## 4 Discussion  
### Biochar-Sediment Interplay  
The study findings revealed that amending DS with sewage sludge-based biochar supplemented DS with P, S, and Ca. Similar patterns considering P content were observed when the studied biochar was applied to the petroleum hydrocarbon-spiked soils at application rates of 3.5 and 7%; specifically, biochar incorporation influenced the increase in P content in soil by 79.3-116 and 26.2-34.5% irrespective of application rates for soils spiked with 10 and 20 g PHs kg\({}^{-1}\), respectively [66]. Whereas, in our study, an increase in P content was linked to biochar application rates, in particular, an increase from 121 to 419%. Despite the assumption that the remediation of DS (ex situ) is closely linked to the remediation of PTE-contaminated soils [92], this may not be entirely accurate, as evidenced by the differing behaviour of the same biochar in DS compared to soil. Indeed, the existing research has focused on changes in the proportions of metal fractions [93; 94; 95], making it challenging to compare the specific impact of biochars on the content of EEs in DS. Figure 6: Revenue of valorising _P. tomentosa_ biomass produced during phytoremediation of contaminated DS via dual-use production for timber and woodchips (Scenario 1). Figures marked in bold indicate the pay-off moment. Figure 7: Revenue of valorising _P. tomentosa_ biomass produced during phytoremediation of contaminated DS via biomass–biogas–electricity value chain (Scenario 2). Figures marked in bold indicate the pay-off moment. Cd, Cu, Zn, and Pb are the most commonly detected PTEs in DS [94]. Consequently, the majority of studies on the biochar-supported remediation of DS have concentrated on these PTEs, and only a few studies have explored phytoremediation with another pollutants. For instance, [PERSON] et al. [93] investigated the potential of biochar derived from palm sawdust (at 550 \({}^{\circ}\)C), at the same application rates as in our study, for the remediation of Cu- and Pb-spiked sediments, with concentrations of 631-644 and 725-731 mg kg\({}^{-1}\), respectively. After 60 days, the bioavailability of Cu and Pb in 10% biochar-amended sediments significantly decreased, while the residual fraction of PTEs increased by up to 87.7%. This phenomenon can be attributed to the highly alkaline nature of the biochar they used (pH of 11.7) in contrast to the biochar investigated in the current study (pH of 9.34) [93], which reduced the availability of hydrogen cations in the DS, thereby promoting the binding of metal ions to ligands and lowering their concentrations [95]. Another study tested biochar produced from _Phyllostchys pubescens_ biomass (at 600 \({}^{\circ}\)C) at various application rates (0.5-15%) in sediments contaminated by Cr, Cu, Zn, and Pb with concentrations of 287, 135, 766, and 44.3 mg kg\({}^{-1}\), respectively, [96]. The incorporation of biochar into DS at application rates higher than 5% resulted in a reduction of the bioavailable fraction of Cu, Zn, and Pb by up to 79.7, 49.8, and 73.2%, respectively, while having no effect on Cr bioavailability. Additionally, the reachability of Zn and Pb decreased, starting at a 3% biochar application rate, and, for Cu, the reduction was observed starting at 5% [96]. [PERSON] et al. [30] investigated the phytoremediation of DS contaminated with Cd (10.9 mg kg\({}^{-1}\)), Zn (510 mg kg\({}^{-1}\)), and Pb (83.6 mg kg\({}^{-1}\)) using _Lolium peremte_ in the presence of corn straw-derived biochar (at 300 \({}^{\circ}\)C) at an application rate of 3%. The incorporation of biochar to DS had minimal effect on Zn fractions, whereas the reducible fraction of Pb decreased, and the residual fraction increased by 7.31% and 8.60%, respectively. These findings contrast with the results of the current study, particularly for Zn, which could be explained by the different feedstock used for biochar production.