Nitrogen Removal Characteristics Of Suspended Sludge And Biofilms In Each Area Of A AAO Oxidation Ditch Process

In recent years,the nitrogen control indicators in the wastewater discharge standards have been particularly strict,and many wastewater treatment plants need to be upgraded to meet the management and control requirements.AAO oxidation ditch process is widely used in urban wastewater treatment plants.Adding a certain amount of fillers to its treatment unit to form mud film mixing is an effective means of upgrading.However,the current method of adding fillers is relatively extensive,and the nitrogen removal mechanism of suspended sludge and biofilm is still unclear.Therefore,it is of great significance to explore the nitrogen removal characteristics of suspended sludge and biofilm in various areas along the AAO oxidation ditch.In this study,fillers were added to each area along the AAO oxidation ditch,and the suspended sludge and biofilm in each area along the route were taken as the research objects to systematically study their nitrogen removal activity and structural characteristics,and analyze the microbial community.The results are as follows:The nitrogen removal activities of suspended sludge and biofilm in each area were investigated.The nitrification ability of the suspended sludge in the middle section of the aerobic area（DO=0.93 mg/L）was the strongest,and the effluent section（DO=1.14mg/L）and the inner reflux section（DO=0.10 mg/L）of the aerobic area were the best filler addition areas for improving the oxidation capacity of NH₄⁺-N and NO₂^--N,respectively.Suspended sludge had strong denitrification ability in the anoxic area,and the middle section of the aerobic area was the best filler addition area for improving the denitrification ability of the process.The suspended sludge and biofilm in the return section of the aerobic area have good potential for anammox.The effects of suspended sludge and biofilm structure characteristics on nitrogen removal activity in each area were explored.The extracellular polymer（EPS）content of the suspended sludge in the anaerobic area and anoxic area was higher（21.57,19.84mg/g）,and the nitrogen removal activity was inhibited,but the EPS content in the suspended sludge in the aerobic area was lower（15.74 mg/g）,which is helpful for nitrification.Excessive EPS could lead to membrane fouling and inhibited the nitrogen removal activity of biofilms,while an appropriate amount of EPS could improve the nitrogen removal activity of biofilms.At the microscopic level,suitable biofilm thickness was the premise of good nitrogen removal ability.The addition of fillers could increase the biodiversity of the system,and different kinds of nitrogen removal bacteria were observed on the suspended sludge and biofilm.High-throughput sequencing was used to study the microbial community structure of suspended sludge and biofilm in various areas.The addition of fillers in the anaerobic area and anoxic area could improve the diversity of system microorganisms,but the increase in the aerobic area was small,and the microbial similarity between the suspended sludge and biofilm with similar nitrogen removal activities was high.At the genus level,the nitrification effect of suspended sludge and biofilm was mainly provided by Nitrospira as the whole-process nitrifying bacteria.The difference in abundance of aerobic denitrifying bacteria,autotrophic denitrifying bacteria and heterotrophic denitrifying bacteria was the reason for the variation of denitrification ability of suspended sludge and biofilm.The relative abundance of Candidatus Brocadia can indicate the anammox potential of suspended sludge and biofilm.