Hyperthermophilic Treatment of Grass and Leaves to Produce Hydrogen, Methane and VFA-Rich Digestate: Preliminary Results

Abstract

In this study, the feasibility of hydrogen and methane production from grass and leaves via hyperthermophilic anaerobic digestion was investigated. The hyperthermophilic treatment of grass at 70 °C resulted in the highest concentrations of volatile fatty acids (TVFA) and reducing sugars in the supernatant of over 21 and 6.5 g/L reported on day 3 and 4 of the experiment. In contrast, hydrolysis and acidification of leaves performed slower and with lower efficiency, as the peak concentrations of TVFA and reducing sugars were observed at the end of the process. However, the highest cumulative hydrogen and methane yields of 69.64 mLH2/gVS and 38.63 mLCH4/gVS were reported for leaves digested at 70 °C, whereas the corresponding maximum productions observed for grass were 50 mLH2/gVS and 1.98 mLCH4/gVS, respectively. A temperature increase to 80 °C hampered hydrogen and methane production and also resulted in lower yields of volatile fatty acids, reducing sugars and ammonia as compared to the corresponding values reported for 70 °C.

Co-Digestion of Kitchen Waste with Grass and Leaves after Hyperthermophilic Pretreatment for Methane and Hydrogen Production

“…In previous studies, the hyperthermophilic preliminary treatment was found to considerably increase the methane and hydrogen yield from grass and leaves in the subsequent mesophilic and thermophilic step and overall process performance. On the other hand, the single-stage anaerobic mesophilic or thermophilic treatment of these materials is characterized by low efficiency due to the specific structure of grass and leaves, which are mainly composed of recalcitrant lignocellulosic compounds [18]. As mentioned above, the hyperthermophilic pretreatment was only applied to grass and leaves as they were much more resistant to biological treatment than kitchen waste.…”

“…Before closure, the top of each tank was flushed with nitrogen gas to provide anaerobic conditions. Considering the earlier experiences, the hyperthermophilic pretreatment time was set for 3 days [18]. After completion of the process, the digested materials were analyzed for pH, soluble COD, ammonium nitrogen, and volatile fatty acids.…”

Isolation and Use of Coprothermobacter spp. to Improve Anaerobic Thermophilic Digestion of Grass

“…The AD process with the addition of isolated bacteria was carried out in a similar manner to the process carried out in previous studies [ 23 ]. The AD was carried out in two variants: I—grass pretreated at 70 °C, mixed with water in the ratio 1:2, with the addition of bacteria (PGB); and II—raw grass mixed with water in the ratio 1:2, with the addition of bacteria (GB).…”