The Performance Of Autotrophic Denitrification Using Iron-sulfide Sludge As Electron Donor

Total nitrogen in the effluent of municipal and industrial wastewater treatment is generally difficult to reach discharge standard.The problems hindering advanced denitrification include unstable C/N ratio in influent wastewater,increased hydraulic loading with increasing reflux ratio,reduced reaction kinetics and high energy consumption,and secondary pollution and high sludge yield resulted from addition of organic carbon source.Therefore,deep denitrification with the advantages of energy-saving and easy operation is urgently needed.Therefore,an additional depth processing unit is required.In view of the low C/N value in secondary biological effluent,additional carbon source?such as methanol,acetic acid,glucose,etc.?is required to complete the heterotrophic denitrification,which is difficult to operate and has high operation costs,then there will be the risk of COD overflow or the nitrite accumulation.Autotrophic denitrification is getting increasingly attention due to its use of inorganic compounds(such as S,S^2-,Fe,Fe²⁺,H₂,etc.)as denitrification electron donor,has the advantage of low sludge production rate and less secondary pollution.Based on the above considerations,chemical iron sulfide sludge which was collected from the pre-treatment of sulfur-containing industrial wastewater was used as a solid-phase electron donor to perform advanced denitrification through autotrophic denitrification in this study.The research contents and results are as follows:?1?Through a series of batch experiments,examined the chemical oxidation characteristics of iron-sulfur sludge,and the influence of dosage of iron-sulfur sludge,sludge concentration,HRT and various physical and chemical factors(T,pH,NH₄⁺-N,HCO₃^-,PO₄^3--P,etc.)on autotrophic denitrification performance of iron-sulfur sludge.In the autotrophic denitrification process of iron-sulfur sludge,when the influent NO₃^--N concentration was 75mg/L,the optimum dosage of iron-sulfur sludge was 0.56 g/L,and the optimum sludge concentration was approximately 4000 mg/L.When the HRT was 8 h,the removal rates of NO₃^--N and TON were 88.50%and 45.09%respectively,and the NO₂^--N accumulation rate reached a maximum value of 46.85%,and the reaction time was to 18 h,the removal rates of NO₃^--N and TON were higher than 90%.The optimum temperature range of this system was25³5°C,the reduction efficiency of NO₃^--N was higher when the range of pH is 6⁹.The limiting concentrations of NH₄⁺-N,Mg²⁺,HCO₃^-and PO₄^3--P were 2.0,0.4,40,and 0.2 mg/L,respectively.When the PO₄^3--P concentration was less than 50 mg/L,the phosphorus removal rate was as high as 99%.The high concentration of NO₃^--N had no effect on the reduction rate of nitrate,but when the concentration of NO₂^--N was higher than 200 mg/L,the activity of sulfur autotrophic denitrifying bacteria was inhibited.?2?Based on the optimized experimental operating conditions,a continuous integrated vertical up-flow biological fluidized bed reactor was established to investigate the long-term denitrification characteristics of the iron-sulfur sludge autotrophic denitrification?ISAD?system.The performance period of ISAD system was over 90 days,the pH of influent was 7.0to 8.0,and the reaction temperature was 20 to 30°C.When the concentrations of NO₃^--N and NO₂^--N in influent were 74.54 mg/L and 1.11 mg/L respectively,the corresponding concentrations in effluent were reduced to 2.78 mg/L and 2.87 mg/L within the HRT of 18 h,the nitrogen loading reached 4.20 mg/?L·h?,the removal rate of TON was as high as 93.36%,in which the reduction rates of NO₃^--N and NO₂^--N,and the accumulation rate of NO₂^--N were12.06 mmol?L/d?,3.70 mmol?L/d?and 7.74 mmol?L/d?respectively,showing the characteristic of high efficiency.When HRT was 12 h,the nitrite in effluent accumulated and the accumulation rate reached 34.05%.The solid sulfide in iron-sulfur sludge was oxidized by microorganisms to soluble sulfate,and electrons were released to reduce nitrate at the same time,forming the flat and acicular secondary mineral crystals.According to the microbial community structure analysis,the dominant strain of the reactor containing iron-sulfur sludge was Proteobacteria,and Thiobacillus which played a major role in denitrification was present in the reactor.?3?Iron-sulfur sludge and pyrite were used as electron donors for autotrophic denitrification respectively,and the feasibility,stability and difference of deep denitrification of wastewater which is lack of organic matter were compared systematically.The reduction rates of NO₃^--N in the process of autotrophic denitrification of iron-sulfur sludge and pyrite were 8.51 mg/?L·h?vs 3.80 mg/?L·h?,the reduction rate of NO₂^--N were 2.67 mg/?L·h?vs 1.59mg/?L·h?,and the accumulation rate of NO₂^--N were 4.60 mg/?L·h?vs 2.09 mg/?L·h?.The ISAD system and UASB reactor for pyrite autotrophic denitrification?PAD?system continued to operate over 90 days,the concentration of NO₃^--N in influent was 70-80 mg/L.When HRT was 18 h,the removal rates of NO₃^--N and TON in ISAD system were 96.30%and 91.51%respectively,while the PAD system gradually lost the ability to reduce nitrate,the removal rates of NO₃^--N and TON were only 13.73%and 13.90%within 40 d.The sludge of ISAD system was formed the flat and acicular secondary mineral crystals by microbiological corrosion.The proportion of sulfur decreased from 15.14%to 1.93%.However,the sludge and elemental composition of PAD system did not change significantly.The dominant bacteria in the ISAD and PAD systems were both Thiobacillus,but the microbial community structures formed in two reactors were different during long-term operation.Iron-sulfur sludge and pyrite are feasible for nitrate reduction under the action of microorganisms.During this process,there will be an accumulation of nitrite.Compared with pyrite,iron-sulfur sludge shows up larger specific surface area,porous surface,high chemical activity,and no fixed crystal form,therefore exerts significant advantages in the removal of total nitrogen and nitrate nitrogen.ISAD system appears to be more potential and practical due to its stable and efficient nitrogen removal rate,stable effluent pH and lower sulfate yield.Therefore,the use of chemical iron-sulfur sludge which is collected from the pre-treatment of sulfur-containing industrial wastewater as a electron donor for depth denitrification could achieve a win-win result of wastewater treatment,it is not necessary to add carbon sources and it could reduce the amount of solid waste disposal while realizing the resource utilization of such chemical sludge,finally reaching the goal of resource utilization in practical engineering applications.