Metagenomics-based interpretation of selective bioaugmentation promoting partial-denitrification coupling with anammox process reactivation in suspended sludge system

The partial-denitrification coupled to anammox process (PD/A) has been emerging as a potential alternative for cost-efficient nitrogen removal from wastewater. Although there has been intensive study on startup and optimization, efficient reactivation and controlling strategy after disturbance of this process still lack essential information. Here, a selective bioaugmentation strategy with low dosage for stepwise recovery of PD and anammox activity were proposed and demonstrated for the first time, and the mechanism of coupling process reactivation was revealed based on metagenomic analysis. A significant increase in nitrite (NO 2 -N) production was obtained after introducing PD inoculum by weight ratio of 7.7 %, with nitrate (NO 3 − -N)-to-NO 2 − -N transformation ratio (NTR) increasing from 15.7 % to 45.0 %, while anammox activity stayed at a relatively low level. The ammonia (NH 4 + -N) removal rate significantly increased by 4.0 times after supplying PD/A biomass by only 1.2 % due to sufficient substrate NO 2 − -N for anammox. Consequently, nitrogen removal efficiency was remarkably improved to 99.2 % and kept stable with low-strength wastewater. Metagenomics analysis revealed that the abundances of genes nar and nap encoding NO 3 − -N reductase were kept at much higher level than nir for NO 2 − -N reductase. Significantly, higher diversity and abundance of genes responsible for carbon metabolism could accelerate the electron generation and transfer to PD, thus stimulating anammox activity, which was the key reason for the rapid reactivation of PD/A process. Furthermore, the substate ratio, nitrogen loading rates and morphological characteristics of sludge have crucial influence on the recovery of biomass activity. The dominant genus Thauera responsible for PD and Candidatus Brocadia for anammox coexisted stably after bioaugmentation. These results provide a comprehensive understanding of reactivating and regulating a novel PD/A process towards application and microbial interaction.