Effect Of Influent Copper Ions On Sludge Treatment Efficiency And The Role Of Extracellular Polymeric Substances

With the development of industrialization,many heavy metals enter the wastewater treatment system and interact with functional microorganisms.Copper(Cu)is widely used in industry and livestock and poultry farming industry,resulting in a variety of industrial wastewater and livestock and poultry farming wastewater containing a large amount of copper iron(Cu2+).As a toxic heavy metal,Cu has significant effects on many denitrification systems,such as short-cut nitrification,denitrifying phosphorus removal and anaerobic ammonia oxidation systems.But so far,the impact of Cu2+ on biological treatment systems is still lacking a systematic understanding.In this paper,the treatment of simulated wastewater by sequence batch reactor(SBR)was studied.On the basis of investigating the effect of Cu2+ on sludge morphology and pollutant removal efficiency,the changes of content and composition of extracellular polymeric substances(EPSs)in sludge were analyzed.The fluorescence characteristics of EPSs were investigated by three-dimensional fluorescence spectroscopy and region analysis.The comprehensive effect of EPSs on sludge nitrification activity in the presence of Cu2+ was analyzed.The changes of microbial community structure and functional microflora were also analyzed.In order to provide reference for further understanding the influence of Cu2+ on biological treatment system.The influent Cu2+ content affects the removal of chemical oxygen demand(COD)and ammonium(NH4+-N)by SBR,especially ammonia nitrogen.Low concentration of Cu2+(1 mg-L-1)slightly inhibited COD and NH4+-N removal.When the influent Cu2+ content was 5 and 9 mg·L-1,the removal rates of COD and NH4+-N decreased by 10.55%and 54.11%,16.50%and 62.12%,respectively.The content and composition of EPSs were affected by the influent Cu2+ content and addition time.The contents of the total extracellular polymeric substances(T-EPSs)and tightly bound extracellular polymeric substances(TB-EPSs)decreased gradually(1 mg·L-1),increased first then stabilized(5 mg·L-1)and increased first then decreased(9 mg·L-1)with the prolongation of Cu2+addition time,respectively.Except for the higher content of influent Cu2+(9 mg·L-1),the content of the loosely bound extracellular polymeric substances(LB-EPSs)showed a decreasing trend.Under other conditions,the content of LB-EPSs was basically stable.The content of protein(PN)and polysaccharide(PS)in T-EPSs and TB-EPSs decreased gradually(1 mg·L-1),increased first then stabilized(5 mg·L-1)and increased first then decreased(9 mg·L-1)with the prolongation of Cu2+ addition time,respectively.The PS content in LB-EPSs decreased gradually to 1.77 mg·L-1,2.58 mg·L-1 and 1.50 mg·L-1 when the influent Cu2+ content was 1,5 and 9 mg·L-1,respectively.The variation of PN in LB-EPSs was related to the influent Cu2+ content.The fluorescence characteristics of EPSs were analyzed by three-dimensional fluorescence spectroscopy and region analysis.The results showed that the fluorescent substances in EPSs were mainly aromatic protein substances and soluble microbial byproduct-like material.After adding Cu2+,the fluorescence intensity and peak position of the two characteristic peaks changed.When the content of Cu2+ in the influent was low(1 mg·L-1),the primeval excitation-emission area volume of the substances in the five regions decreased.With the increase of Cu2+ content in influent water,Cu2+ of 5 mg·L-1 stimulated microorganisms to secrete aromatic protein region ?,aromatic protein region ?,soluble microbial byproduct-like material,and humic acid-like organics.For TB-EPSs and LB-EPSs,the toxicity and fluorescence quenching effect of Cu2+ resulted in the decrease of substance content in five regions when the content of Cu2+ increased to 9 mg·L-1,comparing with 5 mg·L-1.The long-term existence of Cu2+ inhibited the nitrification activity of sludge,and the higher the concentration,the greater the inhibition.Cu2+ induces microbial changes in EPSs to protect microorganisms while also inhibiting sludge nitrification activity.Compared with TB-EPSs,the presence of LB-EPSs has a greater inhibitory effect on sludge activity.The sludge specific aerobic rate(SOUR)decreased with the increase of influent Cu2+ concentration.After removal of TB-EPSs and LB-EPSs,the inhibition rates of nitrification activity increased by 7.99%,22.65%,23.44%and 32.88%,46.56%,64.17%.High-throughput sequencing was used to analyze the changes of microbial community structure and function under different Cu2+ content.Influenced by different concentrations of Cu2+,the species richness of microbial community increased while the diversity decreased in the reactor.With the increase of Cu2+ concentration,the bacteria related to EPSs production,for example,the relative abundance of Rhodocyclaceae,Xanthomonadaceae,Comamonadaceae and Flavobacteriaceae has changed.At the same time,the relative abundance of bacteria associated with pollutant removal(COD and NH4+-N),such as Thauera,Luteimonas and Acinetobacter,also changed.