this work proposed the potential of using pyrolyzed biochar from sewage sludge for soil improvement, and will further contribute to the recycle of P during integrate waste management.

Potential of sewage sludge pyrolyzed biochar as soil amendment: Phosphorous speciation and recovery

256th National Meeting (2018)

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