Sulfate-Informed State Correction Reveals Bed Evolution in Expanded-Bed Sulfur-Based Autotrophic Denitrification

Abstract

Expanded-bed operation has rarely been explored as a hydraulically tunable process for sulfur-based autotrophic denitrification (SAD), where long-term performance is often obscured by continuous media consumption and bed structural drift. Here, we develop an expanded-bed SAD process packed with sulfur and siderite composite particles (1–5 mm) and quantify how bed expansion ratio (BER) regulates nitrate removal across packed, expanded, and incipient fluidized regimes. Moderate expansion (BER 5.4–8%) intensified nitrate removal, with the bed-state-corrected volumetric nitrate removal rate increasing up to 3.7-fold and reaching 10.23 kg-N m^–3 d^–1. To enable fair benchmarking over long-term operation, we introduce a sulfate-informed correction approach that estimates time-resolved effective bed volume and apparent surface availability from routine sulfate production, thereby distinguishing biofilm-associated performance changes from geometric and hydraulic state drift caused by bed evolution. Expanded-bed operation promoted thinner and more viable biofilms (up to 81% live cells), accompanied by elevated detached biomass protein and increased denitrification gene copy numbers quantified by DNA-based qPCR. Overall, the study establishes expanded-bed SAD as a scalable process innovation and provides a practical approach to quantify bed evolution and evaluate long-term performance on a consistent, state-corrected basis.