Effects of phenol on physicochemical properties and treatment performances of partial nitrifying granules in sequencing batch reactors

This study attempts to investigate the effect of phenol on physicochemical properties and treatment performances of partial nitrifying granules (PNGs). Two sequencing batch reactors (SBRs) fed with synthetic ammonium wastewaters were operated in absence (R1) or presence (R2) of phenol. The PNGs in R1 maintained excellent partial nitrification performance and relatively stable physicochemical properties, and exhibited compact and regular shaped structure with a cocci-dominant surface. However, as phenol concentration was stepwise increased from 0 to 300 mg/L in R2, filamentous bacteria appeared and gradually dominated within granules, which in turn resulted in settleability deterioration. Most notably, granules in R2 got easier to agglomerate in the reactor walls and then been washed out with effluent, leading to significant biomass loss, frequent outflow pipe blockage, and eventual system failure. The extracellular polymeric substances (EPS) contents including proteins and polysaccharides in R2 reached 1.8 and 1.7 times of that in R1, respectively, indicating that the presence of phenol played an important role on EPS production. Removal efficiency of ammonium and phenol remained high, but dropped sharply when phenol concentration reached 300 mg/L. Moreover, the failed maintenance of partial nitrification was observed due to the revival of nitrite oxidizing bacteria (NOB) within granules after phenol exposure, which was confirmed by quantitative fluorescence in situ hybridization (FISH) analysis. Overall this study demonstrates that phenol had negative effects on PNGs, and pretreatment to eliminate phenolic substances is recommended when using PNGs for wastewater treatment.