Effect Of Influent Quality On Dissolved Organic Nitrogen Evolution And Characteristics

The textile industry is one of the industries with the largest discharge of wastewater and pollutants in our country,and the problem of total nitrogen pollution in printing and dyeing wastewater is prominent.As the concentration of total nitrogen decreases,the proportion of dissolved organic nitrogen(DON)in the effluent increases,and DON is easily absorbed and utilized by algae,leading to eutrophication of water bodies.Printing and dyeing wastewater usually contains a large amount of azo dyes and urea additives,so that the total nitrogen in the printing and dyeing wastewater contains a large proportion of DON,so the removal of DON is also the key to improve the removal of total nitrogen.As azo dyes are toxic during the treatment process,microorganisms may release soluble microbial products including DON after being stimulated by toxic substances.Therefore,the impact of influent quality on DON is worthy of attention.In this paper,reactive black 5 was used as the azo dye model to simulate dyeing wastewater(RB5 group),urea was used as the organic nitrogen model to simulate printing wastewater(Urea group),and ammonium chloride was used as the nitrogen component to simulate domestic wastewater(NH₄Cl group).Using hydrolysis acidification and sequencing batch reactor activated sludge process as wastewater treatment process,the influence of influent quality on the evolution of inorganic nitrogen and DON was explored.The results showed that the nitrification of activated sludge in the RB5 group was inhibited due to the influence of reactive black 5,so there was residual ammonia nitrogen and the accumulation of nitrite nitrogen in the effluent.In addition,due to the lack of carbon source in the anoxic stage,the three reactors have no denitrification function.The DON concentration of RB5 group increased in the aerobic stage,while that of Urea and NH₄Cl group decreased in the aerobic stage,resulting in a higher proportion of DON/TN in the effluent of the RB5 group than the other two groups,and the proportion of DON/TN in the effluent of the RB5 group is up to 49.0%.After adding carbon source in anoxic stage,the denitrification process was realized,and the TN removal rates of RB5,Urea and NH₄Cl groups were increased by 49%,94%and 86%,respectively,while the DON concentration of Urea and NH₄Cl groups decreased slightly after adding carbon source,and the DON concentration of RB5group increased after adding carbon source,resulting in the improvement of the TN removal rate in the RB5 group lower than the other two groups.In order to assess the environmental risk of DON,the influence of influent quality on the characteristics of DON was analyzed,including the molecular weight,hydrophilicity and hydrophobicity,bioavailability and biodegradability of DON.The results showed that the proportion of low molecular weight DON in the three groups of reactors increased with the progress of the reaction,resulting in the effluent DON being dominated by low molecular weight DON.As the hydrophobic components were easily adsorbed by activated sludge,the DON in the effluent of three reactors were mainly hydrophilic DON.Low molecular weight DON and hydrophilic DON are often more easily absorbed and utilized by algae,which is consistent with the results of this study.DON in the effluent of three reactors are easy to be absorbed and utilized by algae,which is easy to cause eutrophication of water bodies,and are difficult to be biodegraded,with poor biodegradability and easy to remain in water bodies.Using three-dimensional fluorescence spectroscopy,the main source of DON in wastewater was explored.The results show that the DON in the wastewater from the three groups of reactors mainly comes from soluble microbial products or extracellular polymeric substances,and they are mainly tryptophan-like DON.In terms of DON molecular composition,the DON in wastewater is mainly lignin-like DON,with the progress of the reaction,the composition of DON in the Urea and NH₄Cl groups did not change significantly,the proportion of lignin-like DON in the RB5group gradually decreased,and the proportion of tannins-like and condensed aromatic-like DON gradually increased.In addition,the influent quality has an impact on the microbial community structure.At the phylum level,the soluble microbial products and extracellular polymers in each stage of the reaction are mainly derived from the most abundant dominant species Proteobacteria.At the genus level,the abundance of Nitrospira in the sequencing batch reactor of RB5 group was lower than that of the other two groups,resulting in a weaker nitrification capacity of the activated sludge in the RB5 group than the other two groups.