A novel Bio-LOHAS process: Regulating DGAOs-DPAOs interactions and carbon allocation through strategic low-DO aeration for low C/N wastewater treatment.

Conventional biological treatment of low carbon-to-nitrogen (C/N) municipal wastewater is challenged by the need for supplemental carbon sources and high aeration energy. Here, we first introduce a Biological Low Oxygen and High Activated Sludge concentration (Bio-LOHAS) system and evaluate its performance under two low dissolved oxygen (DO) gradients strategies: a monotonically increasing DO profile (M-O) and a reverse profile (O-M). At an influent C/N ratio of 4.41 ± 0.92, the O-M strategy outperformed the M-O strategy, achieving total nitrogen (TN) and total phosphorus (TP) removal efficiency of 77.8 % and 95.8 %, respectively, compared with 64.3 % and 68.9 % under M-O. Integration of in-situ pathway profiling and batch experiments revealed that the O-M strategy facilitated carbon allocation and enhanced microbial synergy. Concurrently, 16S rRNA-based community analysis indicated that the O-M strategy favored the enrichment of denitrifying glycogen-accumulating organisms (DGAOs) and denitrifying phosphorus-accumulating organisms (DPAOs), thereby driving polyhydroxyalkanoates (PHAs) -mediated denitrifying phosphorus removal. Notably, it promoted a dynamic balance between DGAOs and DPAOs, optimized internal carbon source conversion efficiency, and increased microbial network complexity. Metagenomic analysis further confirmed the activation of endogenous denitrification and polyphosphate metabolic pathways, with increased abundance of key functional genes involved in PHAs-glycogen cycling and polyphosphate synthesis. More importantly, the Bio-LOHAS process reduced external carbon demand by 40 % and aeration energy input by 27-33 %. This study demonstrates the potential of the Bio-LOHAS process as a promising and energy-efficient strategy for low C/N municipal wastewater treatment and provides a rational basis for optimizing DO gradients in full-scale applications.