| The low activity and poor stability of partial nitrification process limit its further application in municipal wastewater treatment.Low-intensity ultrasound has been proven to effectively improve the efficiency and system stability of partial nitrification as a means of biological intensification.However,the mechanism of low-intensity ultrasound promoting partial nitrification still needs to be further explored.Therefore,the paper further explored the promotion effect of ultrasound on partial nitrification from two aspects: dissolved oxygen(DO)tolerance and sludge characteristics.The main contents and results are as follows:To investigate the effect of low-intensity ultrasound on DO tolerance in partial nitrification systems,0.10-0.30 W/m L ultrasound was used in a sequencing batch bioreactor(SBR)to study the performance and stability of partial nitrification.The differences in nitrogen conversion,nitrifying bacteria activity,periodic DO curve,extracellular polymeric substance(EPS),and microbial community were investigated.The study found that partial nitrification in the control group collapsed rapidly under DO fluctuations,and the nitrite accumulation rate in the ultrasound group using 0.20W/m L energy density and 10 min irradiation time remained above 83.89%.Under this ultrasound parameter,the gap between the specific ammonia oxidation rate and the specific nitrite oxidation rate was the largest,with 45.77 mg N/g MLVSS·h and 2.36 mg N/g MLVSS·h,respectively.Even in high DO environments,the NOB activity did not recover.Ultrasound had a significant stripping effect on excess EPS,especially loosely bound EPS and proteins,which is beneficial for bioflocculation and mass transfer.In addition,long-term ultrasound irradiation promoted the enrichment of Nitrosomonas(11.42%),strongly inhibited the growth of Nitrotoga(0.09%),and formed a microbial community structure dominated by ammonia oxidizing bacteria.PICRUSt prediction analysis showed that ultrasound could improve the expression level of microbial functional genes,so that microbial community had advantages in membrane transport,signal transduction,metabolism and energy conversion,and the overall trend of metabolic function was R4(0.20 W/m L)> R3(0.15 W/m L)> R5(0.25 W/m L)> R6(0.30 W/m L)> R2(0.10 W/m L)> R1(control group).In addition,ultrasound has different effects on the relative abundance of nitrogen metabolism genes.The relative abundance of amo ABC and hao increased while that of nxr AB decreased,which promoted the stable operation of partial nitrification.Based on the above optimal ultrasound parameters,the effect of ultrasound irradiation on the characteristics of partial nitrification sludge was further analyzed through low-intensity ultrasound coupled SBR.It was found that continuous periodic ultrasound irradiation improved the efficiency of partial nitrification,loosened the sludge structure,and accompanied the phenomenon of sludge uncoupling metabolism.Ultrasound irradiation could increase the activities of ammonia monooxygenase and hydroxylamine oxidoreductase in partial nitrification sludge,which were 81.48% and66.5% higher than those in the control group,respectively.Ultrasound irradiation can reduce the Zeta potential from-29.7 m V to-40 m V,and reduce the contact angle between sludge and water from 97.3 ° to 93.2 °.This increases the surface electronegativity of sludge flocs while reducing their relative hydrophobicity,which is not conducive to sludge aggregation.Three-dimensional fluorescence spectroscopy analysis showed that low-intensity ultrasound irradiation reduced the content of aromatic proteins in EPS,promoted microbial metabolic activity,and led to a significant increase in the proportion of soluble microbial metabolites in EPS.In addition,ultrasound irradiation increases the content of carboxylic groups C=O,C-O,and C-C bonds on the surface of activated sludge,thereby promoting sludge activity. |
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