The low salinity caused by the substitution of seawater and the overuse of deicing salt hinders the treatment of mainstream wastewater by conventional biological nitrogen removal process. In this experiment, the start-up and stable operation of partial nitrification were achieved based on a composite zeolite biofilm (CZB) and 94.1 % of nitrite accumulation efficiency was obtained at low salinity. CZB could still maintain high biofilm proportion, strong adhesion and robust structure to resist the impact of 0.6 % salinity. The enhanced release of C8-HSL from the aqueous phase of the CZB increased the oxygen transfer resistance of the sludge layer, thus reducing the oxygen mass transfer efficiency of the sludge layer and inhibiting NOB more effectively. The augmented release of C6-HSL and C12-HSL in the sludge phase (SP) leads to elevated production of protein and polysaccharides, culminating in the development of a more resilient microbial structure. Meanwhile, C6-HSL and C8-HSL in the SP of CZB promoted gene expression, especially nitrate reductase (Nar), to stabilize the supply of NO2--N. Meanwhile, CZB reinforced the microbial impact resistance by enhancing the tricarboxylic acid cycle, and awakened important gene (nrfA, associated with dissimilatory nitrate reduction to ammonium). This activation played a vital role in maintaining the appropriate NH4+-N to NO2--N ratio in the effluent. Finally, CZB eliminated Nitrospira from the system and increased the relative abundance of Nitrosomonas to 12.8 % at 0.6 % salinity. This method offered valuable theoretical insights for design and optimization of the partial nitrification process for treating mainstream low-salinaty wastewater.
