# Evaluation of Waste Blends with Sewage Sludge as a Potential Material Input for Pyrolysis
## 4 Conclusions  
Comprehensive characterization provided concrete data for the optimization and modeling of the pyrolysis treatment of sewage sludge and its blends with various waste samples. Sewage sludge could be classified as a suitable feedstock for thermochemical conversion processes, with 61.2% organic contents (mass loss between 100 and 600 \({}^{\circ}\)C) and HHV of 11.8 MJ/kg on a dry basis. However, the HHV of sewage sludge is low, and therefore it is suitable to mix sewage sludge with selected waste for further pyrolytic applications, and thus achieve the synergetic effect of disposing of certain types of waste. Waste materials (different types of plastics, waste, and waste paper) were selected based on their availability to eliminate transportation, storage, and handling costs. For pyrolytic applications, the results from thermogravimetric analyses are required in order to know the maximal temperature of thermal decomposition in an inert atmosphere, maximal mass losses, and weight loss rates. The residual mass was in this order, from highest to lowest: SS > PES > PR > TP > RDF > LDPE. Furthermore, 55 waste blends were prepared in various volume ratios. The visual comparison carried out and HHV was measured for each waste blend, and these were key parameters for the selection of suitable waste blends. It is apparent from the thermogravimetric analysis that all waste blends thermally decomposed up to the temperature 500 \({}^{\circ}\)C, with the maximal weight loss rate. The residual mass for waste blends at 900 \({}^{\circ}\)C from TGA was in the following order, from highest to lowest: RDF:SS > 3 PES:SS:PR > 4 LDPE:2 SS:PR > 3 LDPE:SS:PR > 3 RDF:SS:PR > 6 LDPE:SS > 4 LDPE:SS > 3 TP:SS. Thermoplastics with sewage sludge (3 TP:SS) and LDPE with sewage sludge (LDPE:SS) had the lowest residual masses and the highest weight loss rates. On the other hand, the waste blend of polyester waste with sewage sludge and paper rejects (3 PES:SS:PR) had the second highest residual mass and the lowest weight loss rate. The conversion of input materials into oils and gases is strongly dependent on the pyrolysis temperature, which was also confirmed by TGA/DTG analysis. Higher pyrolysis temperatures have always led to higher gas volume yields, as observed in many previous studies [60; 61; 62]. We confirmed that the highest average mass yield was observed during the pyrolysis of plastics. In conclusion, the most suitable combinations of materials and temperature from TGA for the production of pyrolysis oils and gases are the 3 TP:SS and 4 LDPE:SS waste blends. Further research will focus on applied pyrolysis in a semi-pilot plant unit with analyses of outputs. The following are available online at [[https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)]([https://www.mdpi.com/2076-341](https://www.mdpi.com/2076-341)) 7/11/4/1610/s1, Figure S1: Temperatures of maximum weight losses in TGA experiments for inlet waste materials, Figure S2: TGA reproducibility of LDPE waste material, Figure S3: Temperatures of maximum weight losses in thermal TGA experiments for waste blends, Figure S4: Comparison of mass change measured by TGA and by the standard method in a muffle oven, Figure S5: Comparison of mass change measured by TGA and by the standard method in a muffle oven, Table S1: Analytical results of input waste materials, Table S2: Analytical results of waste blends, Table S3: Comparable groups. Conceptualization, L.K. and I.J. ; methodology, L.K. and P.M.; software, P.S. ; validation, I.J. and R.S., formal analysis, I.J. and S.D. ; investigation, L.K., M.D. ; resources, R.S. ; data curation, L.K. and P.S. ; writing--original draft preparation, L.K., J. and P.M.; writing review and editing, L.K., S.D., S.H. and M.D. ; visualization, P.M.; supervision, S.H., I.J. and L.K. ; project administration, S.D. ; funding acquisition, S.H., M.D. All authors have read and agreed to the published version of the manuscript. This research received no external funding. Not applicable.