**4. Conclusions and Future Research**

In this study, thermal and biological pretreatment techniques were applied to two waste materials: sewage sludge and riverbank grass (*Typha latifolia*) and their combination (in the ratio 1:1). Lower and higher temperature thermal pretreatments (at 38.6 and 80 ◦C) and biological pretreatments (at a ruminal temperature of 38.6 ◦C) were studied. Various parameters were measured in the liquid and gas phases.

NPK, TOC and COD values, and the C/N ratio showed that low temperature thermal treatment is preferred, because of its better biodegradation characteristics of waste materials. pH values were mainly in the optimal range, while pretreated grass samples exhibited pH values below the optimal, which could be improved by biological pretreatment and mixing with other waste materials. Additionally, the results of the gas phase showed that the most suitable pretreatment technique(s) for further applications are low temperature thermal and/or biological treatment. The best result in the gas phase was obtained for biologically pretreated sludge, as it yielded the highest concentration of CH4 and the lowest concentration of H2S. Based on the results, it was found that pretreatment technique(s) should be carefully chosen, owing to some inhibitory effects at elevated temperatures, which could exert a negative impact on the further treatment of waste materials.

However, an important consideration for future studies is the further use of pretreated materials which contain sewage sludge, as it may contain heavy metals, pathogens and persistent organic pollutants (POPs). Thus, further handling of these materials is suggested, such as removal of heavy metals, extraction of POPs from sludge, reduction of pathogens and so on. Future studies could be aimed at further hydrolysis and fermentation of pretreated materials, and at identifying and reducing contaminants from treated sewage sludge.

**Author Contributions:** Conceptualization, L.C. and M.S.; methodology, L. ˇ C. and A.P; investigation, R.H. and A.P.; ˇ writing—original draft preparation, R.H. and A.P.; writing—review and editing, L.C. and M.S.; visualization, R.H. ˇ and A.P.; supervision, L.C.; project administration, A.P. and L. ˇ C.; funding acquisition, L. ˇ C. and A.P. All authors ˇ have read and agreed to the published version of the manuscript.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Funding:** The authors acknowledge financial support from the Slovenian Research Agency (research core funding No. P2-0412 and P2-0032 and project No. J7-1816) and the Slovenian Ministry of Education, Science and Sport (project No. C3330-19-952041). The authors also acknowledge support in chemical analytics and consultancy in the field of sewage sludge management from the IKEMA d.o.o. company.
