Impact of readily biodegradable chemical oxygen demand on partial nitritation granules under high-salinity...

Readily biodegradable chemical oxygen demand (COD) challenges the stability of partial nitritation in aerobic granular sludge systems treating high-salinity wastewater. This is especially critical in continuous-flow reactors, where its impacts are not well characterized. This study investigates the response of partial nitritation granules to increasing COD loads in a continuous-flow reactor under sustained high salinity (40 g/L NaCl). A critical COD loading rate threshold of approximately 0.5 kg COD/(m3·d) (influent COD of 150 mg/L) was identified. Below this threshold, a robust granular structure was maintained, with an average size of 686-757 μm, an integrity coefficient exceeding 80%, and an SVI₃ of 46-47 mL/g, while sustaining superior partial nitritation performance (∼100% nitrite accumulation; ammonia oxidation rate: ∼0.7 kg/(m3·d)). In contrast, exceeding this threshold induced severe granular disintegration, and consequently compromised nitritation performance. Importantly, both structural deterioration and functional impairment were reversible upon COD load reduction, highlighting the system's resilience. High-throughput sequencing revealed the enrichment of halophilic heterotrophs (e.g., Marinobacter, Planktosalinus, and Halomonas) with COD introduction. Notably, canonical nitrite-oxidizing bacteria were not detected throughout the experiment, indicating their effective suppression under the high-salinity stress. Instead, the observed nitrate accumulation was primarily attributed to halophilic heterotrophic nitrification. This study provides insights into the effect of readily biodegradable COD on the stability of partial nitritation granules under high-salinity conditions, highlighting the importance of limiting organic loading for partial nitritation granules in continuous-flow processes treating high-salinity wastewater.