High-load nitrite production via the rotating biological contactor with immobilized carrier: Dual pathways of partial nitrification and partial denitrification

Partial nitrification (PN) is crucial for nitrite (NO₂^–-N) supply in anaerobic ammonia oxidation (Anammox), but its efficiency under high-ammonia and stability under low-ammonia conditions remains challenging. This study developed a rotating biological contactor with immobilized carrier (IC-RBC) to achieve stable, high-load NO₂^–-N production by integrating PN and partial denitrification (PD). Under high ammonia conditions, immobilized carriers enriched ammonia-oxidizing bacteria (AOB, 3.9 $\times$ 10⁹ copies/g carrier) while free ammonia (FA) inhibited NO₂^–-oxidizing bacteria (NOB), achieving a NO₂^–-N production rate of 448.2 ± 1.5 mg·(L·h·carrier)⁻¹, with NOB abundance at just 1/20,000 of AOB. At low ammonia levels, reduced FA inhibition increased NOB to 31.1 %, yet PN-PD synergy sustained NO₂^–-N production at 193.4 ± 5.3 mg·(L·h·carrier)⁻¹. PD was primarily driven by organic matter, which promoted denitrifying bacteria in oxygen-limited zones, increasing their abundance from 1.73 % to 18.12 %. Metagenomics revealed a nitrate-to- NO₂^–-N reductase gene ratio shift from 0.2 to 1.5, confirming PD’s critical role in NO₂⁻-N accumulation. The IC-RBC process achieved PN through the periodic spatial alternation inhibition characteristic of FA by the rotating biological contactor, and realized PD through the characteristic of the oxygen-limited zone of the immobilized carrier. These findings highlight IC-RBC’s potential for stable, high-load NO₂^–-N production without complex regulation in Anammox applications.