1 Introduction

Azo dyes are widely used in various industries, and the discharge of azo dye-containing wastewater poses a significant threat to human health and the environment. Microbial decolorization has been recognized as one of the most effective and environmentally friendly methods for treating azo dye wastewater (Pandey et al., 2007; Yin et al., 2016). Immobilization of microorganisms on the surface of a biofilm carrier has become an increasingly popular technology in this field. Microbial immobilization technology employs physical or chemical methods to restrict cell migration, and the carrier provides optimal conditions for microbial reproduction and a protective surface for the biofilm (Mjla et al., 2021; Shao et al., 2017). Embedded or attached microorganisms can create aggregates on the carrier’s surface, enhancing the efficiency and stability of microbial dye treatment (Sarti et al., 2016).
The choice of carrier material is critical to the effectiveness of wastewater biological treatment. Various types of carrier fillers, including thermoplastic plastics, minerals, fiber materials, and waste materials, can be used in the production of carrier fillers (Jagaba et al., 2021). For example, the removal rate of Congo red dye by the low-density polyethylene-polypropylene (LDPE-PP) carrier-based moving bed biofilm reactor (MBBR) using microorganisms isolated from dye-polluted soil as inoculum was as high as 99.2% (Sonwani et al., 2021). The decolorization rate of Anthraquinone Blue RS by a continuous up-flow packed bed reactor using Bacillus flexus TS8, Proteus mirabilis PMS, and Pseudomonas aeruginosa NCH isolated from textile wastewater as inoculum and corn cob biochar as carrier material was up to 90% (Mohanty and Kumar, 2021). Studies have shown that the carrier is a medium for biofilm proliferation, and its surface and material characteristics determine the properties of the biofilm, which may affect the diversity of the biofilm community (Chu and Wang, 2011; Abu Bakar et al., 2020). The type of polymer used in the biofilm carrier has a significant impact on the performance of the reactor (Krsmanovic et al., 2021).
In this study, we investigated the effect of different carriers on the decolorization efficiency and degradation pathway of Reactive Black 5 (RB5) biofilm reactors inoculated with Shewanella india. We employed three different types of carriers, namely almond shell biochar, Moving Bed Biofilm Reactor (MBBR) polyethylene, and Porous Polyurethane Carrier (PPC), and examined their effectiveness in achieving high and stable decolorization rates. We analyzed the microbial community structure and identified highly efficient degradation strains in the reactors to investigate the variations in microbial composition among different carriers. Through 16S rDNA community analysis, we explored the composition of microorganisms on the carrier and highly efficient RB5 degradation bacterial communities in the reactor. The findings from this study will provide guidance for expanding the selectivity of biofilm reactor fillers and improving the decolorization efficiency of azo dyes.