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The valorization study of the largely available corn stover waste biomass after pretreatment with deep eutectic solvent (DES) for biomethane production in one-liter glass bioreactors by anaerobic digestion for 21 days was presented. Ammonium thiocyanate and urea deep eutectic solvent pretreatments under different conditions in terms of the components ratio and temperature were examined on corn stover waste biomass. The lignocellulose biomass was characterized in detail for its chemistry and morphology to determine the effect of the pretreatment on the natural biocomposite. Furthermore, the implications on biomethane production through anaerobic digestion with different loadings of corn stover biomass at 35 g/L and 50 g/L were tested. The results showed an increase of 48% for a cumulative biomethane production for a DES-pretreated biomass, using a solid-to-liquid ratio of 1:2 at 100 °C for 60 min, which is a strong indication that DES-pretreatment significantly enhanced biomethane production.

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The need to embrace a circular economy model for sustainable growth and development is increasing due to the rise in human population and the dwindling natural resources available to meet the demands for energy and food. In this study, anaerobic digestion of rice husk (RH) was carried out under mesophilic conditions to produce biogas and digestates. Two particle sizes (300 and 600 μm) and three dilution ratios (1:4, 1:6, and 1:8) were employed to determine the optimum conditions for biogas production. The best anaerobic digesters (300 μm/1:6 and 600 μm/1:4) in each of the categories produced a cumulative biogas of 3205 + 290 mL and 2310 + 320 mL, respectively. The digestates were separated into solid and liquid fractions and characterized to evaluate their potential as biofertilizers and nutrient sources for microalgae cultivation. The nitrogen and phosphorus contents of the solid fractions (1.00 ± 0.01 and 0.97 ± 0.04) were significantly higher (p < 0.05) than the liquid fractions whereas the liquid fractions had a higher potassium content than the solid fractions. The absence of heavy metals in the digestates confirmed their safety as biofertilizers. The pH values of 4.70 and 5.50 reported in this study for liquid digestates are appropriate for the cultivation of some strains of microalgae that thrive in an acidic medium. The ammonium nitrogen contents of the liquid digestates (0.03% + 0.00% and 0.04% ± 0.00%) were moderate and not as high as some values reported to inhibit the growth of some species of microalgae. However, the brownish color of the liquid digestates could impair microalgae growth; thus, there is a need for dilution to increase light penetration.

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