The Influence Study Of Fe³⁺on Nitrogen And Phosphorus Removal And Its Migration And Transformation Law By A²O Process

A2O(Anaerobic-Anoxic-Oxic)process has become the main wastewater treatment process of nitrogen and phosphorus removal because of its simple flow,convenient operation and management,and less excess sludge.The key to improve the efficiency of nitrogen and phosphorus removal is to increase the activity and bioflocculation of the sludge.Fe is an important element affecting microbial growth and metabolism.A large amount of wastewater containing Fe3+is produced in the industries such as mining,metallurgy,electroplate and iron flocculant production and application.Fe3+entered the sewage treatment plant can affect the activity of microorganisms and the sludge properties.In this study,the effects of Fe3+concentration on the removal efficiency of pollutants and the sludge properties were investigated,the form distribution.The migration and transformation of Fe3+in sludge system were studied.The composition structure of extracellular polymeric substances(EPS)and the change of microbial community structure were analyzed.At the same time,in order to further improve the flocculation performance of the sludge,graphite particles and iron ions were both added to clarify the effect of graphite particles/Fe3+on the performance of the sludge.The results showed that low concentration of Fe3+(<10 mg·L-1)could improve COD and TN removal and enhance the microbial activity.However,high concentration of Fe3+(10-40 mg·L-1)could inhibit them.When Fe3+ concentration is 10 mg·L-1,the removal efficiencies of COD and TN reached the maximum(96%and 89%,respectively),and the activity of dehydrogenase in anaerobic,anoxic and oxic zones were 35.83 mg·(L·h)-1,31.91 mg·(L·h)-1 and mg·(L·h)-1,respectively.With the increase of Fe3+ concentration,the removal efficiency of TP increased.The enhancement of phosphorus removal was achieved by the biological action and chemical adsorption under the low concentrations of Fe3+,while the chemical adsorption of phosphorus was dominant under the high concentrations of Fe3+.Fe in the supernatant was mainly existed in the form of Fe3+.Fe3+ in tightly bound EPS(TB)was the most.Fe3+ in the soluble microbial products(SMP)and loosely bound EPS(LB)layer could migrate to the TB layer.Most of Fe3+ was deposited in the sludge and accumulated in the microorganisms.Low concentration of Fe3+ could improve the flocculation of sludge,increase the particle size of sludge,and enhance the flocculation ability(FA)and Zeta potential.When was Fe3+concentration was 10 mg·L-1,FA,Zeta potential and particle size of the sludge reached the maximum.At this time,FA in each zone was 48.7%,53%and 49.2%.The Zeta potential was-20.552 mV,-18.461 mV,-18.352 mV,respectively,and the sludge particle size was 40.32?m,43.34 ?m,39.72 ?m,respectively.When Fe3+ concentration increased from 10 mg·L-1 to 40 mg·L-1,FA decreased to 26%,29%and 32%and Zeta potential decreased to-49.057 mV,-52.656 mV,-53.671 mV,respectively.Meanwhile,the total amount of EPS increased.The main groups of SMP,LB and TB remained unchanged,and amino acids and protein like organics had been the main components of EPS.It was found that SiO2 crystal was mainly found in the sludge and Fe3+ in the amorphous phase based on X-ray diffraction(XRD)analysis.According to the analysis of fourier transform infrared spectroscopy(FTIR)and specific surface area micropore analyzer,the main functional groups of sludge remain unchanged.The surface area increased and the average pore diameter decreased.The total pore volume changed little.With the increase of Fe3+ concentration,the richness and diversity of the community decreased,but the dominant phylum and dominant genus of the microbial population did not change.Candidatus-Accumulibacter in anaerobic zone,Candidatus-Competibacter and Nitrospira in anoxic zone,Terrimonas in oxic zone were Fe3+-resistant bacteria.At the same time,it was found that K03413 in anaerobic zone,K03088 in anoxic zone and K03832 in oxic zone were the dominant genes under the high concentration of Fe3+ by PICRUSt analysis.Graphite particles and Fe3+ were simultaneously added in the activated sludge system.Fe3+decreased the specific surface area and the negative charges on the surface of graphite particles,and increased their pore size.However,Fe3+ did not change the crystal structure of graphite particles(SiC crystal was main existence form).The conductivity of graphite promoted the degradation and transformation of the pollutants under the action of Fe3+.The main groups of sludge were not affected and mainly included O—H?N—H?C—H?C=O?amide I,O—C—O and C—O.Graphite particles/Fe3+ could promote the bioflocculation.Compared with the sludge without the addition of graphite particles/Fe3+,the particle size and FA increased by 20%and 15%,respectively,and the Zeta potential increased from-63.9102 mV to-53.8723 mV.