The salinity tipping point: determining the impact of stepwise increase in salinity on nitrification and biofilm...

Nitrification is a crucial process in wastewater treatment and is generally inhibited at high salinities. Although packed-bed biofilm reactors (PBBRs) are well-known to facilitate high nitrification rates (NRs ≥ 1 kg N·m^-3·d^-1), the influence of salinity on nitrifying PBBRs remains understudied. This study aims to investigate the effect of stepwise increment of salinity on nitrification process and biofilm community structure in high-rate nitrifying PBBRs. Five PBBRs operated at five different water up-flow velocities (1-20 m/h) and ammonia loading rate of about 1600 g N·m^-3·d^-1 were systematically subjected to incremental salinity shifts from 0 to 35 ‰ over 340 days. All reactors exhibited remarkable resilience and efficiency, achieving nearly complete conversion of ammonia to nitrate, with ammonia-removals consistently >98 % and NRs ≈ 1570 g N·m^-3·d^-1 for 0-20 ‰ salinities. However, a transient accumulation of NH₄⁺ and NO₂^- was observed at intermediate salinity levels of 25 ‰ and 30 ‰, with NRs dropping by about 40 %. Nevertheless, at 35 ‰, all reactors demonstrated exceptional nitrification performance, with efficiencies exceeding 95 % and NRs ≈ 1527 g N·m^-3·d^-1. Nitrosomonas emerged as the dominant ammonia-oxidizing genus, increasing in abundance from 2-7 % at 10 ‰ salinity to 9-21.3 % at 35 ‰. The nitrite-oxidizing community underwent a shift in dominance from Nitrospira (3-8 % at 10 ‰ to 0.65-2.3 % at salinity 35 ‰) to Phycisphaerales genus (which was undetected at 10-20 ‰ salinity, to 4.2-6.2 % and 15 % at higher salinity levels of 30 and 35 ‰). This study shows the possibility for realising efficient high-rate nitrification up to seawater-salinities using PBBRs, which is desirable across several sectors.