# Biogas Production Enhancement through Chicken Manure Co-Digestion with Pig Fat
## 3 Results and Discussion
### Biogas Production at Different OLRs  
Biogas production was carried out in anaerobic digestion under mesophilic conditions. This digestion process involves a metabolism hierarchy between microbial populations, where the hydrolysing bacteria, acetogens, acidogens, and methanogens are the leading players. All microbes differ in morphology, optimum growth conditions, final product formation, sensitivity to changing microenvironments, and require personal conditions to survive. All those conditions are widely described in scientific literature, but authors continue new findings in relations between microbes and investigate obstacles in their lives [44; 45; 46; 47]. Therefore, the monitoring and controlling of essential biogas production parameters ensures a healthy balance between microbial populations responsible for different processes. The task of achieving a steady, effective, and productive anaerobic digestion process is a driver in finding improvements for biogas production in both our and other studies [48; 49; 50]. The first and highly important parameter of biogas production is the biogas yield at any biogas production scale. Figure 2 depicts the daily raw biogas production rate throughout the entire experiment. The four stages presented in the diagram of clustered columns indicate the biogas production change at different OLRs. In the first experiment stage, the biogas production started at OLR 3.0 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\) on the basis of a WWTP inoculum, adding diluted chicken manure. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Parameter Description**} & \multicolumn{2}{c}{**Material Analysed**} \\ \cline{2-3}  & **RM** & **FAT** \\ \hline Total solids (TS), \% & 37.76 \(\pm\) 0.62 & 99.98 \(\pm\) 0.01 \\ Volatile solids (VS), \% & 26.45 \(\pm\) 0.27 & 99.96 \(\pm\) 0.02 \\ Total carbon (C), \% & 26.8 \(\pm\) 0.3 & 86.0 \(\pm\) 0.1 \\ Total nitrogen (N), \% & 2.36 \(\pm\) 0.12 & 0.93 \(\pm\) 0.10 \\ C/N ratio & 11:1 & 93:1 \\ Total sulphur (S), \% & 0.39 \(\pm\) 0.28 & 3.16 \(\pm\) 0.15 \\ Ammoniacal nitrogen (TAN), \% & 0.55 \(\pm\) 0.15 & n/d \\ pH & 7.80 \(\pm\) 0.03 & n/d \\ \hline n/d—Not determined. & & \\ \end{tabular}
\end{table} 
Table 2: Physical and chemical parameters of raw material (wet matter basis). On the first day of the experiment, the biogas production rate was the lowest, but increased within the following days. However, the growth was not steep, and the biogas production range reached the highest value of 491.2 L/kg VS on the seventh day of the experiment. This slower growth of biogas yield can be explained due to the change of organic matter of feeding material from WWTP sewage to chicken manure. When the feeding material changes, the microbe population needs time to adapt to new organic structures and for the new population development, as reported by literature [34,35]. After a run-up of ten days, the biogas production rate stabilised at the average value of 434.2 \(\pm\) 34.7 L-kg VS\({}^{-1}\), which became a reference for further experiments. In the second stage, when OLR was increased to 3.5 kg VS-(m\({}^{3}\)-day)\({}^{-1}\) by fat addition, biogas production started slowly to grow and reached the highest value of 557.2 L -kg VS\({}^{-1}\), but finally settled on an average biogas production rate of 494.4 \(\pm\) 42.2 L-kg VS\({}^{-1}\). This slow growth of biogas production rate can be explained due to the change of organic matter of feeding material after fat addition. The microbe population needs time to adapt to new organic structures and time for the new population to be developed because microbe populations differ in different environments, as reported by [51]. Some studies reported this phenomenon [38,52,53] too, and according to the authors, there is a lag in the biological activity of microorganisms living on certain digestion products in the biogas reactor [43,51,54].