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Dewatering of sewage sludge (SS) was the prerequisite for saving its drying energy and sustaining its stable combustion. Hydrothermal treatment (HT) has been a promising technology for improving SS dewaterability with high energy efficiency. However, the knowledge of phosphorus (P) transformation and heavy metals (HMs) behaviors in the combined HT and incineration process was still lack. P fractions, P-bioavailability, HMs speciation, and their environmental risk in the ash samples from this combination process were evaluated and compared with those from the co-incineration of SS and CaO. The combination process was superior to the latter one in the light of P and HMs. CaO preferred to enhance the transformation of non-apatite inorganic phosphorus (NAIP) to apatite phosphorus (AP) initially with enriched P and increased P-bioavailability in the resultant ash samples. The combination process further reduced the values of risk assessment code and individual contamination factor with the increment of the stable F4 fraction in HMs. Significant reduction of potential ecological risk was observed with the lowest global risk index of 43.76 in the combination process. Optimum CaO addition of 6% was proposed in terms of P and HMs. The work here can provide theoretical references for the potential utilization of P from SS to mitigate the foreseeable shortage of P rocks. [Display omitted] • Hydrothermal treatment combined with incineration of sewage sludge was studied. • P enrichment and bioavailability in ash were enhanced by combination process. • Reduced mobility and ecological risk of heavy metals were observed in ash. • Combination process was superior to co-incineration of sewage sludge and CaO. • CaO preferred to enhance the transformation of NAIP to AP in combination process. [ABSTRACT FROM AUTHOR]