Study On The Effect Of Calcium And Iron Ions On Phosphorus Removal Pathway In Biological Phosphorus Removal System

Wastewater from urban wastewater treatment plants in many regions in China contains high concentrations of calcium ions derived from tap water or industrial wastewater and iron ions derived from the addition of bio-chemical synergistic phosphorus removal agents or industrial wastewater.At present,the effects of these calcium and iron ions on the phosphorus removal efficiency and pathways of biological phosphorus removal systems lack systematic and in-depth research.In response to the above problems,this study took biological phosphorus removal system as the research object,to explore the influence of influent calcium and iron ion concentration on the phosphorus removal pathway of biological phosphorus removal system.SMT phosphorus extraction method,Nuclear Magnetic Resonance Spectroscopy(³¹P NMR)and Scanning Electron Microscopy Energy Spectroscopy（SEM-EDS）analysis were utilized to investigate the phosphorus form and content of the sludge in the biological phosphorus removal system under different Ca²⁺and Fe³⁺concentrations,further analyzing the chemical precipitation components in the sludge and revealing how the phosphorus removal pathways of the system response to metal ions.Meanwhile,three-dimensional fluorescence spectroscopy（3D-EEM）and Fourier infrared spectroscopy（FTIR）analysis were employed to explore the phosphorus removal mechanism of extracellular polymer substances（EPS）in the biological/chemical phosphorus removal system under different Ca²⁺and Fe³⁺concentrations.Furthermore,16S r RNA high-throughput sequencing technology were applied to reveal the effect of Ca²⁺and Fe³⁺on the functional flora and microbial mechanism of biological phosphorus removal system.The main research conclusions were as follows:（1）The influent Ca²⁺concentration had a significant impact on the phosphate and organic matter removal pathway in the biological phosphorus removal system.As the Ca²⁺concentration increased,the phosphorus removal through the biological path decreased,and the removal through the chemical path increased.Under the influent Ca²⁺concentration of 40±10、80±10、120±10、160±10 mg/L,21.0%,26.3%,33.6%,and 45.5%of PO₄^3--P were removed by chemical pathways respectively.³¹P NMR results showed that with the increase of influent Ca²⁺concentration,the proportion of poly-P peak area in the aerobic end sludge decreased from 34.71%to 0.28%,which meant that the biological components in the sludge decreased,and the chemical components increased.SEM-EDS results showed that the precipitation products in the biological phosphorus removal system mainly contained calcium phosphate,calcium carbonate,hydroxyapatite and magnesium phosphate.16S r RNA high-throughput sequencing results showed that Ca²⁺had a significant impact on the biological phosphorus removal function in the system.With the increase of Ca²⁺concentration,the total relative abundance of phosphorus accumulating organisms（PAO）Rhodocyclaceae and Comamonadaceae decreased from15.66%to 5.57%,and the relative abundance of Glycogen accumulating organisms（GAO）Rhodospirillaceae decreased from 2.57%to 0.43%;the relative abundance of aerobic heterotrophic bacteria Chryseolinea increased from 0.20%to 6.57%.The degradation pathway of organic matter changed from PAO/GAO in anaerobic period to PAO and heterotrophic bacteria in anaerobic and aerobic periods respectively.（2）The biological effect in the biological phosphorus removal system significantly enhanced the chemical phosphorus removal effect of Ca²⁺.During the anaerobic period,anaerobic beginning（aerobic end of the last cycle:the biodegradation of organic matter made pH rise from 7.54±0.02 to 8.65～8.16）had a higher pH,meanwhile,the PAO released phosphorus,increasing the liquid PO₄^3--P concentration.These biological effects promoted the PO₄^3--P removal through chemical calcium precipitation.Under the influent Ca²⁺concentration of 40,80,120,160 mg/L,at the end of anaerobic phase,the liquid PO₄^3--P concentration of the biological phosphorus removal system decreased by 0,5.4%,12.1%and 25.2%relative to the maximum phosphorus release of each system.³¹P NMR results showed that,comparing with the sludge of aerobic phase,the peak area of poly-P at the end of anaerobic phase decreased by 20.16%,and the peak area of ortho-P increased by 4.51%.（3）As Ca²⁺concentration increased,the phosphorus removal efficiency of EPS in the system weakened.Under the influent Ca²⁺concentration of 40±10、80±10、120±10、160±10 mg/L,EPS contributed 26.1%,20.2%,16.4%,and 13.2%,respectively for the phosphorus removal of the system.During the cycle operation of the reactor,the content of PO₄^3--P in the EPS showed a trend of anaerobic increase and aerobic decrease.3D-EEM analysis results showed that as the concentration of Ca²⁺increased,the content of protein-like and microbial metabolic by-products in EPS decreased.（4）The Fe³⁺concentration had a significant impact on the phosphate and organic matter removal pathway in the biological phosphorus removal system.As the influent Fe³⁺concentration increased,the phosphorus removal through the biological phosphorus removal path decreased,and that through the chemical path increased.³¹P NMR results showed that with the increase of influent Fe³⁺concentration,the proportion of poly-P peak area in the sludge at the end of aerobic period decreased from 34.71%to 10.4%;the proportion of ortho-P increased from 12.70%to 21.42%.SEM-EDS results showed that the precipitation products of the system mainly contained ferric oxide,iron-phosphorus polymer and iron phosphate.16S r RNA sequencing results showed that Fe³⁺had a significant impact on the biological phosphorus removal function of the bacterial flora in the system.As the influent Fe³⁺concentration increased from 0 mg/L to 42±2 mg/L,the total relative abundance of PAO Rhodocyclus and Bdellovibrio in the system decreased from 1.10%to 0.21%;the relative abundance of GAO Defluviicoccus decreased from2.51%to 0.21%;the relative abundance of aerobic heterotrophic bacteria Chryseolinea increased from 0.20%to 2.06%.The organic matter degradation pathway changed from PAO/GAO in anaerobic period to PAO and heterotrophic bacteria in anaerobic and aerobic periods respectively.（5）The biological effect in the biological phosphorus removal system significantly enhanced the chemical removal effect of Fe³⁺.During the anaerobic period,anaerobic beginning（aerobic end of the last cycle:the biodegradation of organic matter made pH rise from 7.32±0.22 to 8.65～8.15）had a higher pH,meanwhile,the PAO released phosphorus,increasing the liquid PO₄^3--P concentration,which promoted the removal of PO₄^3--P through the adsorption of ferric oxides and the formation of iron phosphate precipitation.In the aerobic period,During the aerobic period,the heterotrophic bacteria degraded organic matter and CO₂ blow-off increased the liquid pH,which promoted the PO₄^3--P removal by Fe³⁺in the system.Under the influent Fe³⁺concentration of 14±2,28±2,42±2 mg/L,at the end of anaerobic,the concentration of PO₄^3--P in the system decreased by 5.9%,10.3%and 22.9%relative to the maximum phosphorus of each system.³¹P NMR results showed that,comparing with the sludge of aerobic phase,the peak area of poly-P at the end of anaerobic phase decreased by 10.40%,and the area of ortho-P increased by 1.63%.（6）As Fe³⁺concentration increased,the phosphorus removal efficiency of EPS in the system weakened.Under the influent Fe³⁺concentration of 0,14±2,28±2,42±2 mg/L,EPS contributed 26.1%,8.7%,5.5%,and 2.4%,respectively for the phosphorus removal of the system.During the cycle operation of the reactor,the content of PO₄^3--P in the EPS showed a trend of anaerobic increase and aerobic decrease.3D-EEM results showed that with the increase of Fe³⁺concentration,the content of protein-like,humic acid and microbial metabolic by-products in EPS was greatly reduced.