Study On Simultaneous Removal Of Nitrogen And Phthalates From Wastewater Treatment Plant Effluent Using Solid-phase Denitrification System

The excess of phthalate esters（PAEs）micropollutants and nitrogen in the aqueous environment poses an increasingly serious environmental problem.Adding solid carbon source to enhance the denitrification of low C/N sewage can solve the problems of low denitrification efficiency and difficulty in meeting the total nitrogen standard of effluent,which is one of the hotspots of current research.However,little attention has been paid to the removal of phthalic acid esters while using plant carbon sources for deep denitrification of tail water.In this paper,the tail water containing both nitrate and PAEs was used as the research object,and Arundo donax was used as the additional plant carbon source for solid phase denitrification.The properties of the carbon source filler and its performance of simultaneous denitrification and removal of phthalic acid esters were analyzed by characterization analysis,batch adsorption experiments,batch biodegradation experiments,small-scale biofilter experiments,and high-throughput sequencing.The effects of different phthalic acid ester stresses on the denitrification effect were explored,the path of PAEs biodegradation was studied,and the operating parameters of the biofilter were optimized.The optimum hydraulic retention time（HRT）and water quality conditions containing dibutyl phthalate（DBP）and butyl benzyl phthalate（BBP）were determined.The main findings are as follows:（1）The adsorption of DBP and BBP on Arundo donax satisfied the pseudo-second-order kinetic model,which was dominated by chemical adsorption,and the adsorption rate was affected by the internal diffusion of particles.The surface roughness of Arundo donax adsorbed DBP and BBP was the largest,the number of holes was the largest and the distribution was relatively uniform,which was more suitable for the adhesion and growth of microorganisms.（2）In the anaerobic environment dominated by suspended sludge,the presence of phthalates in the water promotes nitrate removal,and the system with DBP and BBP has the best denitrification effect.However,the concentration of nitrite nitrogen and ammonia nitrogen in the effluent of the blend system with the simultaneous presence of two PAEs was higher than that of the other three groups,because the reaction of nitrate dissimilatory reduction to ammonium（DNRA）occurred due to insufficient carbon source.DBP and BBP achieved excellent removal efficiency in both the single system with only one PAEs and the binary system with two PAEs,and their removal mainly depended on the adsorption of Arundo donax and the biodegradation of activated sludge.（3）In the biofilm-dominated anoxic denitrification filter,the best condition for nitrate removal was HRT 3 h,and the addition of 60 ppb phthalate in the water did not affect the overall denitrification effect of the system.However,the addition of DBP alone can promote the removal of NH₄⁺-N,and the addition of BBP alone can effectively reduce the accumulation of NO₂^--N.The removal rate of DBP in the single and binary systems reached more than 91%,and BBP could always be completely removed.In the range of HRT=3、2.5、2 h,the denitrification effect of the reactor decreased with the shortening of hydraulic retention time,because HRT would directly affect the contact time and mass transfer process between denitrifying bacteria and substrate.（4）The phthalic acid ester in the water body further affects the denitrification capacity of the system by affecting the distribution and proportion of the microbial community structure.The microorganisms are mainly denitrifying bacteria and PAEs degrading bacteria.According to the measured degradation intermediates of DBP and BBP,it was speculated that the general pathway of BBP biodegradation was as follows:BBP was first degraded by microorganisms to produce DBP,then decomposed into dimethyl phthalate and monoethyl phthalate,and further generated phthalic acid（PA）.PA was metabolized into benzoic acid under anoxic conditions,and finally completely degraded into carbon dioxide and water.The biological metabolic process of BBP includes the degradation of DBP.