# Greenhouse Residues' Potential for Biogas Production
## 3 Results
### BMP Evaluation  
The BMP varied between 210 and 357 NmL g\({}^{-1}\) VS or 170 and 289 NmL g\({}^{-1}\) COD (Figure 2). There is no correlation between the time of sampling (March, May, August, end of crop), the type of residue (leaves, suckers, stalks), and the plant (tomato, cucumber) with the BMP, either expressed per g of VS or g of COD. Comparing the maximum yield of 350 NmL of CH\({}_{4}\) per g of COD consumed with the BMP values expressed in terms of g COD (Figure 2) reveals the high anaerobic biodegradability of the various residue types (up to 82%). In all cases, the suckers had the highest methane yield compared to the leaves and stalks of the two plants. The highest yield expressed per VS was found in cucumber suckers (357.1 \(\pm\) 33.8 NmL CH\({}_{4}\) g\({}^{-1}\) VS), while tomato suckers had the highest yield when expressed per FM. These results were consistent with the chemical analysis, where the VS and COD content of the suckers were higher for both plants, while tomato leaves were the lowest in content (Table 3). Specifically, the methane potential of tomato suckers was 23.3% higher than their leaves, and was 21.1% in the cucumber's case. In total, the methane yield of the cucumber's residues was 16.6% higher than the total yield of the tomatoes. \begin{table}
\begin{tabular}{c c c c c c c} \hline  & **T.L.** & **T.S.** & **T.St.** & **C.L.** & **C.S.** & **C.St.** \\ \hline Humidity (\%) & 89.6 \(\pm\) 0.6 & 88.8 \(\pm\) 3 & 85.8 \(\pm\) 1.7 & 91.6 \(\pm\) 1.3 & 91 \(\pm\) 1.9 & 92.3 \(\pm\) 2.8 \\ TS (\% FM) & 10.4 \(\pm\) 0.6 & 11.2 \(\pm\) 3 & 14.2 \(\pm\) 1.7 & 8.4 \(\pm\) 1.3 & 9 \(\pm\) 1.9 & 7.7 \(\pm\) 2.8 \\ VS (\% TS) & 69.5 \(\pm\) 2.6 & 85.9 \(\pm\) 2.5 & 86.1 \(\pm\) 1.4 & 71.6 \(\pm\) 4.3 & 83.8 \(\pm\) 1 & 81.3 \(\pm\) 0.5 \\ COD (g kg\({}^{-1}\) VS) & 1179 \(\pm\) 64 & 1239 \(\pm\) 129 & 1215 \(\pm\) 7 & 1251 \(\pm\) 83 & 1304 \(\pm\) 49 & 1157 \(\pm\) 14 \\ TKN (g kg\({}^{-1}\) VS) & 38 \(\pm\) 3.9 & 60.4 \(\pm\) 4.5 & 11.7 \(\pm\) 0.5 & 47.3 \(\pm\) 2.1 & 62.6 \(\pm\) 7.4 & 15.3 \(\pm\) 1.2 \\ P (g kg\({}^{-1}\) VS) & 69.1 \(\pm\) 16 & 70.5 \(\pm\) 18 & 26.8 \(\pm\) 0.5 & 80.5 \(\pm\) 25.4 & 110.9 \(\pm\) 18 & 47.5 \(\pm\) 5 \\ Acid Insoluble Acid-insoluble lignin (\% e.f.s.) & & & 10 \(\pm\) 1.1 & & 10.1 \(\pm\) 1.4 \\ Acid-soluble lignin (\%e.f.s.) & & & n.d. & & 16.4 \(\pm\) 0.6 \\ \hline \end{tabular}
\end{table} 
Table 2: Mean values and standard deviations of the physicochemical characteristics of the residual biomass. By eliminating the seasonality factor from the evaluation of the BMP, all BMP values were taken into account per residue type and averaged (Figure 3a). The null hypothesis of all BMPs being equal was rejected at a 95% level (\(p\) = 1.8 \(\times\) 10\({}^{-6}\)) according to the one-way analysis of variance (ANOVA) test. By performing a \(t\)-test pairwise, it was found that the BMP of the leaves was statistically different from the BMP of the suckers (\(p\) = 0.027 and \(p\) = 9.3 \(\times\) 10\({}^{-5}\) for the tomatoes and cucumbers, respectively). The BMP of the cucumber leaves was statistically different from the BMP of the tomato leaves (\(p\) = 0.0058), but this was not the case for the BMP of the cucumber and tomato suckers (\(p\) = 0.081). By comparing the leaves/suckers with the stalks, it was found that the BMP of the tomato suckers and the cucumber leaves and suckers was statistically different from the BMP of the stalks (\(p\) < 0.05). Only the BMP of the tomato leaves and tomato and cucumber stalks were statistically similar. Finally, no statistical difference was recorded for the BMP between the tomato and cucumber stalks (\(p\) = 0.804). Although the BMP expressed per mass of VS indicates that these residues are promising feedstocks in a biogas plant, the low TS content (or high moisture content) results in a low energy density expressed as NL of methane per kg of fresh matter (Figure 3b).