Successful control of low-temperature sludge bulking in a WWTP by installing iron-carbon polyurethane packing in the return-sludge channel.

Low-temperature-induced filamentous sludge bulking has long represented a major operational challenge during winter operation of wastewater treatment plants, and practical engineering systems still lack a simple, stable, and reliable control strategy. At a municipal wastewater treatment plant where winter sludge bulking recurrently occurred and the sludge volume index (SVI) exceeded 200 mL g-1, this study proposed and implemented a bypass regulation strategy without altering the existing process configuration. Specifically, iron-carbon polyurethane packing (ICPP), composed of iron-carbon spheres coupled with polyurethane sponge, were installed in the sludge return corridor. The strategy synergistically improved sludge settleability through multiple mechanisms, including oxidative stress induced by iron-carbon micro-electrolysis, selective interception of filamentous bacteria by the packing structure, and intermittent drying effects associated with return sludge operation. During the winter period, while the Control Group(CG) exhibited severe bulking with SVI values ranging from 200 to 350 mL g-1, the Treatment Group(TG) successfully maintained SVI within 60 to 120 mL g-1. The abundance of Kouleothrix (Type 1851) was maintained at 8.58% in the TG, compared with 24.87% in the CG. This full-scale validation demonstrates that regulation via ICPP installed in the sludge return channel can serve as a low-intervention, implementable, and stable control strategy for winter sludge bulking. The approach provides a practical engineering solution and conceptual framework for maintaining stable wastewater treatment performance under cold-season conditions.