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.

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 safe application 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.

Solid-state organosolv pretreatment was carried out on rice husk (RH) at various concentrations and temperatures for biogas enhancement. Pretreatments were conducted at 100 and 120°C for 60 min with acid-catalysed ethanol at 1, 3 and 5% concentration. Samples pretreated with 3% ethanol at 100 and 120°C for 60 min produced more biogas compared with raw RH, although the difference between the 3% pretreated sample at 120°C was not significant. The samples were designated in accordance with the ethanol concentration and reaction temperature. 3E100 produced 5545 ml of biogas followed by 3E120 with 2550 ml, whereas the control RH produced 2500 ml. First-order kinetic model and the modified Gompertz model were used to determine biodegradability constants and predicted cumulative biogas production. The Chen and Hashimoto model was used to determine the critical hydraulic retention time (HRT) for 3E100 to determine the minimum retention time needed to prevent washout when biogas production critical was run on a continuous basis. The biodegradability constant (k_o) ranged between 0.2688 and 0.9185 d⁻¹ with 3E100 having the highest value for k_o which confirmed optimum utilisation of substrate. Comparison between experimental and predicted data revealed that Gompertz model showed a good fit in describing cumulative biogas production of RH and pretreated RH at different concentrations and temperatures. HRT for 3E100 was calculated critical to be approximately 10 days. Pretreatment of RH at 3% concentration of ethanol at 100°C for 60 min greatly enhanced cumulative biogas production with 121.80% increase in comparison with the raw sample.

Production of fuels and chemicals from renewable biomass is crucial to sustainability of bio-based economy. In this study, anaerobic digestion of corn stover (CS) was carried out to produce biogas under mesophilic conditions. The study also investigated the potential of corn stover digestate ash as a potential source for biosilica production. A cumulative volume of 930 mL of biogas was produced with methane content of 85.87%. Higher and lower calorific values of biogas were calculated to be 36.30 MJ/m3 and 32.70 MJ/m3, respectively. Calcination of raw CS (RCS) and the digestate (CSD) was carried out at 600 °C for 3 hours. The samples generated, raw corn stover ash (RCSA) and corn stover digestate ash (CSDA) were characterized using X‐ray fluorescence (XRF), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscope (SEM), and X‐ray diffractometer (XRD). The XRF results revealed that CSDA biosilica content was 60.49%, which was 34.84% higher than RCSA. There was reduction in most of the metallic impurities in CSDA compared to RCSA. Potassium oxide was drastically reduced from 12.34% to 1.60%, while Al2O3 was reduced from 7.17% to 6.95%. SEM images showed the changes in the surface morphology of RCSA and CSDA, while FTIR spectra identified the changes in the key functional groups present in the samples. The study showed that CS and its digestate could serve as profitable renewable feedstocks for bioenergy production and bio-based chemicals like silica. Keywords: Anaerobic Digestion; Biogas; Biosilica; Corn Stover; Digestate; Calorific Value