Effect of long-term ammonia deficiency on nitrification performance and microbial dynamics in downflow sponge biofilm reactor

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The downflow sponge biofilm (DSB) reactor has a promising nitrification capacity for ammonium (NH4+-N)-polluted raw water treatment. However, the highly fluctuating NH4+-N levels in raw water may pose a challenge to a stable nitrification process. It is unclear how the DSB reactor contends with NH4+-N deficiency. This study evaluated the robustness of a DSB reactor under long-term NH4+-N abundant and NH4+-N deficient conditions. Four NH4+-N deficient periods were investigated, comprising two 14-day partial NH4+-N deficient phases, one 56-day phase involving both partial and complete NH4+-N deficient, and one 90-day complete NH4+-N deficient phase. Repeated 14-day partial NH4+-N deficient phases had no impact on nitrification performance. However, shifting from partial to complete NH4+-N deficient condition resulted in a delay in NOx−-N production when NH4+-N was replenished. Following an additional 90 days of NH4+-N deficient phase, NH4+-N removal performance recovered faster than nitrite (NO2−-N) removal performance, which took 5 and 29 days, respectively. Furthermore, long-term NH4+-N deficiency resulted in a nominal loss of biomass in the DSB reactor. These findings suggested that the nitrification capacity of the DSB reactor is robust when subjected to long-term NH4+-N deficient conditions. The 16S rRNA amplicon sequencing results revealed that the growth of Nitrosomonas was suppressed, but Candidatus Nitrosotenuis predominated in NH4+-N deficient conditions, while Nitrospira persisted throughout the study. The observed persistence of Nitrospira may be attributed to their diverse metabolic capabilities, which include mixotrophy and complete ammonia oxidation metabolisms. Collectively, the co-existence of Ca. Nitrosotenius, Nitrosomonas, and Nitrospira contributed to stable NH4+-N removal performance in the robust DSB reactor. The DSB reactor shows promise for use in raw water treatment with highly fluctuating NH4+-N availability.