Exploiting differential recovery kinetics of nitrifiers after extreme free nitrous acid shocks enable rapid nitritation start-up in acidic granular sludge systems.

Granular sludge holds promise for acidic nitritation but achieving it remains challenging. Conventional free nitrous acid (FNA)-based strategy, which relies on selective inhibition of nitrite-oxidizing bacteria (NOB) over ammonia-oxidizing bacteria (AOB), is ineffective in acidic granular systems. This study revealed that in acidic granular sludge, AOB exhibited higher sensitivity to low pH/FNA than NOB, due to metabolic proton dependence and physical protection of inner-layer NOB by granular architecture. To overcome this, we developed a novel suppress-and-recover strategy exploiting differential recovery kinetics of AOB and NOB after transient extreme inhibition. Short-term exposure to extreme conditions (e.g., pH 2.7 with 11 mg NO2--N/L for one hour) completely inactivated both functional groups; however, during subsequent recovery, AOB rapidly regained activity while NOB remained suppressed, driving nitrite accumulation efficiency (NAE) to ∼ 100 %. Long-term continuous-flow reactor operation validated rapid nitritation start-up within one day and identified inhibition frequency as key parameter for long-term nitritation stability. Daily partial-sludge inhibition sustained stable nitritation, whereas less frequent application led to NOB resurgence. High-throughput sequencing revealed elimination of the dominant NOB Candidatus Nitrotoga and retention ofNitrosomonasAOB. This work provides a robust and operationally flexible pathway for achieving acidic nitritation in granular sludge systems.