Assessment of Liquid and Solid Digestates from Anaerobic Digestion of Rice Husk as Potential Biofertilizer and Nutrient Source for Microalgae Cultivation

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 mu m) 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 mu m/1:6 and 600 mu m/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.

Automated summary

The need for a circular economy model is increasing due to the growth of the human population and the decrease in natural resources. This study focused on the anaerobic digestion of rice husk to produce biogas and digestates. Optimum conditions for biogas production were determined based on particle size and dilution ratio. The resulting digestates were characterized and found to have potential as biofertilizers and nutrient sources for microalgae cultivation. The absence of heavy metals in the digestates confirmed their safety for application. The pH values and nitrogen content of the digestates were suitable for the cultivation of some microalgae strains.