Study On The Mechanism Of Action Of Activated Sludge Extracellular Polymers In The Process Of Antibiotic Adsorption

A plenty of antibiotics produced in people's daily life will enter into wastewater treatment plants along with the sewage network system.It is showed that two main pathways of antibiotics removal in wastewater treatment plants are adsorption and biodegradation processes.Due to the existence of extracellular polymeric substances(EPS)in activated sludge,it may affect the adsorption process of antibiotics.However,up to now,the mechanism of antibiotics removal by EPS in activated sludge is still unclear,and the effects of different components of EPS on antibiotics adsorption removal are still unknown.Therefore,in this study,trimethoprim(TMP),a typical antibiotic,was selected as the target,through the in situ adsorption experiment,enzyme treatment to study the mechanism of different components of activated sludge EPS on TMP adsorption process for improving the removal of antibiotics in activated sludge system and protecting the ecological environment and human health.The main research results are as follows:1.In this study,three different activated sludge,the actual wastewater treatment plant activated sludge(WWTP AS),lab cultivated nitrifying activated sludge(NAS)and lab cultivated anaerobic ammonium oxidation sludge(AN)were selected as adsorbents for the adsorption experiment.Through the EPS analysis of three kinds of activated sludge,it was found that the EPS content of NAS sludge was the highest(86.69 mg/g VSS),and that of WWTP AS sludge was the lowest(22.95mg/g VSS).The components results of EPS showed that polysaccharide was the main component in NAS activated sludge with 47.2 mg/g VSS,while protein was the main component in AN and WWTPS AS sludge,whose concentration were 33 mg/g VSS and 11.1mg/g VSS,respectively.Three dimensional excitation-emission matrix fluorescence spectroscopy results showed that there were two main fluorescence peaks in three activated sludge EPS(Peak I,Ex/Em=270-280/325-335;Peak II,Ex/Em=225-235/325-335),which were belonged to the tyrosine and tryptophan aromatins,respectively.2.The study showed the adsorption process of antibiotics by activated sludge was affected significantly before and after EPS extraction.After EPS extracted from three kinds of activated sludge,the adsorption capacity of TMP for AN,WWTP As and NAS were increased from 28.33,5.37 and 2.98?g/g VSS to 149.62,12.93 and7.87?g/g VSS,respectively.These results indicated that the presence of EPS in activated sludge would inhibit the adsorption of TMP.After EPS extraction,the adsorption rates of TMP for NAS and AS were decreased from 0.125 and 0.756?g/g·min to 0.023 and 0.173?g/g·min,respectively,while the adsorption rates of TMP for AN was increased from 0.034?g/g·min to 0.214?g/g·min.It was found that the TMP adsorption process of activated sludge before and after EPS extraction can be expressed by a quasi-second-order kinetic model,and the TMP adsorption of activated sludge is a chemical adsorption process.Furthermore,the results also show that the protein content of EPS is positively correlated with the adsorption capacity and adsorption rate.3.The inhibition mechanism of EPS on activated sludge adsorption of TMP was explored.After EPS extraction,the zeta potential of the three kinds of activated sludge was decreased significantly from-16.97 m V?-13.70 m V to-30.07 m V?-26.73 m V.At the same time,the sludge contact angle decreased from 81°?84.6°to58.1°?62.1°.The results showed that after EPS extraction,the hydrophilicity of sludge was increased,while the hydrophobic protein content on sludge surface was decreased.Infrared spectrum results analysis showed that the protein related functional groups(1641 cm^-1 and 3435 cm^-1)were significantly reduced after CER extraction.Therefore,we conclude that after EPS extraction,activated sludge released more adsorption sites bonded with cell membrane,and TMP replaced the protein embedded in cell membrane improving the adsorption capacity of activated sludge on TMP.4.To explore the effect of different components of EPS on TMP adsorption,activated sludge EPS was treated with enzyme.Two different enzyme,?-amylase and proteinase K were selected for enzymatic hydrolysis of two main components of EPS,polysaccharide and protein components.The effect of proteinase K treatment was not obvious.The protein content and polysaccharides content were decreased from 31.25and 15.2 mg/g VSS to 21.53 and 8.22 mg/g VSS after proteinase K treatment(1200U).After?-amylase treatment(1000U),the protein content and polysaccharides content were decreased from 31.25 and 18.75 mg/g VSS to 9.2 and7.83 mg/g VSS.With the mixed enzyme treatment(1200U proteinase K and 1000U?-amylase),the protein content and polysaccharides content were decreased from31.25 and 15.2 mg/g VSS to 10.76 and 6.03 mg/g VSS.The results of EPS components after enzyme treatment indicated that EPS could be effectively removed by different enzyme.5.The quasi-second-order kinetic model can describe the TMP adsorption process of activated sludge after enzymatic hydrolysis.With the decrease of EPS content,the adsorption capacity of activated sludge increased gradually(from2.17?g/g VSS to 12.43?g/g VSS).The maximum adsorption capacity of antibiotics to activated sludge was 12.43?g/g VSS after CER treatment.The adsorption rate coefficient k₂ of antibiotics was decreased steadily(from 0.617?g/g.min to0.015?g/g.min).Protein plays an important role in TMP adsorption process.With the increasing number of protein adsorption sites on cell membrane,the adsorption capacity of activated sludge was enhanced.Using MATLAB to build multiple regression model,we can get Q_E=11.8284-0.3686X1-0.6943X2+0.2510X3(X1,X2and X3 represent the amount of protein,humic acid and polysaccharide in sludge EPS respectively).The established multiple regression model was used to verify the TMP adsorption process with data in other studies,and the results showed that the model could predict the adsorption process well with low concentration of antibiotics.