# Biogas Production Enhancement through Chicken Manure Co-Digestion with Pig Fat
## 3 Results and Discussion
### Biogas Production Indicators  
The change of the methane yield M\({}_{\rm fm}\), specific methane yield M\({}_{\rm bs}\), and M\({}_{\rm ws}\) on an average daily biogas basis at different OLRs is given in Figure 4. In our study, the methane yield M\({}_{\rm fm}\) (Figure 4a), the methane yield M\({}_{\rm ts}\) (Figure 4b), and the methane yield M\({}_{\rm ws}\) (Figure 4c) increased at every OLR after fat addition. The highest methane yield obtained from the daily biogas basis corresponded to the highest OLR of 4.5 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\). At this OLR, the M\({}_{\rm fm}\), M\({}_{\rm ts}\), and M\({}_{\rm ws}\) were on average 154.6 \(\pm\) 3.1 L-kg FM\({}^{-1}\), 343.2 \(\pm\) 6.8 L-kg TS\({}^{-1}\) and 441 \(\pm\) 7.1 L-kg VS\({}^{-1}\), respectively. However, the change of FM mass (biomass) at this OLR was comparatively low because the added fat increased the FM mass by 5.26% only. The experiment results also showed that the methane production curve resembled the biogas production curve. This similarity was due to the counting of methane yield by Formula (4), where two essential components (biogas yield and methane concentration) played a role and determined methane yield. Biogas yield increased more considerable than methane concentration, and the methane yield curve repeated the curve character of biogas yield. Although methane concentration increased too, its increase was too weak to remarkably influence methane yield curve character. According to the studies reviewed [66; 67; 68; 69], carbon-rich material addition to the digested substance significantly affected the methane concentration increase, but we could not increase methane concentration in our experiment as expected. From our point of view, the C\({}_{\rm m}\) is a very important parameter if biogas is used in biogas upgrading and biomethane is produced. As much methane raw biogas consists of less energy and other expenditures,  
Figure 4: Methane yield: (**a**) CH\({}_{4}\) yield from FM; (**b**) specific CH\({}_{4}\) yield from TS; (**c**) specific CH\({}_{4}\) yield from VS.  
we will need further methane purification technologies. Despite the fact that we had a low increase of C\({}_{\rm m}\), overall, the methane yield increase met our expectations. Figure 5 illustrates the average energy value of biogas obtained, E\({}_{\rm b}\), and the average energy value obtained from feeding material E\({}_{\rm fm}\) at different OLRs was investigated. In evaluating the energy potential of biogas produced, the experiment showed that the energy of one cubic meter of biogas increased at every OLR. The result was the best at the highest OLR of 4.5 kg\(\cdot\)(m\({}^{3}\cdot\)day)\({}^{-1}\), reaching 25.9 \(\pm\) 0.2 MJ\(\cdot\)(m\({}^{3}\))\({}^{-1}\). The highest biomass energy value obtained from one kilogram of feeding material was 6.43 \(\pm\) 0.05 MJ\(\cdot\)kgFM\({}^{-1}\) at the same OLR of 4.5 kg\(\cdot\)(m\({}^{3}\cdot\)day)\({}^{-1}\). The reference energy values were obtained in the first stage at OLR 3.0 kg\(\cdot\)(m\({}^{3}\cdot\)day)\({}^{-1}\); 23.1 \(\pm\) 0.6 MJ\(\cdot\)(m\({}^{3}\))\({}^{-1}\), and 2.63 \(\pm\) 0.07 MJ\(\cdot\)kgFM\({}^{-1}\), respectively. In our study, the energy of biogas increased at every OLR. This increase was not proportional to the rise of fat addition, showing a synergistic relationship between the primary feeding material and added supplement. The other studies also describe a similar synergetic phenomenon [70,71]. Basically, fat addition to chicken manure improved biogas production, and an increase in many parameters were observed (Table 3). According to the methodology of this study, the increase (in percent) of OLR and the feeding material mass directly depended on added fat quantity. Still, the specific methane yield Mvs was not proportional to OLR increase. At OLR 3.5 kg\(\cdot\)(m\({}^{3}\cdot\)day)\({}^{-1}\), the Mvs and OLR ratio was around 3:2, but at the OLR 4.5 kg\(\cdot\)(m\({}^{3}\cdot\)day)\({}^{-1}\), this ratio became much higher and reached more than 4:2.