Research On The Effect Of Anaerobic Digestion And Photocatalysis On The Inactivation Of Enteric Indicator Bacteria And Antibiotic Resistance Genes In Dairy Wastewater

Dairy wastewater contains high concentration of organic matter.In addition,dairy wastewater is an important reservoir of pathogens and antibiotic resistance genes（ARGs）.Pathogens may cause a series of diseases,such as foot-and-mouth disease and diarrhea and so on.Antibiotic resistance genes,as an emerging environment pollutant,posing universal health threats to the human and animals.Serious biological risks might be caused if the dairy wastewater is discharged directly.Dairy wastewater is usually treated by anaerobic digestion which can convert biodegradable organic matter to methane as renewable energy.However,anaerobic digestion has a poor effect on refractory organics such as lignocellulose,and its impact on antibiotic resistance genes is very complicated.On the other hand,as a green and efficient method,the photocatalytic can cause active oxygen species to attack the pollutants in wastewater.However,the impact of photocatalysis on the biological safety of dairy wastewater is still unclear,and the photocatalytic materials is difficult to separate from the aqueous solution.In response to the above problems,this thesis first investigated the effect of the pretreatment（ultrasonic,ozone and ultrasonic combined ozone pretreatments）on pathogens inactivation,the reduction of ARGs in dairy wastewater,and on their further fate and methane production during anaerobic digestion of the pretreated dairy wastewater.In addition,this thesis investigated that the effect of photocatalysis,which using TiO₂-based floating materials and g-C₃N₄-based floating materials as photosensitizers,on the inactivation of enteric indicator bacteria in dairy wastewater.1.The effects of ultrasonic pretreatment,ozone pretreatment and ultrasonic combined ozone pretreatment（ultrasonic/ozone）on pathogens and ARGs in dairy wastewater,as well as the subsequent anaerobic digestion process were studied.The results showed that all three pretreatments could reduce the content of coliforms and enterococci in dairy wastewater,especially ultrasonic/ozone pretreatment,total coliforms and enterococci were reduced by 99% and 92% after 30 min ultrasonic/ozone pretreatment.Pretreatments could not decrease ARGs in absolute concentration,but could decrease ARGs in relative abundance.In the subsequent anaerobic digestion,compared to the control group,ultrasonic pretreatment had little effect on the methane production while methane production increased more than 10% with ozone and ultrasonic/ozone pretreatments.Pretreatment-anaerobic obviously inhibited the enrichment of ARGs in relative abundance.This study showed that ultrasound/ozone pretreatment can enhance methane production and prevent the enrichment of ARGs.2.TiO₂-based floating material was obtained by loading TiO₂ on ethylene-vinyl acetate.The effect of photocatalysis,which using TiO₂-based floating materials as photosensitizers,on the inactivation of enteric indicator bacteria in dairy wastewater was studied.The results showed that the floating material had poor adsorption capacity for enteric indicator bacteria,but photocatalytic performance was good.Under the condition of light intensity of 800m W/cm²（UV254）,the amount of inactivated enteric indicator bacteria exceeded 4 orders of magnitude within 4 hours when 1 g/L of TiO₂-based floating material（TiO₂ net content）was added to the reaction system.The material concentration,light intensity and the concentration of organic matter in the reaction system can affect the efficiency of the floating material to inactivate enteric indicator bacteria by photocatalysis.The inactivation efficiency of enteric indicator bacteria decreased when the floating material was used in actual dairy wastewater.The main mechanism to inactivate enteric indicator bacteria was that the floating material absorb photon to generate·OH andO₂·^-,and these active oxygen species attack cells to cause them death.In the process,·OH andO₂·^-contribute equally.This research showed that the photocatalytic method,using TiO₂-based floating material as photosensitizers,could inactivate enteric indicator bacteria,thereby reducing the biological risk of dairy wastewater,and also solved the problem that photocatalytic materials were difficult to separate from aqueous solutions.3.g-C₃N₄-based floating material was obtained by loading g-C₃N₄ on ethylene-vinyl acetate.The effect of photocatalysis,which using g-C₃N₄-based floating materials as photosensitizers,on the inactivation of enteric indicator bacteria in dairy wastewater was studied.The results showed that the floating material has poor adsorption capacity for enteric indicator bacteria,but photocatalytic performance was good.The material concentration,light intensity and the concentration of organic matter in the reaction system can affect the efficiency of the floating material to inactivate enteric indicator bacteria by photocatalysis.The inactivation efficiency of enteric indicator bacteria decreased when the floating material was used in actual dairy wastewater.The main mechanism to inactivate enteric indicator bacteria was that the floating material absorb photon to generate ¹O₂,·OH andO₂·^-,which played mine role in enteric indicator bacteria inactivation.Among these radicals,O₂·^-was dominant contribute radicals,followed by ¹O₂,and the least contribution is·OH.The photocatalytic method,using g-C₃N₄-based floating material as photosensitizers,could inactivate enteric indicator bacteria,thereby reducing the biological risk of dairy wastewater,and could use visible light,and also solved the problem that photocatalytic materials were difficult to separate from aqueous solutions.