Formation and characterization of aerobic granular sludge in the treatment of tempering lubricant wastewater by hydrolysis acidification-aerobic process

Tempering lubricant is widely used for the surface treatment of cold rolled strip in steel industry. It causes tempering lubricant wastewater (TLW) discharge, which is characterized as high concentration of organic matter, poor biodegradability, and high concentrations of total nitrogen (TN). In this study, the performance of combined hydrolysis acidification-aerobic process for treating real TLW was assessed. It also examined the formation process and characteristics of aerobic granular sludge (AGS) in the aerobic reactor. Additionally, the characteristics of microbial community were analyzed. The results of 210 days monitoring showed that the hydrolysis acidification-aerobic process had a good effect on the treatment of TLW, with final TOC and TN (NO2-, NO3-, NH4+) removal efficiencies reached 84.8% and 86%, respectively. Meanwhile, AGS was formed in the aerobic reactor. It was also found that higher organic loading rate contributed to AGS formation, while shorter settling time had a positive impact on forming compact and stable AGS. Additionally, Ca2+ and Mg2+ also participated in AGS formation. According to the microbial community analysis, Proteobacteria, Bacteroidota and Firmicutes were dominant in the hydrolysis acidification reactor, which were beneficial for the degradation of refractory organic substances. In the aerobic reactor, the dominant bacteria such as Proteobacteria, Patescibacteria and Chloroflexi contributed to TN removal and the stability of AGS.