Enhanced anaerobic mono-digestion and co-digestion of crop residues by NaOH alkali pre-treatment: digestion performance and microbial community dynamics

Abstract

Abstract The main objective of the study was to determine the effect of different dosages of NaOH alkali pre-treatment on the lignocellulosic structures and the methane production of crops’ residues. In this regard, NaOH pre-treatment was implemented with different dosages on three different crops’ residues: wheat, rye, and rice straws. Substrates were soaked into NaOH solutions at different dosages with solid-to-liquid ratios of 100 and 200 gTS/L to investigate both conditions. The anaerobic digestion process was conducted using AMPTS II under mesophilic conditions. By 8% NaOH pre-treatment dosage, the reduction of hemicellulose and lignin was 81% and 62% for rice straw, 86% and 69% for wheat straw, 80% and 74% for rye straw, respectively. The concentrations of sCOD in the liquid fraction after pre-treatment were regarded as 3 times higher than in untreated samples. Pre-treatment with 8% NaOH dosage increased the methane yield of rice, wheat, and rye straws by 55%, 40%, and 46% respectively. Samples from digesters that produced the highest methane yield were chosen for further molecular analysis. Compared to untreated digesters, 16S rRNA gene amplicon sequencing revealed more diverse microbial communities in NaOH pre-treated digesters.