Valorization of green market waste for sequential biohydrogen and biomethane production

High organic carbon wastes can be used for biofuel production for reducing raw material costs. Green market wastes (GMW) are one of the raw material candidates to generate biofuel by fermentation with low-cost production and high yield of fuel formation. Biohydrogen and biomethane are potential gaseous energy sources that can be obtained through sequential fermentation of GMW. In this study, the ground and hydrolyzed GWM (a mixture of lettuce, parsley, spring onion, and dill) solid-liquid mixture was first valorized for biohydrogen, then the organic acid-rich residue of biohydrogen fermentation was subjected to biomethane production. Biohydrogen and biomethane were produced at mesophilic (37 degrees C) and thermophilic (55 degrees C) temperatures. The initial GWM amount varied between 1000 and 5000 g ww at mesophilic biohydrogen production. The highest cumulative hydrogen formation (CHF) and the rate were obtained at 5000 g ww as CHF = 8.9 L and 916 mL H2 /L day, respectively. Thermophilic biohydrogen fermentation was conducted at 1000 and 5000 g ww and CHF reached 14.2 L at 5000 g ww GMW. The yield of hydrogen formation was 1.6 times higher at thermophilic than at mesophilic conditions. The residues from two fermentation temperatures of 5000 g ww GMW were used to produce biomethane under both fermentation conditions. Mesophilic and thermophilic conditions resulted in cumulative biomethane formations (CMF) of 8.3 L and 5.8 L, and biomethane production yields of 0.17 mL CH4/g VFA and 0.13 mL CH4/g VFA, respectively. The findings revealed that GMW is a suitable substrate for efficient biohydrogen and biomethane production.