Synchronous N And P Removal By Pyrite-based Autotrophic Denitrification From Secondary Effluent

Nowadays,eutrophication has become a serious environmental issue throughout the world.And the secondary effluent from municipal wastewater treatment plant is one of the main reasons of causing and aggravating the eutrophication due to its large discharge and the much higher concentrations of nitrogen and phosphorus than the limit value of causing eutrophication.Therefore,the advanced treatment of nitrogen and phosphorus in the secondary effluent is greatly necessary.In this study,pyrite and organic residues in the secondary effluent were used asthe co-electron donors in mixotrophic denitrification system to remove nitrogen and phosphorus in the secondary effluent synchronously.The phosphorus removal capacity of pyrite and the feasibility of removing low concentration of nitrate and phosphorus from the secondary effluent were evaluated.The effects of pyrite particle size,initial nitrate and ammonium concentrations and carbon source conditions on the nitrate and phosphate removal performance in the mixotrophic denitrification system were investigated.Meanwhile,phosphate removal mechanism and denitrification process in the mixotrophic denitrification system were discussed by means of material balance calculation,microbial community analysis,XPS characterization of pyrite before and after the reaction.Furthermore,the bioavailability of fulvic acid?FA?in the secondary effluent was studied.The results showed that:Pyrite shows the ability of removing phosphorus and could remove 75.3%of PO₄^3--P within 6 h.Using pyrite as the electron donor of autotrophic denitrification to remove nitrate and phosphorus in the synthesized secondary effluent,NO₃^--N with the initial concentration of 12 mg/L could be removed by 98%within 6 days,NO₃^--N concentration in the effluent was 0.26±0.05 mg/L;PO₄^3--P with an initial concentration of 1.5 mg/L was completely removed within 24 h,but the effluent PO₄^3--P concentration was 0.39±0.27 mg/L.According to the results of the influencing factors,the larger the size of pyrite particle is,the more difficult it was to be chemically and biologically oxidized,resulting in poorer phosphate removal efficiency.However,when the pyrite particle size used was less than 1.00 mm,it showed no significant influence on the denitrification performance of the system.While when the pyrite particle size increased to greater than 1.00 mm,the denitrification ability was inhibited.And higher initial NO₃^--N concentration and NH₄⁺-N concentrations had adverse effects on denitrification rates.Under the condition of different carbon sources,pyrite-based mixotrophic denitrification system could achieve simultaneous nitrate and phosphate removal.The FA simulation of the secondary effluent organic matter also could undertake denitrification process,during which FA were first broken down into microbial available protein-like material before being used,accompanied with a trend of humification.In addition,when there were both organic and inorganic carbon sources in the denitrification system,the pyrite-based autotrophic denitrification and heterotrophic denitrification displayed a synergistic effect.The mixotrophic denitrification process was a two-stage reaction when the organic residues in the secondary effluent were simulated by glucose.Heterotrophic denitrification mainly occurred in the first 36 h,with the main functional bacteria genera of Simplicipira and Cloacibaterium.And autotrophic denitrification mainly occurred after36 h,the major functional genera were Sulfurimonas and Thiobacillus.The calculated contribution of heterotrophic denitrification was 55%⁶2%.PO₄^3--P in the system was mainly removed by combining with Fe³⁺produced by chemical and biological oxidation of pyrite?Dechloromonas and Ferritrophicum?to form FePO₄ precipitation and being adsorbed by Fe?OH?₃.The research results of this study provided a theoretical basis for the practical application of pyrite autotrophic denitrification technology,and contributed to developing a cost-effective method for nitrogen and phosphorus advanced treatment.