Evolution Of Extracellular Polymeric Substances And Settling Property Of The Activated Sludge With Calcium Ion Addition

Activated sludge wastewater treatment has been widely used for biological treatment of municipal and industrial wastewaters.The activated sludge settleability directly influences the normal operation of the sewage treatment system.At present,great attention has been paid to accelerate the start-up period and enhance floc properties and structural stability in activated sludge reactors with the aid of inorganic chemical agents such as calcium ion.In particular,extracellular polymeric substances(EPS),as major components of activated sludge,have a significant influence on the formation and stable operation of aerobic sludge in the field of wastewater treatment.Despite the indisputable central role of Ca2+ in the floc structure of activated sludge,surprisingly,few studies were found dedicated to the relationship between EPS and Caa·i The laboratory-scale sequencing batch reactors(SBRs)were operated to investigate the effect of Ca2+ on the physicochemical properties and evolution of EPS of sludge during SBR set-up and stability period.At the same time,many analytical methods had been used including three-dimensional excitation-emission matrix fluorescence spectroscopy(3D-EEM),fourier transform infrared spectroscopy(FT-IR),nuclear magnetic resonance(NMR),X-ray photoelectron spectroscopy(XPS)to confirm the binding between Ca2+ and EPS.Results from this study provided a fundamental knowledge basis for the improvement of the activated sludge settling properties with calcium ion addition.The main conclusions were as follows:1.In view of the different effectiveness of various methods of EPS extraction from activated sludge,the efficacies of extracting extracellular polymeric substances(EPS)from activated sludges using centrifugation,glutaraldehyde,formaldehyde + NaOH,EDTA and ultrasonic + cation exchange resin(CER)were compared.Results showed that the method of ultrasonic + CER was most effective in extracting EPS for sludge.2.During SBR start-up and stability period,different concentrations of Ca2+(0,50,100,150,200 mg/L)were added to investigate the influence of Ca2+ on activated sludge EPS content and its composition.Compared with the control system(non-calcium ion addition),the amount of EPS and polysaccharides(PS)were increased under the condition of Ca2+ dosage 150 mg/L,but there was no significant difference in protein.The influence of Ca2+ on LB-EPS and TB-EPS was similar to EPS,and the change of PS in EPS was mainly effected by TB-EPS.Ca2+ obviously increased the content of activated sludge EPS and polysaccharides.3.Three-dimensional excitation-emission matrix fluorescence spectroscopy(3D-EEM),fourier transform infrared spectroscopy(FT-IR),and nuclear magnetic resonance(NMR)technology were used to study the influence of Ca2+ on the functional groups of EPS during the reactor start-up and stability period.The analysis of EPS using 3D-EEM showed that response values of fluorescence peak D(Ex/Em= 200~250/250-300 nm)which represent tyrosine protein-like was enhanced with the addition of 150 mg/L Ca2+ in influent,which played an important role in the process of activated sludge flocculation.Combined the FT-IR technology,the EPS in characterization of tyrosine peak at 1517 cm-1 response values increased with 150 mg/L Ca2+.In addition,the functional group-OH absorption peak intensity was high in 3400 cm-1 which represent polysaccharide.And the amount of tyrosine in sludge with 150 mg/L Ca2+ was increased by 47.9%compared with the control system from amino acid content analysis.Further analysis with 1H-NMR indicated that hydrogen spectrogram was higher in characterization of polysaccharides in 5.0 ppm to 3.0 ppm which region was on behalf of-OH.Ca2+ can promote the production of tyrosine and polysaccharide in the activated sludge EPS,which improve the performance of the activated sludge sedimentation.4.The activated sludge settling property,microbial activity and the removal rate of pollutants were improved after the addition of different dose of Ca2+ The addition of 150 mg/L Ca2+ in the influent significantly increased the mixed liquid suspended solids(MLSS)and the mixed liquid volatile suspended solids(MLVSS).The activated sludge microorganism,dehydrogenase activity and the removal rate of pollutants improved,and floc structure stability of activated sludge was strenghed with the addition of Ca2+.5.The effect of EPS and Ca2+ on the activated sludge settling property was detacetea in this study.The settling property of activated sludge decreased after the LB-EPS and TB-EPS extracted with different concentration of Ca2+(0,50,100,150,200 mg/L)in influent,tightly bound EPS was more hydrophobic than loosely bound EPS.And the settling property of activated sludge increased significantly with 150 mg/L Ca2+ treatment after EPS extracted.The content of Ca2+ in activated sludge and EPS increased with the increasing of Ca2+ in influent.The Ca2+ content in activated sludge and EPS was 21 mg/g VSS and 25 mg/L with 150 mg/L Ca2+ in influent.And there was no significant difference with 200 mg/L Ca2+ in influent.The results of X-ray photoelectron spectroscopy(XPS)analysis showed that the main form of Ca2+ in activated sludge and EPS was Ca3(PO4)2 and CaCO3 which observably improved activated sludge settling performance.The mechanism of 150 mg/L Ca2+ in influent significantly improving the activated sludge was:on the one hand,Ca2+promoted the content of EPS and changed its components,which help the ludge biofloculation.On the other hand,the main form of Ca2 in sludge and EPS were Ca3(PO4)2 and CaCO3,which increased the floc density,and improved the activated sludge settling property.Results from this study provided a fundamental knowledge basis for the improvement of the activated sludge settling properties and microbial activity with calcium ion addition.