## 2 Materials and Methods
### Analytical Monitoring of the Process  
The separated ORF after autoclaving was analyzed in terms of pH, COD, total nitrogen, ammonium nitrogen, phosphorus, total solids and volatile solids. The analyses were performed by the Institute of Soil Science and Plant Cultivation (IUNG), State Research Institute in Pulawy, Poland. As presented above, at the start of the experiment, the inoculum was analyzed to measure the pH, COD, ammonium, alkalinity, total solids concentration and phosphates.  
Figure 2: Experimental stand. (1) Electric engine, (2) fermenter with tooling, (3) shutof valve/substrate feeder, (4) manometer, (5) biogas flow meter, (6) biogas outlet, (7) water jacket feed valve, (8) watercoat, (9) reactor chamber, (10) stirrers, (11) PT-100 temperature sensor, (12) shutof valve/post-fermentation outlet, (13) water jacket drain valve.  
The post-fermentation residues were analyzed in terms of total solids and volatile solids concentration. The liquid phase of the post-fermentation residues (obtained by centrifuging the digestate samples for 10 min at 9000 rpm) was analyzed with regard to pH, COD, ammonium nitrogen, phosphates and volatile fatty acids (VFAs). The analyses of post-fermentation residues were performed at one-day intervals during the first 20 days and then at two-day intervals.  
Analyses of total solids, volatile solids, COD, alkalinity, ammonium nitrogen and phosphates conducted by IUNG and in our laboratories were all performed according to standard methods for examination of water and wastewater, APHA [17]. According to the methodology of the IUNG, total nitrogen was analyzed by elemental analysis using a FLASH 2000 elemental analyzer (Thermo Scientific, USA), phosphorus content was analyzed by the spectrophotometric method based on an ammonium molybdate reaction.  
VFAs (acetate, propionate, isobutyrate, butyrate, isovalerate, valerate, isocaproate and caproate) were analyzed using a gas chromatograph (GC, Varian 3800, Australia) equipped with a capillary column (Factor-Four VF-1 ms, 30 m \(\times\) 0.25 mm i.d., 1.0 lm film; Varian). A volatile acid standard mix (Supelco, USA) was used as the standard. The flame ionization detector (FID) was operated at 280 \({}^{\circ}\)C with an injection port temperature of 250 \({}^{\circ}\)C. The split ratio was 1:100. The initial column temperature was set to 100 \({}^{\circ}\)C, then raised at a rate of 20 \({}^{\circ}\)C/min to 200 \({}^{\circ}\)C and finally held at 200 \({}^{\circ}\)C for 1 min. Helium was used as the carrier gas at a flow rate of 1.0 mL/min. Samples were prepared for VFA analysis according to [PERSON] et al. (2010).  
To determine the biological stability of the ORF, the AT4 analysis was performed. The AT4 analysis (performed in triplicate) is a respiration test employing a nanometric method, which was performed with the Oxitop Control measuring system. Waste samples were incubated at 20 \({}^{\circ}\)C in the presence of a 1M sodium hydroxide solution (for CO\({}_{2}\) absorption) for four days.  
The volume and composition of the biogas produced during fermentation were monitored. This analysis was carried out daily on averaged samples collected in Tedlar bags. The gas composition, including oxygen, carbon dioxide and methane content, was measured using a GA 2000 PLUS gas analyzer (Geotech).