Salinity-induced microbial acclimation and kinetic responses in continuous self-circulating granular sludge process

This study evaluated the performance and inhibition kinetics of a novel continuous-flow aerobic granular sludge (AGS) reactor (Zier) treating saline wastewater (1%-3% salinity). At 3% salinity and a carbon-to-nitrogen ratio of 2.3, selective inhibition of nitrite-oxidizing bacteria (NOB) maintained a stable partial nitrification-denitrification (PND) pathway, achieving 82% total nitrogen removal. At 1%-2% salinity, NOB activity recovered. This shifted the system to full nitrification and reduced nitrogen removal efficiency. Additionally, the sludge achieved successful granulation and exhibited excellent settleability at all salinity levels. Kinetic analysis showed that the salt tolerance of ammonia-oxidizing bacteria (AOB) increased with acclimation salinity. The half-maximal inhibitory concentration (IC₅₀) values for AOB acclimated at 1%, 2%, and 3% salinity were 27.3, 57.1, and 71.0 g/L, respectively. The Aiba model showed that acclimated NOB were more salt-tolerant than AOB at lower salinities, but it was unsuitable for higher salinities. Instead, the Luong model accurately described salt inhibition across all sludges. AOB adapted to higher salinities exhibited cooperative-type inhibition (n > 1). This provides a tolerance buffer but risks sudden functional collapse beyond critical thresholds. Conversely, NOB exhibited progressive inhibition (n < 1) and higher tolerance limits, demonstrating superior stability during salinity fluctuations. The Zier system achieves efficient nitrogen removal from saline wastewater via the PND pathway, and these kinetic findings offer essential strategies for process control.