Effect of pre-anoxic and post-anoxic on sludge structure: critical insights into carbon utilization, pollutants removal, and microbial community.

Anoxic zone played essential roles in nutrient removal in biological wastewater treatment processes. This study compared the impacts of pre-anoxic (PreA) and post-anoxic (PostA) configurations on sludge characteristics, nutrient removal performance, and microbial succession. Results showed that shifting from PreA to PostA deteriorated sludge settleability, with sludge volume index (SVI30) increasing from 71 to 192 mL·g-1. Meanwhile, sludge structures became looser and more hydrophilic, accompanied by a decrease in the protein (PN) to polysaccharides (PS) ratio of extracellular polymeric substances (EPS) from 0.78 to 0.62. Denitrification batch tests revealed that PostA promoted soluble organic matter release, with the release rates of 1.17 and 1.54 mg COD·(g·h)-1 under PreA and PostA, respectively. In addition, PostA denitrification relied more on the EPS-derived soluble products as electron donors, including tyrosine- and tryptophan-like substances, which contributed to sludge structural loosening. Furthermore, the sludge bulking under PostA was not attributed to filamentous bacteria proliferation, as their relative abundance remained unchanged (from 3.00 % to 3.03 %), but by the loose and hydrophilic sludge. Overall, the layout of anoxic zone regulated the sludge structure by altering the distribution of carbon sources and electron acceptors, reshaping microbial metabolic functions rather than community composition.