Study On The Regression Of Sulfamethoxazole And Methoxybenzidine In Wastewater Treatment System And Its Effect On Biological Nitrogen Removal System

The combination of sulfamethoxazole and trimethoprim drugs,known as cotrimoxazole,is one of the most common types of sulfa drugs.Cotrimoxazole can effectively kill a variety of pathogenic bacteria and is widely used to treat bronchitis,suppurative tonsillitis,urinary tract infections,pharyngitis,dysentery,and pneumonia and so on.As a new kind of micro-pollutants in many PPCPs,it has the characteristics of wide production,widespread presence in the environment and poor biodegradability.At present,the studies of sulfamethoxazole and trimethoprim are mainly focus on the microbial degradation of antibiotics,the adsorption of sulfamethoxazole and trimethoprim by synthetic materials,and the effect of single antibiotic on nitrification or denitrification process.However,there are few studies on the fate of sulfamethoxazole and trimethoprim in sewage treatment plants and the potential impact on sewage biological treatment systems,especially compound pollution.In this study,two kinds of antibiotics,sulfamethoxazole and trimethoprim,were taken as target pollutants to study the adsorption and degradation status of these two drugs in sewage treatment plants.Based on this,we explored the effects of sulfamethoxazole and trimethoprim on biological denitrification and microbial community structure.Therefore,the following three aspects were studied:the adsorption and degradation of sulfamethoxazole and trimethoprim in sludge;the short-term and long-term effects of sulfamethoxazole and trimethoprim on biological denitrification and the changes in microbial community structure under long-term effects.The relevant research results are as follows:（1）The adsorption of sulfamethoxazole and trimethoprim in the secondary sedimentation tank sludge is a rapid adsorption process.The pseudo-second-order kinetics adsorption model can better describe the adsorption behavior of sulfamethoxazole and trimethoprim on the sludge.The adsorption process includes physical adsorption and chemical adsorption,but the main process is chemical adsorption.Thermodynamic studies showed that the sludge has good correlation with the three isotherm models for the adsorption of sulfamethoxazole and trimethoprim,and the adsorption processes are thermal reactions.In the presence of two antibiotics,the adsorption of sludge on them has a competitive effect.Sulfamethoxazole is more gradual than the trimethoprim isotherm,indicating that the adsorption affinity of sludge to sulfamethoxazole is higher.Sulfamethoxazole is more favorable for adsorption under acidic conditions,while trimethoprim is more favorable for adsorption removal under neutral conditions.The increase in ionic strength does not have a significant effect on adsorption.（2）In the short-term experiment,10 mg·L^-1 trimethoprim and sulfamethoxazole had the strongest inhibitory effect on nitrification.The inhibition of nitration by the combination of trimethoprim and sulfamethoxazole is significantly more potent than when they are applied alone.When trimethoprim and sulfamethoxazole were used separately and mixed,the maximum inhibition rates of ammonia oxidation of sludge were 42.1%,55.2%,and 64.0%,respectively.Trimethoprim had a significant inhibitory effect on denitrification,while sulfamethoxazole had a weak inhibitory effect on denitrification.At the same time,sulfamethoxazole and trimethoprim are themselves degraded by microorganisms during the denitrification process.Sulfamethoxazole is more easily degraded than trimethoprim during nitration,and trimethoprim is more easily degraded than sulfamethoxazole during denitrification.（3）Sulfamethoxazole and trimethoprim significantly inhibited the function of nitrifying bacteria in the initial period of long-term experiments,and accumulated ammonia nitrogen and nitrous acid in the effluent water.With the increase of contact time,the nitrifying bacteria gradually adapted to the environment in which the two antibiotics existed,and completely restored the nitrification function after 40 days.Due to the influence of antibiotics,the microbial community structure in the sludge has undergone significant changes,and the role of antibiotics has led to a decrease in the abundance of bacteria in the beta-proteobacteria and gamma-proteobacteria.The abundance of functional bacteria AOB and NOB in the process of long-term exposure to antibiotics showed a rapid reduction and gradual recovery,and the initial abundance ratio of AOB and NOB in the blank group was 1.92,1.97,respectively.After 35 days exposure to antibiotics,the proportions were 0.75 and 1.27,respectively.During the long-term exposure（80 days）,there was no significant difference between the blank groups at each stage.The samples in the experimental group gradually moved away from the blank group at 35 days after exposure to antibiotics.After 80 days of contact,the experimental group showed a tendency toward close to the blank group,indicating that the long-term exposure to antibiotics in the environment,the nitrification function of AOB and NOB can be gradually restored.