The overlooked impact of polyether defoamer on sludge dewatering after anaerobic digestion.

Chemical defoamers are universally dosed to mitigate foaming in anaerobic digestion (AD) systems, yet their residual impacts on downstream sludge dewaterability remain a black box. This study reveals that a ubiquitous polyether defoamer (PD) profoundly deteriorates AD sludge dewaterability. With 0.3% (v/v) PD addition, capillary suction time and bound water surged by 63.9% and 536.7%, respectively. To unravel this, a comprehensive mechanism was decoupled across the liquid, solid-liquid interface, and solid dimensions. In the liquid phase, PD triggered a 414.3% surge in soluble COD and enriched hydrophilic proteins. Molecular dynamics (MD) simulations demonstrated PD actively binds to these proteins, constructing a robust hydrogen-bond network that reduced the water diffusion coefficient by 20.0%. At the solid-liquid interface, PD plummeted the sludge's relative hydrophobicity from 27.02% to 5.86% and heightened electrostatic repulsion, severely exacerbating water retention. In the solid phase, rheological analysis revealed PD intensified the matrix's apparent viscosity and structural network strength, inducing a cohesive, gel-like transformation. Spatially, three-dimensional computed tomography (3D-CT) visually confirmed that this structural reconfiguration collapsed interconnected drainage channels, plummeting effective porosity from 3.21% to 0.75% and exponentially increasing capillary resistance. Consequently, while defoamers ensure upstream AD stability, their accumulation creates a severe bottleneck for subsequent sludge volume reduction. These insights urge the optimization of dosing strategies and the development of novel dewatering-friendly defoamers to alleviate operational burdens in wastewater treatment plants.