# Biogas Production Enhancement through Chicken Manure Co-Digestion with Pig Fat
## 3 Results and Discussion
### Fat Influence on Digestated Substrate Changes
#### 3.5.2 TS Reduction and VS Consumption  
The TS reduction and VS consumption in the digested substrate after being drained from the reactor are given in Figure 6. The best performance of TS reduction and VS consumption was observed in the stage of 4.0 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\). The TS reduction and the VS consumption were the greatest and reached 70.3 and 74.7%, respectively. However, the worst performance of TS reduction and VS consumption was observed in the stage of 4.5 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\). At this OLR, we expected to get much better TS reduction and VS consumption because biogas yield reached the highest level of the entire experiment. However, in our experiments, we found the VS consumption at OLR of 4.5 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\) less effective than at organic load rates of 3.5 k\({}^{8}\) and 4.0 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\). The TS reduction and the VS consumption at OLR 4.5 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\) reached 63.1 and 66.4%, respectively. It was even below what was observed at the lowest OLR of 3.0 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\) when only diluted chicken manure was digested. This indication of worse digestion stopped us from going to higher OLR, avoiding reactor overload. Overload of the reactor with further OLR increases can lead to decreased biogas production. Other authors [49; 50; 75; 76] described such reactor overloading processes in their studies. In our study, the best performance of TS reduction and VS consumption was at the OLR of 4.0 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\).  
The changes in the TS and VS concentration at the end of each research experiment are given in Table 4. Both parameters indicate around the same values in the first three experiments (OLR3.0, OLR3.5, and OLR4.0): TS reduction is around 5.43%, and VS consumption is about 3.70%. In the last experiment at OLR of 4.5 kg VS-(m\({}^{3}\cdot\)day)\({}^{-1}\), TS reduction and VS consumption became significantly worse at 7.11 and 5.03, respectively. However, the biogas production was still increasing, but a further increase in TS and VS concentration in the digestate could impact biogas production.  
The similar change in the tendency and character of TS and VS justifies that TS and VS are relevant parameters. The results of TS measurement we can obtain within a few hours after the digestate is drained, the instruments for TS evaluation are pretty simple, not expensive, and every biogas plant and laboratory can afford to have them on hand. However, the VS measurement results are more complicated, requiring costly instruments, and obtaining the results after a few days. Many biogas plants and even some academic laboratories cannot afford VS analysers and buy outsourcing services from larger-scale laboratories instead. Obtaining analysis this way extends VS measurement time and does not allow the making of any rapid corrections to the digestion process if VS analysis indicates the bad performance of the digestion process. Relying on our experiment and following other authors [76], we recommend monitoring TS reduction constantly or regularly as often as possible in real-scale biogas plants as a measure for reactor overload prevention.