## 4 Conclusions and Future Work  
In this study, self-sustaining double-stage circularity is successfully realized through the utilization of sunflower agriculture's waste in bio-fertilizers. The applicability of different Bulgarian SHA as a single source or in mixture for the production of bio-fertil with 30 wt. % K\({}_{2}\)O and 5 wt. % P\({}_{2}\)O\({}_{5}\) is established. The process parameters of a full-scale industrial site for the production of granulated material were investigated, and optimal operating conditions were defined. The most important parameters were identified as the moisture content of raw SHA delivered by other industrial sites, where the burning of sunflower husk was performed, the technical water flow during the mixing process, and the drying conditions. The established optimal values are a moisture content in the raw SHA in the range of 5-15 wt. %; a water flow rate in the humidifier at ambient temperature in the range of 2-18 L/min; a temperature of the inlet hot air for drying in the range of 180-220 \({}^{\circ}\)C; a drying time of 20 min; and a cooling time of 15 min. After setting the optimal operating regime of the full-scale technology at a 4 t/h load, the obtained final product was characterized by a granule size of 2-5 mm, a moisture of approx. 5 wt. %, and 2.5 kgf crush resistance. The study demonstrates that the proposed process for producing bio-fertilize complies with all safety, quality, and environmental standards established by local and international regulations (such as Regulation (EC) No 2003/2003 on fertilizers in the EU). The proposed technological approach is economically feasible due to the cost-free raw material and the transport costs that are covered by the suppliers. Suppliers also economically benefit because in a linear economic model they have to pay for both the transportation and disposal of sunflower husk ashes because they are treated as a waste product according to the current legislation. It is important to note that both the economic and environmental benefits depend on the geolocation of the supplier and the bio-fertilizer producer. In terms of the process's carbon footprint, a short distance for transportation is preferred. The proposed approach in the study could be applied directly as a technological solution in different agricultural regions, but the economic, logistical, and legislation challenges have to be individually assessed. The next stage of investigation will consider the evaluation of the carbon footprint of proposed bio-fertilizer material during its whole life cycle from the generating of sunflower husk ash to the final stage of commercial production. Other challenges that we will address \begin{table}
\begin{tabular}{l l l l} \hline \hline  & \multicolumn{3}{c}{**Polychlorinated Dibenzo-p-Dioxin**} \\  & & **ng/kg** & \\ \hline
2.3.7.8-TCDF & \textless{}0.100 & 1.2.3.4.7.8-HxCDD & \textless{}0.149 \\
2.3.7.8-TCDD & \textless{}0.061 & 1.2.3.6.7.8-HxCDD & \textless{}0.125 \\
1.2.3.7.8-PeCDF & \textless{}0.073 & 1.2.3.7.8.9-HxCDD & \textless{}0.131 \\
2.3.4.7.8-PeCDF & \textless{}0.070 & 1.2.3.4.6.7.8-HpCDF & \textless{}0.061 \\
1.2.3.7.8-PeCDD & \textless{}0.148 & 1.2.3.4.7.8.9-HpCDF & \textless{}0.092 \\
1.2.3.4.7.8-HxCDF & \textless{}0.133 & 1.2.3.4.6.7.8-HpCDD & \textless{}0.087 \\
1.2.3.6.7.8-HxCDF & \textless{}0.137 & OCDF & \textless{}0.590 \\
2.3.4.6.7.8-HxCDF & \textless{}0.142 & OCDD & \textless{}0.167 \\
1.2.3.7.8.9-HxCDF & \textless{}0.232 & 1.2.3.4.7.8-HxCDD & \textless{}0.149 \\ \hline \hline \end{tabular}
\end{table} 
Table 9: Long-lived polyhalogenated organic compounds in granulated sunflower husk ash. in our future work will be the identification of nitrogen-enriched by-products, which will be suitable for mixing with sunflower husk ash for NPK bio-fertilization. Conceptualization, D.Z.-F.; methodology, D.Z.-F., V.N. and L.K. ; software, K.F. ; validation, D.Z.-F. and K.F. ; formal analysis, V.N. and L.K. ; investigation, D.Z.-F., V.N. and L.K. ; resources, D.Z.-F.; data curation, D.Z.-F. and K.F.