Study On The Deep Nitrogen And Phosphorus Removal Performance Of Biological Denitrification Using Modified Pyrite As Electronic Donor

With the increasingly strict requirements of the society for water environment protection,it has become a trend for the total nitrogen emission limit to reach the four standards of surface water quality.The main characteristics of the municipal sewage plant inflow is the low mass concentration ratio（C/N）of organic carbon and nitrogen.It is necessary to add carbon sources to make up for the lack of carbon sources for biological denitrification and denitrification,which increases the operating costs.Therefore,finding an economical and efficient alternative carbon source has become an urgent technical problem to be solved.Pyrite autotrophic denitrification（PAD）has three main advantages:no external carbon source,lower sludge productivity and lower operating costs.In order to solve the problem of long reaction time of pyrite in actual sewage treatment,the biological nitrogen and phosphorus removal performance of modified pyrite as electronic donor was explored.In this paper,natural pyrite is modified by acid solution,alkaline solution and calcination and mixed calcination.The crystal structure,physical properties and micro-morphology of pyrite are analyzed by modern analysis and test methods such as X-ray powder diffraction（XRD）,scanning electron microscopy（SEM）,specific surface area analyzer（BET）and magnetic intensity（VSM）.The study of nitrogen and phosphorus removal with acid modified pyrite as electron donor was investigated by static experiment.The acid-modified pyrite can be obtained by ball-milling natural pyrite to nanometer scale and soaking it in the solution with 50%acid concentration for 12 hours,which can achieve 90%denitrification efficiency and 4.2mg/（L·h）denitrification rate.It can also be seen from XRD and SEM that the crystallinity of pyrite soaked in 50%HCl decreases and the surface morphology changes,which is conducive to the removal of nitrogen.In terms of phosphorus removal,the removal rate of acid-modified pyrite for phosphorus solution with concentration of 10 mg/L and 40 mg/L is75.6%and 24.2%after 7 hours.The study of nitrogen and phosphorus removal with alkali modified pyrite as electron donor was investigated by static experiment.The nano-scale natural pyrite was soaked in60%alkaline solution for 12 hours to obtain alkali-modified pyrite with a nitrogen removal rate of 96.80%.Under SEM,it was observed that the surface of pyrite soaked in this concentration showed obvious erosion and mineral dissolution.In the aspect of phosphorus removal,the removal rate of 10 mg/L and 40 mg/L phosphorus solution by alkali modified pyrite is 85.9%and 31.7%respectively.The study of nitrogen and phosphorus removal with calcined pyrite as electron donor was investigated by static experiment.The natural pyrite was milled to nanometer scale and calcined at 600℃for 2 hours without oxygen to obtain modified pyrite.Through XRD and SEM analysis,pyrrhotite was formed and showed porous structure.In the system of calcining pyrite at 600℃,the nitrate removal end point can be basically reached within 3.5hours,and the denitrogenation rate is 96.36%.However,pyrite calcined at higher temperature will have higher crystallinity and cannot obtain better denitrogenation effect.Compared with pyrite after acid and alkali modification,pyrite calcined at 600℃has greatly improved in phosphorus removal.The removal rate of 10 mg/L phosphorus solution is 95.60%,but the removal effect of higher concentration phosphorus solution is still not ideal.The study of nitrogen and phosphorus removal with mixed calcined pyrite as electron donor was investigated by static experiment.Natural pyrite was milled to nanometer scale,mixed with dimethyl sulfoxide,and then calcined at 600℃for 2 hours without oxygen to obtain modified pyrite.XRD,SEM and BET analysis results show that the pyrite with high crystallinity is transformed into pyrrhotite with low crystallinity(Fe_0.95S_1.05)through modification and has a porous honeycomb structure.The specific surface area increases from 0.82 m²/g to 10.54 m²/g,and the pore size increases from 7.70 nm to 22.41 nm.At the same time,VSM results show that the magnetization of mixed calcined pyrite is 12 emu/g.The effect of different physical and chemical factors on the biological nitrogen removal performance of mixed calcined pyrite was investigated.The results showed that the average particle size of mixed calcined pyrite was 0.2μm.Under the conditions of dosage of 500 mg/L,temperature of 35℃and p H=7.0,the mass concentration of NO₃^--N in the simulated sewage decreased from 18.50 mg/L to 0.36 mg/L,the reaction time was shortened from 4.5 h to 0.75 h before the modification,and the removal rate of NO₃^--N was increased to 98.05%from 42.80%before the modification,indicating that the mixed calcined pyrite can be used as an electronic donor by denitrifying microorganisms efficiently.The analysis of denitrification kinetics showed that the kinetics of autotrophic denitrification（MNPAD）reaction with mixed calcined pyrite as electron donor was in line with the zero-order reaction characteristics（R²>0.97）.In terms of phosphorus removal,because the specific surface area and pore size of mixed calcined pyrite have increased significantly compared with those before modification,the adsorption capacity of higher concentration phosphorus has been greatly improved,and the removal rate of 40 mg/L phosphorus solution has reached 79.13%.Figure[33]Table[6]Reference[107]...