# Greenhouse Residues' Potential for Biogas Production
## 4 Discussion  
Most studies involving biogas production from vegetable residues concern either the lignocellulosic part of the plant or peels, pomace, and rotten vegetables. However, only a few have concentrated on the leaves and suckers. In the present study, the BMP of tomato residues (leaves, suckers, and stalks) was averaged at 242.0 \(\pm\) 33.1, 298.3 \(\pm\) 56.9, and 233.1 \(\pm\) 27.2 NL CH\({}_{4}\) kg\({}^{-1}\) VS, respectively, while the BMP of cucumber residues (leaves, suckers, and stalks) was averaged at 284.9 \(\pm\) 17.4, 344.9 \(\pm\) 21.9, and 229.6 \(\pm\) 16.8 NL CH\({}_{4}\) kg\({}^{-1}\) VS, respectively (Figure 3a). The averages were calculated from the BMP of all samples taken throughout the cultivating season. Statistical comparisons showed that the leaves yielded lower methane than the suckers for each crop. There was no difference between the BMP of suckers from tomatoes or cucumbers, but a statistical difference was found for the BMP of leaves between the two crops. The stalks of both crops yielded lower methane than leaves and suckers, except for the tomato leaves, which exhibited similar BMP as the stalks. It should be noted that the BMP of the tomato leaves collected in March was lower than all other samples and caused a high variance of the average calculated for the BMP of tomato leaves throughout the cultivating season. Finally, there was no statistical difference in the BMP between the tomato and cucumber stalks. As shown in the following discussion, the tomato and cucumber crop residues' BMP determined in the present work lie at the higher range of values reported in the literature for the tomato and cucumber, as well as for other vegetable crop residues. The BMP values are similar to what [PERSON] et al. [45] determined by using a mixture of stems, leaves, and stalks at fresh or ensiled conditions as residues from tomato and cucumber plants taken from a greenhouse of the University of Life Sciences in Lublin. The fresh and ensiled tomato biomass yielded 301 \(\pm\) 5 L CH\({}_{4}\) kg\({}^{-1}\) VS and 238 \(\pm\) 7 L CH\({}_{4}\) kg\({}^{-1}\) VS, respectively, while the fresh and ensiled cucumber biomass yielded 280 \(\pm\) 5 L CH\({}_{4}\) kg\({}^{-1}\) VS and 166 \(\pm\) 4 L CH\({}_{4}\) kg\({}^{-1}\) VS, respectively. The fresh residues used in their study and the leaves and suckers used in the present study had a similar BMP, but the stalks of both plants in the present study yielded a lower methane value. The TS content of the fresh residues in the study of [PERSON] et al. (10.77 \(\pm\) 1.11 and 17.42 \(\pm\) 0.54%), the VS content (74.70 \(\pm\) 7.17% TS and 81.97 \(\pm\) 0.41% TS), and the TKN content (4.38 \(\pm\) 0.16 and 3.47 \(\pm\) 0.13% TS) for cucumber and tomato fresh residues, respectively, were at similar levels with the results of the present work (Table 2), except for the tomato residues, which were drier (17.42 \(\pm\) 0.54%) in the study of [PERSON] et al. [45]. Moreover, the net heat energy production corresponding to the methane produced was evaluated according to [PERSON] et al. [45], who accounted a 20% utilization of the heat for heating the digester. The obtainable heat energy ranges from 4.84 (tomato leaves) to 8.32 (cucumber suckers) MJ kg\({}^{-1}\) TS and can be compared to the low heat calorific value of tomato and cucumber residues (12.60 \(\pm\) 0.47 and 11.12 \(\pm\) 0.53 MJ kg\({}^{-1}\) TS, respectively) as determined in [45]. [PERSON] et al. [57] reported lower yields, 124 \(\pm\) 6 L CH\({}_{4}\) kg\({}^{-1}\) VS from tomato (_Lycopersicon esculentum_ Mill.) crop residues and 118 \(\pm\) 5 L CH\({}_{4}\) kg\({}^{-1}\) VS from cucumber (_Cucumis sativus_ L.) crop residues from a greenhouse in China. The BMP vessels were not mixed continuously, which may have affected the yield. The residues included stems, vines, residual fruit, and leaves, which were drier (higher TS content of tomato and cucumber residues at 17.5 \(\pm\) 0.1 and 21.9 \(\pm\) 0.1%, respectively) than in the present work (7.7-14.2 \(\pm\) 0.6-3%).