Thrace Greenhouses S.A. cultivates 5000 t y−1 cucumbers and 3500 t y−1 tomatoes hydroponically on 170,000 m2 of land. During cultivation (from February to November), suckers and leaves are removed regularly, producing 7,000 – 8,000 t y−1 residual green biomass. The anaerobic digestion technology was applied at laboratory scale, to explore the potential of valorising the generated residues for biogas production. The latter ranged between 18 and 30 NL methane per kg of fresh biomass (suckers or leaves, respectively). Considering a typical mixture of suckers and leaves produced during the cultivating season, the methane produced annually was estimated at 140,000 m3. The biogas can be used as a fuel in a combined heat and power (CHP) system (cogeneration). CHP was considered the central unit in a hybrid energy system (HES) integrated with batteries, aiming to provide energy autonomy to the greenhouses. Moreover, the flue gas from the CHP containing 10% CO2 and 9% O2 could be used as a source of CO2 in the greenhouses. In this case, trace gases (NOx: 401±37 ppm, NO:168±15 ppm, NO2: 150±18ppm, CO: 401±60, SO2: 1.4±2.2) present in the flue gas should be removed and/or diluted with air before entering the greenhouses.

Agricultural residues are intensively evaluated as potential feedstocks for biogas plants. Vegetable crops generate massive residues during and at the end of their growing seasons. A greenhouse facility in Greece, Thrace Greenhouses S.A., generates 7000–8000 t per year of residual green biomass, resulting from the hydroponic cultivation of tomatoes and cucumbers on 170,000 m2 of land from February to November. The crop residues included leaves, suckers, and stalks. The biochemical methane potential (BMP) estimation was realized on samples taken in March, May, and August, as well as at the end of cultivation (November). Suckers, leaves, and stalks of both plants yielded a range of 221–357, 210–296, and 225–250 NL kg−1 VS, respectively. t-test statistical analysis showed that the BMP of the leaves and suckers were statistically different for tomato and cucumber plants. The BMP of stalks was lower than the other residue types except for the tomato leaves. The diauxic behavior of the specifi‘1c methane production curves indicated that the two-phase Gompertz model (TGM) was the most suitable. The model fitting showed that leaves and suckers, in spite of having a higher BMP than the stalks, exhibited a lower maximum specific methane production rate constant than the stalks during the first phase, which may indicate the presence of inhibitory or slowly biodegradable compounds in leaves and suckers in comparison to the stalks.

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