Performance Evaluation of Lab-Scale Fungal Batch Reactor and Genomic Insights of a Newly Isolated Aspergillus niger AZ2 for Black Liquor Treatment

The pulp and paper industry generates large volumes of black liquor, a lignin-rich effluent requiring effective treatment before disposal. In this study, a lab-scale fungal batch reactor (FBR) was designed and operated using Aspergillus niger AZ2 as a lignin-degrading biocatalyst. The reactor achieved 54% lignin reduction, 66% reduction in chemical oxygen demand (COD), 57% phenol removal, and 62% color reduction, demonstrating strong treatment efficiency. Phytotoxicity tests showed a significant improvement in seed germination. These results highlight the potential of the fungal-based reactor as a simple, cost-effective, and eco-friendly strategy for black liquor treatment under alkaline conditions. The genome-based functional analysis of A. niger revealed a diverse repertoire of lignin-degrading enzymes, including laccase, peroxidases, oxalate peroxidases, and pyranose oxidases. Pathway mapping further highlighted genes associated with the breakdown of aromatic compounds in auxiliary metabolic processes supporting efficient lignin mineralization. CAZymes profiling revealed a strong lignin-degrading potential, with the genome encoding abundant glycoside hydrolases (GHs) (≈500) and auxiliary activity enzymes (AAs) (≈220), alongside a substantial number of glycosyltransferases (GTs) (≈195). An unusually low GH/AA ratio (~2.3:1) indicates enhanced oxidative capacity and lignin-modifying potential in this non-white-rot A. niger strain. Fewer carbohydrate esterase (CEs), polysaccharide lyases (PLs), and carbohydrate-binding modules (CBMs) were identified. This enzyme repertoire highlights the organism's ability to degrade polysaccharides and lignin efficiently. The findings demonstrate the strong ligninolytic potential of A. niger AZ2, highlighting its suitability for black liquor treatment and bioremediation.