# Biogas Production Enhancement through Chicken Manure Co-Digestion with Pig Fat
## 4 Conclusions  
Though fat is a biodegradable organic material, it decomposes very slowly, and anaerobic digestion of single fat is impossible. However, the research results showed that pure pig fat used in co-digestion with chicken manure was feasible, justifying fat waste utilisation possibility through anaerobic co-digestion. Pig fat as a supplement for primary substrate increased organic load rate in the biogas reactor and significantly enhanced biogas production, contributing to biogas yield, methane concentration, and overall methane yield increase in the biogas at all organic load rates investigated. In our case, fat addition directly and significantly impacted the rise of biomass energy obtained. The highest biogas yield of 246.9 \(\pm\) 5.8 L\(\cdot\)kg VS\({}^{-1}\), the highest methane concentration of 62.3 \(\pm\) 0.5%, and the highest biomass energy of 6.43 \(\pm\) 0.05 MJ\(\cdot\)kg FM \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Parameter**} & \multicolumn{4}{c}{**Experiment**} \\ \cline{2-5}  & **OLR3.0** & **OLR3.5** & **OLR4.0** & **OLR4.5** \\ \hline TS, \% & 5.28 \(\pm\) 0.46 & 5.68 \(\pm\) 0.31 & 5.32 \(\pm\) 0.21 & 7.11 \(\pm\) 0.59 \\ VS, \% & 3.95 \(\pm\) 0.28 & 3.72 \(\pm\) 0.19 & 3.42 \(\pm\) 0.12 & 5.03 \(\pm\) 0.22 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Digestate TS and VS concentration. Figure 6: TS reduction and VS consumption. obtained corresponded to the highest OLR of 4.5 kg VS-(m\({}^{3}\)-day)\({}^{-1}\). However, the increase in the biogas calorific value was weak because fat addition did not change the methane concentration significantly. The inhibitory indicators such as hydrogen sulphide concentration in biogas and total ammonia nitrogen in the digestate were presented in low concentrations and did not play an essential role in biogas production quality. The pH of digestate being on a stable level (varied in the range of 7.8-8.0) also justified that fat addition did not show any signs of possible process inhibition. Despite overall biogas production increase at each OLR investigated, the TS reduction and VS consumption were the greatest at an OLR of 4.0 kg VS-(m\({}^{3}\)-day)\({}^{-1}\), reaching 70.3 and 74.7%, respectively. However, the TS reduction and the VS consumption dropped at OLR of 4.5 kg VS-(m\({}^{3}\)-day)\({}^{-1}\) to the lowest level of the entire research, reaching 63.1 and 66.4% only. This drop implied further OLR increases, with such pig fat addition approaches to limit the effective digestion process and, subsequently, biogas production. According to our research results, the TS reduction and VS consumption could be highlighted as the monitoring indicators for biogas production performance as an additional measure for predicting the efficiency of substrate co-digestion. Optimising costs of expensive substrate material analysis, TS only can be analysed by controlling digestion process efficiency instead of both (TS and VS) because TS and VS changes in the digestate have the same tendencies and are related parameters. In our work, waste management problems, the demand for renewable energy, and the lack of appropriate utilisation technology of fats still exist. The extensive literature on the so-called FOG (fat, oil, grease) utilisation issues shows that scientists are still searching for new, sustainable, and effective ways to solve such global problems. Our research targeted the implementation of anaerobic co-digestion in existing and new-built chicken manure driven biogas plants. The results of our work provide new knowledge about the process investigated, revealed parameters to be controlled for obtaining additional value in a commercial scale, and environmental benefits in having a new option to solve the FOG utilisation problem. Conceptualization, K.N. and E.B. ; methodology, E.B. and B.\(\check{Z}\). ; investigation, M.R. ; resources, E.B. and V.Z. ; writing--original draft preparation, E.B. ; writing--review and editing, K.V. All authors have read and agreed to the published version of the manuscript.