Effect of Alkali Pretreatment on BioMethane Potential of Lignocellulosic Crop Residues in Kenya.

Introduction

The global transition towards low-carbon, circular bio-economies has accelerated interest in anaerobic digestion (AD) for renewable energy generation from organic wastes. In Kenya, agriculture contributes approximately 33% of GDP directly and over 50% indirectly, producing an estimated 15-20 million tonnes of crop residues annually. However, these lignocellulosic residues remain underutilized due to their high lignin content, cellulose crystallinity and poor biodegradability. This study evaluated the effect of alkaline pretreatment on the anaerobic digestibility and biomethane potential (BMP) of corn stalk, wheat straw and rice straw in Kenya under AD conditions.

Methods

Biomass samples were milled (<2 mm) and pretreated with 0.5 M NaOH at 25°C for 72 h, followed by washing to neutral pH and drying at 65°C to constant weight. Untreated and pretreated samples were characterized by proximate analysis. Batch AD experiments were conducted in duplicate using 500 mL digesters inoculated with digestate from a commercial biogas plant in Karen, Nairobi. Digesters operated at 37 ± 2°C for 30 days with an inoculum-to-substrate ratio of 1.2. Biomethane production was monitored daily by liquid displacement.

Results

Alkali pretreatment enhanced methane production in all substrates. Rice straw achieved the highest BMP, increasing from 189.9 to 249.4 mL CH₄/g VS (31.3%). Wheat straw exhibited the greatest relative improvement, increasing from 79.6 to 130.3 mL CH₄/g VS (63.7%), while corn stalk increased from 100.2 to 140.4 mL CH₄/g VS (40.1%). These results demonstrate that alkaline pretreatment effectively improved lignocellulosic biodegradability and methane recovery.

Conclusion

Valorization of Kenya's abundant agricultural residues through optimized alkaline pretreatment and anaerobic digestion presents a sustainable strategy for renewable energy generation, reduced open-field biomass burning, improved waste management and greenhouse gas mitigation. These findings provide a practical basis for scaling decentralized biogas technologies and advancing Kenya's circular bioeconomy and clean energy transition.