<p>Although microbial immobilization has been widely applied in wastewater treatment, the functional differences between suspended sludge and carrier-attached biofilms remain poorly understood. In this study, we investigated the microbial community structure and potential metabolic differences between suspended sludge (MIS) and polyurethane foam (PUF)-attached biofilms (MIC) in an immobilized biochemical tank (MI) from a chemical fiber plant. Compared to the conventional activated sludge process (CAS), the MI demonstrated significantly enhanced removal efficiencies of 39.4% for COD and 83.3% for BOD. The richness, diversity and unique microorganisms of MIS were higher than those of MIC. The dominant genera in MIS were Aridibacter, Diaphorobacter, Nostocoida, Pirellulaceae, Mucilaginibacter, and Rhodanobacter, while the dominant genera in MIC were Mucilaginibacter, Aridibacter, Nostocoida, Gemmata, Meiothermus, and Mycobacterium. Although the major genera were consistent, their relative abundance varied. Metabolic pathway analysis indicated that MIS showed stronger contributions to the transport of organic pollutants, while their role in nitrogen removal in the wastewater was greater than that of attached microorganisms. In contrast, carbon removal primarily occurred on the MIC. Moreover, the intensity of stochastic processes in shaping bacterial communities was observed as CAS (R² = 0.427) &gt; MIS (R² = 0.261) &gt; MIC (R² = 0.26), suggesting that the carriers enhanced the exposure of microbial communities to deterministic processes. These findings offer concrete theoretical support for the engineering application of microbial immobilization technology in treating industrial wastewater by elucidating key mechanistic insights.</p> Graphical abstract <p></p>