# Biomethane Potential Test: Influence of Inoculum and the Digestion System
## 3 Results and Discussion
### Impact on Specific Methane Yield and Methane Percentage
#### 3.2.2 Effect of the Inoculum  
Figure 3A shows the specific methane yield for all inoculum/substrate combinations tested in digestion system D-HBT1. In Figure 3B, inoculum I-LRS is used as a reference and the specific methane yield of all the other inocula is related to the mean value of I-LRS. The deviation of the specific methane yield when using different inocula differed for each substrate as also shown in other publications [17; 18]. The CV differed between 1.9% and 4.8%. Substrate S-HAY is the only substrate that showed no statistical differences for different inocula in the results. Substrates S-HAY and S-CON had the lowest CV, both with 2.3% and 1.9%. Both substrates were homogenous standard substrates, which could be the reason for the small CV. Substrates S-DMS has two well adapted inocula, namely I-BGP and I-LRD, characterized by a higher percentage of _Firmicutes_. However, both produced a significantly lower yield than I-WWP7. Other studies report that inocula, which have already adapted to the feedstock, produce a higher methane yield [16; 18]. This cannot be confirmed by our data and more research will be needed to clarify the reasons. \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **I-LRS** & **I-BGP** & **I-LRD** & **I-WWP7** & **I-WWP14** \\ \hline \multicolumn{5}{c}{**Specific methane yield, L**} \\ \multicolumn{5}{c}{**kg\({}^{-1}\)**} \\ Blank & \(22\pm 1^{\text{a}}\) & \(48\pm 1^{\text{a}}\) & \(77\pm 3^{\text{a}}\) & \(66\pm 1^{\text{a}}\) & \(60\pm 3^{\text{a}}\) \\ Blank + TE & \(20\pm 1^{\text{a}}\) & \(49\pm 1^{\text{a}}\) & \(81\pm 4^{\text{a}}\) & \(65\pm 2^{\text{a}}\) & \(58\pm 4^{\text{a}}\) \\ S-DMS & \(361\pm 4^{\text{a}}\) & \(342\pm 8^{\text{a}}\) & \(346\pm 4^{\text{a}}\) & \(354\pm 7^{\text{a}}\) & \(354\pm 10^{\text{a}}\) \\ S-DMS + TE & \(357\pm 10^{\text{a}}\) & \(343\pm 10^{\text{a}}\) & \(341\pm 9^{\text{a}}\) & \(371\pm 11^{\text{a}}\) & \(349\pm 11^{\text{a}}\) \\ \hline \multicolumn{5}{c}{**Methane percentage, \%**} \\ Blank & \(64\pm 2^{\text{a}}\) & \(56\pm 1^{\text{a}}\) & \(58\pm 0^{\text{a}}\) & \(73\pm 1^{\text{a}}\) & \(78\pm 1^{\text{a}}\) \\ Blank + TE & \(68\pm 2^{\text{a}}\) & \(56\pm 1^{\text{a}}\) & \(59\pm 1^{\text{a}}\) & \(72\pm 0^{\text{a}}\) & \(79\pmResults of S-MCC showed statistical differences between I-WWP and all other inocula, which is also reflected in the highest CV (4.8%). [PERSON] et al. [18] also reveals the biggest CV for S-MCC (around 5%). Yields achieved with I-LRS and I-BGP were around 370 LCH4 \(\text{kg}^{-1}\text{{}_{\text{oDM}}}\). This value is expected as it falls within the range of average values between 363 and 371 LCH4 \(\text{kg}^{-1}\text{{}_{\text{oDM}}}\) determined by calculation and inter-laboratory tests [7]. It is also 99% of the theoretical value [5]. Inocula I-WWP7 and I-WWP14 produced more methane, reaching 384 and 393 LCH4 \(\text{kg}^{-1}\text{{}_{\text{oDM}}}\), respectively, which is 103-105% of the theoretical value. However, [PERSON] et al. [49] also report such a high spec. methane yield (392 LCH4 \(\text{kg}^{-1}\text{{}_{\text{oDM}}}\)) for substrate S-MCC. One possible reason could be the lack of carbon in inocula I-WWP7 and I-WWP14. Adding S-MCC as a source of carbon might improve the C/N-ratio and lead to a higher degradation of the inoculum. Values of CV up to 4.8% can be explained by various influences. All the inocula had different microbial community compositions. This can result in a variance of substrate consumption for microbial growth [17]. Additionally, a stimulation of inocula by adding a carbon-rich substrate can result in a higher methane potential of the inoculum than measured in the blanks. An overestimation of the methane yield is a possible consequence [18]. The methane percentage of the different inocula is presented in Figure 3C. The CV between the inocula was less than 3.3%. The methane percentage showed an almost similar behavior for all substrates. Inocula I-LRS and I-LRD did not show statistically significant differences to each other.