# Greenhouse Residues' Potential for Biogas Production
## 5 Conclusions  
Tomato and cucumber residues are produced massively from hydroponic greenhouses throughout the cultivating period starting from January and ending in November, especially from April to August. The primary part of residues consists of leaves and suckers, while the whole plant (consisting mainly of stalks) is removed at the end of the cultivating season. The BMP of tomato residues was assessed to be 242.0 \(\pm\) 33.1, 298.3 \(\pm\) 56.9, and 233.1 \(\pm\) 27.2 NL CH\({}_{4}\) kg\({}^{-1}\) VS for leaves, suckers, and stalks, respectively, while the BMP of cucumber residues was found to be 284.9 \(\pm\) 17.4, 344.9 \(\pm\) 21.9, and 229.6 \(\pm\) 16.8 NL CH\({}_{4}\) kg\({}^{-1}\) VS for the leaves, suckers, and stalks, respectively. Fitting the BMP curves with the two-phase modified Gompetz model showed that a diauxic methane production occurred. The methane production was at a higher rate in the first than in the second phase, suggesting a readily biodegradable fraction in the biomass. Moreover, it is worthwhile to mention that the methane produced from the stalks took place faster in the first phase, although the stalks are considered less biodegradable due to their lignin content. Therefore, there may be inhibitory compounds in the leaves and suckers that slow the process but do not limit the ultimate methane yield.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app13095445/s1](https://www.mdpi.com/article/10.3390/app13095445/s1)]([https://www.mdpi.com/article/10.3390/app13095445/s1](https://www.mdpi.com/article/10.3390/app13095445/s1)), Figure S1: Replicate BMP curves of samples from tomato leaves; Table S1: Parameter values of the TGM fitting the BMP curves of samples from tomato leaves; Figure S2: Replicate BMP curves of samples from tomato suckers; Table S2: Parametervalues of the TGM fitting the BMP curves of samples from tomato suckers; Figure S3: Replicate BMP curves of samples from cucumber leaves; Table S3: Parameter values of the TGM fitting the BMP curves of samples from cucumber leaves; Figure S4: Replicate BMP curves of samples from cucumber suckers; Table S4: Parameter values of the TGM fitting the BMP curves of samples from cucumber suckers; Figure S5: Replicate BMP curves of samples from tomato and cucumber stalks after removal of the whole plant; Table S5: Parameter values of the TGM fitting the BMP curves of samples from tomato and cucumber stalks after removal of the whole plant.  
Methodology, K.G. and K.S.; software, K.G. and K.S.; investigation, K.G.; resources, A.P.; writing--original draft preparation K.G., M.M. and K.S.; writing--review and editing, K.G. and K.S.; supervision, K.S. All authors have read and agreed to the published version of the manuscript.  
This research was primarily funded by the Hellenic Foundation for Research and Innovation (H.F.R.I.) under the \"Third Call for H.F.R.I. Scholarships for PhD Candidates\" (project: 68263). It was also financially supported by the Thrace Greenhouses SA, who were the beneficiary of the project titled \"Employing circular economy principles on the management of residual plant biomass from greenhouse crops\" within the Operational Programme \"Eastern Macedonia and Thrace\" 2014-2020, under Contract No. AM\(\Theta\)P7-0063005, co-funded by Greece and the European Union.  
Not applicable.  
The authors are enormously grateful to the Thrace Greenhouses S.A. ([[https://www.thracegreenhouses.com/gr/en/home/](https://www.thracegreenhouses.com/gr/en/home/)]([https://www.thracegreenhouses.com/gr/en/home/](https://www.thracegreenhouses.com/gr/en/home/))) for supplying the feedstocks used in the experiments.  
The authors declare no conflict of interest. The funders had no role in the design of the study, the collection, analyses, or interpretation of data, the writing of the manuscript, or the decision to publish the results.