Unveiling the overlooked impact of idle phase on partial nitrification in sequencing batch reactor: Insights from...

As an energy-efficient bioprocess, partial nitrification (PN) has attracted considerable attention. Although numerous operational parameters have been studied to optimize PN performance, it remains unclear whether the operational mode (sequencing batch and continuous systems) fundamentally influences PN performance. In this study, representative sequencing batch reactor (SBR) and continuous stirred tank reactor (CSTR) were compared over 128 days. When treating identical municipal wastewater, the nitrite accumulation ratio (NAR) ranged from 2.4% to 4.3% in CSTR, while it reached 96.3% in SBR. Significantly, under the same community structure, transitioning from CSTR to SBR triggered an immediate 14-fold increase in NAR, rising from 4.3% to 59.2%. Metatranscriptomic analysis showed that nitrification-related genes were significantly up-regulated on the 3rd day after transition to SBR operation, with amoA exhibiting the greatest increase (3.2-fold). Typical SBR cycle-based functional gene expression emphasized the significant role of idle phase, which has traditionally been considered a passive period. Downregulation of amoA during idle phase was less pronounced compared to nxrB. Upon entering the active feeding phase, amoA transcription rates surged more rapidly than nxrB. The idle phase in SBR is a crucial 'priming' stage, positioning SBR as a more advantageous approach for achieving PN in real-world wastewater treatment applications.