**Robert Hren, Aleksandra Petroviˇc, Lidija Cuˇ ˇ cek \* and Marjana Simoniˇc**

Faculty of Chemistry and Chemical Engineering, University of Maribor, Smetanova ulica 17, SI-2000 Maribor, Slovenia; robert.hren1@student.um.si (R.H.); aleksandra.petrovic@um.si (A.P.); marjana.simonic@um.si (M.S.)

**\*** Correspondence: lidija.cucek@um.si; Tel.: +386-2-229-44-54

Received: 3 April 2020; Accepted: 1 May 2020; Published: 4 May 2020

**Abstract:** Pretreatment of waste materials could help in more efficient waste management. Various pretreatment methods exist, each one having its own advantages and disadvantages. Moreover, a certain pretreatment technique might be efficient and economical for one feedstock while not for another. Thus, it is important to analyze how parameters change during pretreatment. In this study, two different pretreatment techniques were applied: thermal at lower and higher temperatures (38.6 ◦C and 80 ◦C) and biological, using cattle rumen fluid at ruminal temperature (≈38.6 ◦C). Two different feedstock materials were chosen: sewage sludge and riverbank grass (*Typha latifolia*), and their combinations (in a ratio of 1:1) were also analyzed. Various parameters were analyzed in the liquid phase before and after pretreatment, and in the gas phase after pretreatment. In the liquid phase, some of the parameters that are relevant to water quality were measured, while in the gas phase composition of biogas was measured. The results showed that most of the parameters significantly changed during pretreatments and that lower temperature thermal and/or biological treatment of grass and sludge is suggested for further applications.

**Keywords:** waste materials; sewage sludge; riverbank grass; rumen fluid; pretreatment of waste; determination of parameters
