# Biogas Production Enhancement through Chicken Manure Co-Digestion with Pig Fat
## 3 Results and Discussion
### Main Changes in Biogas Composition  
The methane concentration in biogas is undoubtedly the second important parameter of biogas production. However, any single gas in biogas composition plays a role when biogas is used either for burning, purification, or other purposes. Almost in all further use of biogas, because it is treated as a substitute for natural gas, biogas contaminants such as hydrogen sulphide (H\({}_{2}\)S) and water vapour have to be always removed. According  
Figure 2: Daily biogas production at different OLRs. to the literature [49], in conventional continuously fed biogas reactors or biogas plants under anaerobic conditions, the methane concentration usually varies between 55% and 65%. In some cases, the methane concentration can reach up to 70% [55], depending on feeding material composition, digestion conditions, and activity of the microbiological community [56; 57; 58]. In a review [59], we can even find numbers of methane concentrations reaching up to 80%. The CH4 and H2S concentrations (Cm and Cs) of daily biogas produced have changed throughout the research (Figure 3). In the stage of OLR 3.0 kg VS-(m3-day)\({}^{-1}\), the Cm was the highest at the beginning of the experiment, when we started feeding the rector with diluted chicken manure only, but within several days this slightly dropped, and the value settled on an average of 57.8 \(\pm\) 1.4%. This value of Cm, such as biogas production rate, was used as a reference for the following experiments. The drop of Cm value can be explained due to the change of organic matter of feeding material [34; 35], such as the run-up of biogas production rate was explained. In the second stage, when we added fat to the feeding material for an OLR increase to 3.5 kg VS-(m3-day)\({}^{-1}\), the Cm value lifted and, after reaching the highest level of 61.1%, settled on an average level of 59.0 \(\pm\) 1.3%. The ramping up of Cm value at the beginning of the stage was longer than in other stages, indicating the bacterial community change, referring to the publications [60; 61]. In the third stage, after OLR increased, the Cm value lifted too and reached the highest level of 63.2%. The lowest Cm value was reached at a level of 59.0%, and the average-level of 61.5 \(\pm\) 1.0%. In the fourth stage of the experiment, at OLR of 4.5 kg VS-(m3-day)\({}^{-1}\), we observed the highest average Cm value of 62.3 \(\pm\) 0.5%. Meanwhile, the Cs value in biogas had reversed curve character when compared to the Cm value curve. The H2S always presents in biogas because sulphur presents in feeding material in more or fewer amounts [35; 38; 49; 62; 63]. Even a low concentration of H2S is toxic to methanogens, and researchers are still trying to find techniques that could mitigate the sulphur impact on biogas production [62; 64]. This means that biogas production requires the constant monitoring of H2S concentration and finding control measures to avoid inhibition of the digestion process. Fat addition did not increase H2S concentration in biogas, and we did not need additional measures to be applied for H2S reduction. If the Cm value rose at any OLR increase, the Cs value dropped with every OLR increase in all our experiments. This decrease in Cs can be explained because the sulphur percentage in feeding material mass did not change after fat addition. The sulphur formation rate was left at the same level while the CH4 and CO2 production in biogas increased. This increased amount of biogas diluted H2S gas and diminished H2S concentration. The second parameter influencing the H2S concentration in biogas is the pH of the substrate, because the pH of the substrate determines gaseous H2S and ionic HS\({}^{-}\) relations in the substrate [65]. In our experiment, the H2S change strongly corresponded to the reverse of the Cm change (\(p<0.05\)). The authors of another study [35] claim similar results. In the first stage of our  
Figure 3: Changes of Cm and Cs values during experiments. research, the C\({}_{\rm s}\) value was the highest, but during the experiments almost halved.