Municipal sewage sludge is featured by both pollution risks and resource attributes. However, existing collaborative disposal technologies suffer from several inevitable defects including heavy metal residues, pollutants control, and limited nutrients utilization. Nevertheless, thermal carbonization technology offers several advantages for resource recovery and utilization by converting sewage sludge into high value-added biochar. To clarify its potential of sewage sludge char (SSC) utilization in soil amendment, this work aimed to comparatively analyze the speciation of N, P, K nutrients and heavy metals in dried sewage sludge (DSS), and SSC produced from a pilot-scale thermal carbonization device. The results showed that the total nutrient element in SSC was increased to 10.02 % (as the form of N + P₂O₅+K₂O), in comparison with DSS. Specifically, total N element in SSC accounted for 3.68 %, and its speciation shifted towards more stable pyridine/pyrrole-N. Total P element increased to 4.11 %, primarily existing as the form of biologically-available inorganic-P. Total K element was slightly decreased to 0.58 % but the relative proportion of available-K content was increased. Moreover, pilot-scale thermal carbonization process also showed a satisfactory fixation effect on heavy metal elements, and it promoted the transformation into oxidizable and residual form with a lower leaching risk level. Critically, with SSC improving soil structure and nutrient content (e.g., N, P, K, and organic matter), maize pot experiments further confirmed its promoting effect on plant growth, including accelerated germination, increased plant height and root/stem biomass. This pilot-scale thermal carbonization method provided a cleaner utilization paradigm for municipal sewage sludge valorization.

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