Mechanisms of partial denitrification in an integrated fixed-film activated sludge (IFAS) system: From progressive startup to stabilization

Abstract

The rapid startup and long-term stability of partial denitrification (PD) processes are essential prerequisites for successful PD/Anammox systems. Bioaggregates play a significant role in impacting mass transfer efficiency and shaping the microbial community structure within the PD process. Thus, the startup and stable operation of the PD process were investigated in an integrated fixed-film activated sludge (IFAS) by inoculating activated sludge and biofilm from anoxic tanks of a wastewater treatment plant. During the initial stage of startup, a sudden increase in NO3−-N concentration induced a rapid enzyme stress response, leading to a significant increase in total NaR activity (1.38 ± 0.0 μmol·(L·min)−1 to 25.94 ± 0.23 μmol·(L·min)−1), concomitant with the inhibition of total NiR activity (13.93 ± 0.48 μmol·(L·min)−1 to 4.25 ± 0.26 μmol·(L·min)−1), thereby inducing the rapid accumulation of NO2−-N (13.39 mg·L−1) within 6 days. During the later stage of startup, enzyme stress response was subsided but the partial denitrifiers were gradually enriched, consequently leading to continuous accumulation of NO2−-N. The results of ex-situ activity showed that flocs exhibit superior PD performance in contrast to biofilm throughout the entire operational period. Meanwhile, the total extracellular polymeric substances (EPS) content and community structure in both flocs and biofilm indicated that the migration of EPS-secreted partial denitrifiers (Thauera) from flocs to biofilm was obliged by substrate deficiency during the stable operating stage (days 35–100). Therefore, the startup of PD may be sequentially mediated by two distinct mechanisms, enzyme stress response and functional bacterial enrichment, with microbial community migration occurring during the stable phase.