*2.1. Feedstock Preparation and Characterization*

The following feedstock materials were used in the experiments: riverbank grass *Typha latifolia* (G), sewage sludge (S) and rumen fluid (R). Sewage sludge was taken from a local municipal wastewater treatment plant (latitude and longitude coordinates of 46◦24 29.3 N and 15◦52 51.0 E) employing a tertiary biological treatment of wastewater in sequential basins with the capacity of 68,000 population equivalents (PE) and a maximal inflow of 350 L/s wastewater. The *Typha latifolia* grass (cattail) was gathered near the Dravinja riverbank near city Ptuj, Slovenia (coordinates of 46◦21 01.5 N and 15◦49 48.8 E). The *Typha latifolia* grass was chopped into pieces smaller than 1 × 0.5 cm before being used in experiments. Rumen fluid was acquired from a nearby slaughterhouse. Before it was used in experiments, it was filtered through fabric in order to remove larger particles. All feedstock materials were gathered in the middle of May 2019.

For the characterization of feedstock materials, the following analytical methods were used: SIST EN 16168:2013 for determining total nitrogen content using dry combustion [20], SIST EN ISO 11885:2009 by the inductively coupled plasma optical emission spectrometry (ICP-OES) analyzer for the total phosphorus content [21], SIST EN 13137:2002 for total organic carbon content based on catalytic oxidation combustion technique [22] and SIST EN 14346:2007 for the dry matter content [23]. The content of heavy metals was determined by the ICP-OES analyzer according to the EN 16170:2016 method [24]. For the determination of total dry solids content (TS), substrates were dried in a laboratory drying chamber (ED 115, producer Binder) at 105 ◦C until constant weight.
