Effects of residual ozone on the performance of microorganisms treating petrochemical wastewater

This study investigated the effects of residual ozone on the performance of microorganisms treating petrochemical wastewater using batch experiments with low and high ozone dosages (5.0 mg/L and 50.0 mg/L, respectively). The results indicated that the low residual ozone concentration significantly increased COD removal by 24.21% in the biological process compared to control group with no ozone residual, while the high residual ozone concentration showed the opposite effect. In the reactor with low residual ozone concentration (0.45 mg/L), the amount of loosely bound (LB)-extracellular polymeric substances (EPS) in the activated sludge decreased by 23.23%, while the amount of tightly bound (TB)-EPS increased by 129.16% compared to the none-ozone residual reactor. In addition, the low residual ozone was found to improve the bioactivity of activated sludge by 139.73% in the first 30 min of the biological process. In the reactor with high residual ozone concentration (0.91 mg/L), both LB- and TB-EPS of the activated sludge increased, while bioactivity decreased. This implies that low residual ozone in a bio-reactor can enhance microbial activity by increasing contact between the pollutants and cells by removing LB-EPS covering the outer layer of the sludge. The microorganisms in the sludge samples could be classified into three groups representing those that are susceptible to ozone, tolerant to low dose of residual ozone, and resistant to high dose of residual ozone. The resistant bacteria Gemmatimonadaceae uncultured became predominant, with a relative abundance of 11.37%, under low residual ozone conditions, while it decreased at high ozone concentrations. The results showed that a certain amount of residual ozone could stimulate the activity of microorganisms by altering the EPS fraction and structure of the microbial community, and thus it is important for the removal of refractory organics from wastewater in the ozone-biological process.