Denitrification And N₂O Conversion Characteristics Of SBR Systems Under Acidic Conditions

N₂O,the intermediate product of biological denitrification process,is a strong greenhouse gas and a potential energy source.The recovery of nitrogen energy（in the form of N₂O）from wastewater treatment processes is of increasing interest.Previous studies have shown that pH is an important factor affecting N₂O conversion during denitrification.In this paper,we take acidic nitrogen-containing organic wastewater as an entry point,compared the characteristics of microbial population,sludge morphology and N₂O release in the denitrification system with nitrate as electron acceptor and nitrite as electron acceptor.Aims to provide a basis for the enrichment recovery or reduction of N₂O release in the denitrification process.The conversion and influence pattern of N₂O during the start-up and operation of denitrification system with nitrate as electron acceptor under acidic conditions were investigated using SBR with KNO₃and sodium acetate as the main substrates,and the main conclusions were as follows.（1）During the domestication process,N₂O conversion tended to increase as the initial pH decreased,and the conversion rate reached 9.81%at an initial pH of 5.0,which was 18.2 times higher than that at pH 7.0.（2）The effect of constant pH on N₂O conversion was investigated and it was found that the N₂O conversion rate increased from 0.41%to 60.62%as the pH decreased from7.0 to 5.0,indicating that N₂O enrichment could be achieved at low pH.Further study using microelectrodes revealed that the net volume consumption rate of N₂O decreased with decreasing pH,indicating that the inhibition of N₂O reduction at low pH was the main reason for its enrichment.（3）The characteristics of microbial populations in the system were studied by high-throughput sequencing,and the abundance and diversity of microorganisms decreased after acidic pH domestication;the relative abundance of Pseudomonas spp.（39.57%）was the highest before domestication,and Thauera spp.（relative abundance rose from 2.41%to 38.52%）became the dominant bacteria after domestication.（4）The study of sludge morphology in the system revealed that granular sludge was formed at 180d with a median particle size of 161.3μm,indicating that acidic pH is prone to granular sludge formation.The N₂O conversion characteristics of floc and granular sludge under acidic pH conditions were compared,and the nitrate reduction rate of granular sludge(13.04 mg·h^-1·g^-1VSS)was found to be 1.92 times higher than that of floc sludge;while its N₂O conversion rate（7.21%）was only 1/9 of that of floc sludge,indicating that granular sludge could reduce the release of N₂O.The denitrification and N₂O conversion characteristics were investigated in the SBR system under acidic conditions when nitrite was used as the electron acceptor,and the main conclusions were as follows.（1）N₂O conversion rate increased from 2.07%to 7.17%as the initial pH decreased（from 6.5 to 5.0）.The effect of constant pH showed that the N₂O conversion rate tended to increase and then decrease as the pH decreased from 7.0 to 5.0,and the maximum N₂O conversion rate was 26.26%at pH 5.8.（2）The dominant genus of denitrification system with nitrite as the electron acceptor was unclassified＿k＿＿norank＿d＿＿Bacteria（65.60%）,followed by Thauera（20.22%）,which was very different from the denitrification system with nitrate as the electron acceptor,indicating that the electron acceptor had a great influence on denitrifying bacteria species.（3）The effect of free nitrous acid（FNA）on N₂O conversion was studied by adding different nitrite concentrations under the premise of pH 5.8.When the FNA was not higher than 0.84 mg·L^-1,the N₂O conversion was linearly and positively correlated with the FNA concentration with the correlation coefficient as high as 0.996,and the highest conversion rate was 42.56%;when the FNA concentration continued to rise to 1.54mg·L^-1,the N₂O conversion rate decreased to 35.29%.（4）The effect of anthraquinone-2-sulfonate（AQS）on nitrite reduction and N₂O release was investigated,and it was found that the nitrite reduction rate and N₂O conversion rate showed an increasing trend with the increase of AQS dosage.When the dosage was 0.20 mmol·L^-1,the nitrite reduction rate was 2.65 mg·h^-1·g^-1VSS,which was 1.15 times higher than that before the dosage;the N₂O conversion rate was 54.10%,which was 1.27 times higher than that before the dosage,indicating that AQS could improve the nitrite reduction rate and promote the release of N₂O.