Abstract:The escalating challenges of rapid industrialization and increasingly stringent environmental regulations have placed immense pressure on existing wastewater treatment plants (WWTPs), particularly those within industrial parks. These facilities often face the dual imperative of expanding their treatment capacity to accommodate rising influent volumes and simultaneously upgrading their processes to meet higher effluent quality standards, while constrained by limited physical space. This study addresses these critical issues by presenting a comprehensive evaluation of an in situ upgrading strategy for a WWTP in a major industrial park in Nanjing, China. The technical approach involved the integration of a Moving Bed Biofilm Reactor (MBBR) process to enhance the performance of the traditional Anaerobic-Anoxic-Oxic (AAO) system. The study aimed to systematically evaluate the feasibility and efficacy of the MBBR-AAO hybrid for achieving significant capacity expansion and improving effluent quality to meet the stringent Class Ⅳ surface water standards. The results demonstrated a rapid and successful system start-up, with a mature and stable biofilm layer established on the MBBR carriers within 30 days during favorable summer temperatures. Under a demanding hydraulic loading rate increased to 150% of the original design capacity (corresponding to an influent flow rate of 1.5 m3/h), the hybrid system exhibited exceptional resilience. During warm seasons, the treated effluent consistently and reliably met all Class Ⅳ surface water quality standards. However, the system performance was most critically tested during cold seasons. Despite the inhibitory effects of low temperatures on microbial activity, the MBBR-enhanced process demonstrated remarkable robustness. The effluent chemical oxygen demand concentration was slightly elevated above the Class Ⅳ standard limit, with an average value of (30.87 ± 2.54) mg/L, while ammonia, total nitrogen, and total phosphorus concentrations all met the stringent requirements. This outcome confirms that the MBBR-enhanced process effectively achieved the dual goals of in situ capacity expansion and substantial effluent quality improvement for the complex industrial park wastewater. Analysis of the microbial community structure revealed a pronounced seasonal shift in community composition and assembly. During warm seasons, the biofilm community exhibited significantly higher α-diversity, with a notable enrichment of various thermophilic heterotrophic bacteria. Conversely, the microbial community shifted dramatically in cold seasons, with a clear enrichment of cold-tolerant microorganisms. Notably, the significant proliferation of the genus Nitrospira was crucial for sustaining effective nitrification under low-temperature conditions. The assembly of the microbial community in warm seasons was predominantly governed by deterministic processes. In contrast, community assembly during cold seasons shifted towards a greater influence of stochastic processes. Concurrently, microbial network analysis indicated that the interspecies interaction network became simplified in winter and spring, likely enhancing overall community resilience and stability. In conclusion, this study offers a robust and scalable solution applicable to WWTPs worldwide that are facing similar challenges of expansion and stringent effluent quality improvement. Close-