| Partial nitrification is an essential pre-process of new nitrogen removal process.There is a certain proportion of industrial wastewater mixed in urban sewage,which increases the risk of heavy metal impact on urban sewage treatment plants.High concentration of heavy metals will lead to the collapse of partial nitrification and the effluent quality will exceed the standard.Therefore,it is necessary to study how to improve the stability of partial nitrification system under the impact of heavy metals.Low intensity ultrasonic treatment can improve the performance of wastewater biological treatment,which has been proved to be an effective means to enhance partial nitrification.Therefore,the paper investigated the effects of low-intensity ultrasound on the partial nitrification operation,sludge characteristics,the response level of antioxidant system and the migration and transformation of Cd in the process,and revealed the effects of low-intensity ultrasound on the microbial community structure.The main research contents and results are as follows:Ultrasound can accelerate the activity recovery of PN sludge after Cd impact.Through batch experiments,taking mature short-cut nitrification sludge domesticated in laboratory as the research object,and selecting specific oxygen uptake rate（SOUR）as the characterization index,the activity changes of partial nitrification sludge impacted by Cd under different ultrasonic intensity,operation mode and irradiation time were investigated.It was found that ultrasound can accelerate the recovery of PN sludge activity after Cd impact,especially improve the activity of ammonia-oxidizing bacteria（AOB）after inhibition.Compared with the control group,the activity of AOB increased by 220.89%when the ultrasonic intensity was 0.20 W/m L,the working time was intermittent for 2 s and the irradiation time was 15min.The PN sludge impacted by Cd stress at different irradiation time was studied.It was found that ultrasound accelerated the process of Cd discharge and transformation to steady state,and it also improved the response level of antioxidant system,reduced the oxidative stress caused by Cd impact,and finally reduced the negative effects caused by Cd stress.Ultrasound can improve the stability of partial nitrification sludge under the impact of Cd.On the basis of batch experiments,the operation and sludge characteristics of control group and ultrasonic group before and after Cd impact were investigated.The results showed that,compared with the control group,after 35 days of ultrasonic irradiation,the half inhibitory concentration(IC50)of AOB in ultrasonic group increased by 18.60%,indicated that the tolerance to Cd was enhanced.Under the impact of 10 mg/L Cd,the accumulation rate of nitrite nitrogen and AOB activity in ultrasonic group decreased less and recovered more quickly.The total extracellular polymeric substances（EPS）content of sludge increased by 46.29%,but the types of functional groups remained unchanged.The increase of protein and polysaccharide components made Cd more adsorbed on the surface,which prevented its inflow.In addition,low-intensity ultrasound improved the antioxidant capacity of microorganisms in normal state,and the activity of antioxidant enzymes increases more rapidly under the impact of Cd,so that the toxic effects of heavy metals can be eliminated more quickly.The microbial flora structure was partial nitrification sludge were screened by continuous and periodic ultrasonic irradiation,which made the richness of microbial community slightly decreased and the original dominant strains were further enriched.At the same time,microorganisms with heavy metal resistance have also been strengthened.At the phylum level,low-intensity ultrasound mainly increased the relative abundance of Bacteroidetes and Ignavibacteriae,and made Verrucomicrobia,Acidobacteria and Planctomycobacteria become new dominant flora.At genus level,low intensity ultrasound mainly increased the relative abundance of aerobic denitrifying bacteria such as Ohtaekwangia. |
