Study On The Mechanism Of Combined Effect Of Microplastics And Ofloxacin On Anaerobic Digestion Of Waste Activated Sludge

The variety of pollutants entering wastewater treatment plants tends to diversify due to the continued consumption and misuse of large quantities of chemicals.Some emerging contaminants are not only difficult to be biodegraded,but are also highly hydrophobic,such as microplastics and ofloxacin,which are not effectively eliminated in the wastewater treatment process and are therefore detected in large quantities in the watse activated sludge generated from wastewater treatment.During anaerobic digestion of the waste activated sludge,the pollutants adsorbed or contained in it will inevitably affect the digestion process.To explore the anaerobic treatment of waste activated sludge under complex pollution,this paper combined polystyrene(PS)microplastics and polyvinyl chloride(PVC)microplastics with ofloxacin,respectivel y.The study was carried out with waste sludge as the digestion substrate,and the mechanism of their combined effects was investigated in depth on the basis of the single effect of each separately.The main findings were as follows.(1)Three experimental conditions were set: single PS microplastic,single ofloxacin,and combined contamination.By conducting biochemical methane potential(BMP)test to examine the methanogenesis with time separately,the influence of pollutants on methanogenesis was analyzed by kinetic models.Based on the influence behavior,dissolved organic matter(DOM),extracellular polymers(EPS)and microbial communities in the system were examined,while four different metabolic stages of solubilization,hydrolysis,acidification and methanogenesis were simulated so as to comprehensively identify the impact mechanisms.The results showed that single PS microplastics inhibited methane production much less strongly than ofloxacin.The combined contaminants showed synergistic effects,leading to the decrease of methane production rate and the shortening of effective gas production time,respectively,with the dominance of inhibition by ofloxacin.Mechanistic investigation revealed that under both single ofloxacin and combined conditions,the humic substances in DOM increased greatly,the efficiency of the four stages were significantly inhibited,especially the acidification process,and the structural change o f EPS components was not conducive to the release of hydrolase.Single PS microplastics increased methane production rate at the beginning of the reaction,and caused relatively less effect of the four stages.Microbial community analysis revealed that the distribution of communities in the combined group was more similar to that in the single ofloxacin group,where the abundance of acidification bacteria Hydrogenispora and acetic acid methanogen Methanosaeta was significantly reduced,indicating that the metabolic pathway was obviously affected.(2)Three experimental conditions were set: single PVC microplastic,single ofloxacin,and combined contamination.Based on the main body of BMP tes ts,a series of studies including kinetic analysis,stage simulati on experiments,EPS detection and microbial community identification were conducted,and the mechanism of single and combined effects was investigated.The results showed that methane production was significantly inhibited by ofloxacin.PVC microplastics did not have a significant effect at 50 mg/L level and promoted production at the 200 mg/L level.The cumulative methane yield of the combined group was comparable to that of the control,revealing the mitigating effect of PVC microplastics on the strong in hibition by ofloxacin within the combined system.Mechanistic analysis indicated that the single addition of PVC enhanced the efficiency of solubilization.Compared to the single ofloxacin group,the combined group showed significant enhancement in solubil ization and hydrolysis processes,while it was still suppressed by the toxicity of oxfloxacin in acidification and methanogenesis processes.PVC microplastics weakened the effect of oxfloxacin formation on TB-EPS,further illustrating the mitigation mechan ism.The dominant microbial community comparison revealed that the changes of acidification bacteria Bacteroidetes and methanogenic archaea Methanosaeta in the combined group were more affected by ofloxacin,and the acidification and acetic acid nutrient-based methanogenic pathways were severely suppressed.