Study On Impact Of Nanoscale Zero-valent Iron,Fe²⁺ And Fe³⁺ On The Properties Of Aerobic Granular Sludge

Aerobic granular sludge?AGS?has the unique advantages of less sludge bulking,strong impact resistance,high organic load bearing and synchronous nitrogen and phosphorus removal.Nanoscale zero-valent iron?nZVI?is widely used in the removal of organic pollutants,nutrients and heavy metals in sewage due to its strong reducing activity and adsorption capacity,and it is easy to be oxidized into Fe²⁺or Fe³⁺in aerobic biological treatment system.An appropriate amount of nZVI will improve the activity of sludge microorganisms,but the accumulation of a large amount of nZVI will have potential adverse effects.Therefore,it is necessary to discuss the relationship between the dose of nZVI,Fe²⁺and Fe³⁺and the effect of AGS,so as to provide theoretical reference and basis for the better application of nZVI in AGS system.In this paper,three sequential batch reactor?SBR?,namely R₁,R₂ and R₃,were used to add different concentrations of Fe³⁺,nZVI and Fe²⁺into the influent water,and the effects of different concentrations of Fe³⁺on AGS formation and nitrification characteristics were investigated.The impact of different concentrations of nZVI and Fe²⁺on mature AGS is discussed.The main research contents and conclusions are as follows:?1?The addition of Fe³⁺did not promote the process of sludge granulation in SBR,but had great influence on the morphology of sludge granulation.Compared with R₃?control?,R₁?1 mg/L?and R₂?10 mg/L?could promote the secretion of more extracellular polymers?EPS?,and the COD removal rate of AGS in R₁ was the highest with the NH₄⁺-N degradation rate.In R₁,R₂ and R₃,SOUR for nitrosification increased by 131.80%,124.90%and 11.00%,respectively,while SOUR for nitrification increased by 239.60%,282.10%and 146.20%,respectively.Nitrification kinetic parameters were:v_m1>v_m2>v_m3,indicates that R₁ is easier to realize efficient nitrification process.The results of high-throughput sequencing showed that the addition of Fe³⁺could improve the denitrification and carbon removal performance of AGS.?2?The dosage of nZVI increased from 0 mg/L to 25 mg/L,the removal rate of NH₄⁺-N was significantly enhanced by AGS.TN removal was promoted when the dosage of nZVI was less than 100 mg/L,and the concentration of nZVI was 5 mg/L,the removal rate of NH₄⁺-N and TN in effluent by AGS was the highest,which was 22.75%and 51.77%higher than the control group,respectively.The measurement results of the ammonia oxidation rate of sludge show that:when the dosage of nZVI is 5 mg/L,the ammonia oxidation rate of AGS is the highest and the promoting effect is most obvious,while when the concentration of nZVI is higher than 50 mg/L,AGS ammonia oxidation is inhibited.?3?The accumulation and loss in the content of LB-EPS and TB-EPS were observed respectively with the increase of nZVI dosage.When the concentration of nZVI was 25 mg/L,the content of AGS secreted LB-EPS was the highest and the content of TB-EPS was the lowest.Fourier transform infrared spectroscopy?FTIR?results show that the increase in nZVI concentration has a significant effect on the protein and polysaccharide components in TB-EPS.When the dosage of nZVI increased from 0 mg/L to 50 mg/L,the adsorption ratio of AGS to nZVI gradually increased,and when the dosage of nZVI was higher than 50 mg/L,the adsorption ratio of AGS to nZVI gradually decreased,but the iron content in the mud phase still increased significantly.Scanning electron microscopy?SEM?results showed that a large amount of nZVI was adsorbed on the AGS surface,which caused a large reduction of filamentous bacteria in the microbial flora,affecting the microbial ecology of the sludge surface.?4?When the dosage of Fe²⁺was increased from 0 mg/L to 100 mg/L,AGS had no effect on the removal of NH₄⁺-N,but the removal of TN was promoted.When the concentration of Fe²⁺was 50 mg/L,AGS has the highest removal rate of TN,which is 17.89%higher than the control group.The results of ammonia oxidation rate measurement showed that when the dosage of Fe²⁺was increased from 0 mg/L to 100 mg/L,the ammonia oxidation rate of AGS increased slightly compared with the control group,while when the dosage of Fe²⁺was higher than 100 mg/L,the ammonia oxidation rate of AGS is significantly reduced.?5?The content of LB-PN and TB-PN secreted by AGS increased first and then decreased with the increase of the dosage of Fe²⁺,while the content of LB-PS and TB-PS decreased slightly compared with the control group.When the dosage of Fe²⁺was 50 mg/L,AGS secreted the highest levels of LB-EPS and TB-EPS,which were 33.09%and 10.18%higher than the control group,respectively.The three-dimensional fluorescence spectrum showed that the concentration of Fe²⁺in the influent did not affect the main components of LB-EPS,and the high concentration of Fe²⁺would change the main components of TB-EPS in AGS.FTIR results show that the change of Fe²⁺concentration in the influent has no effect on the functional group of LB-EPS,but has a significant effect on the protein and polysaccharide components in TB-EPS.The distribution of iron in the mud phase and the water phase shows that when the dosage of Fe²⁺increases from 0 mg/L to 300 mg/L,the adsorption ratio of Fe²⁺by AGS increases continuously.The SEM results show that the AGS surface adsorbs a large amount of particulate matter,which significantly changes the micro-ecological environment on the surface of the sludge.