Reasearch On The Conditions Of Iron Matrix Autotrophic Biological Denitrification Process

In recent years,the problem of nitrate pollution in water bodies has aroused people's attention and research,treatment of nitrate with heterotrophic denitrification and sulfur autotrophic denitrification will lead to the introduction of secondary pollutant of organic matter and sulfate into water,so denitrification with zero valent iron was carried out,the iron resultant ferric hydroxide was precipitated from water,the denitrification can be applied to treat surface water,ground water and urban wastewater with nitrate pollution.The reactor type selection,reactor operation parameters and conditions of autotrophic denitrification process with zero valent iron as matrix are introduced without secondary pollutant.The operation of a stirred upflow reactor,an internal reflux upflow reactor and an external reflux upflow reactor was observed,the external reflux reactor that was suitable for a biological reaction with zero valent iron and nitrate as substrate was determined,Iron autotrophic denitrifying bacteria were obtained by inoculating sewage sludge from municipal wastewater treatment plants.The use of reflux causes ferric iron to produce ferrous iron substrate with zero valent iron and provides slurry mixing power,the reaction process effectively transfers nitrate to nitrogen that removes from water,the chemical reaction between iron and nitrate to produce ammonia is avoided.The external reflux upflow reactor can solve the problem of iron mineralization in process of autotrophic denitrifying that activated sludge was packaged by produced ferric hydroxide material,realizing that the process of autotrophic denitrification has been carried out for long-term.In operation,in order to better realize the design principle of reactor and increase the nitrogen removal rate of nitrate,the operation control parameters and reaction conditions of the reactor were determined,the operation control parameters include the rising velocity,volume load,the zero valent iron dosage and the dosing interval,The reaction conditions include temperature,pH,dissolved oxygen,and organic matter.The main conclusions of this research are as follows:?1?Inoculating activated sludge from municipal wastewater treatment plants in an upflow upflow reactor,The reduced iron powder as an electron donor was used for denitrification,When the hydraulic retention time is 26 hours,In the stable period,66.9%of the nitrate conversion variables were gaseous nitrogen,The average nitrogen concentration of ammonia in effluent was 3.24 mg L-1,The rate of nitrogen removal was 29.3 g·?m³·d?^-1,Tring to minimize the chemical reaction,The transformation of nitrate is mainly produced by iron autotrophic denitrifying microorganisms and nitrogen is produced,Therefore,the operation of a autotrophic denitrifying reactor with zero valent iron as the substrate can be achieved.?2?On the supplied mode of the reaction substrate ferrous iron,because two valence iron is easy to oxidize,so the utilization ratio is low,the two valent iron was not selected as the substrate addition of the reactor,but select zero valent iron,ferric iron is the reaction product,the formation of ferrous iron by the use of reaction between ferric iron and zero valent iron.In the reactor,they can be contacted by stirring and refluxing to produce two valent iron.Comparison of the operation of a stirred up flow reactor with an internal reflux upflow reactor,the upflow reactor with internal reflux is superior to the stirred upflow reactor in terms of operational stability.In operation,both achieve the stable period,when the water retention time is 26 hours,the nitrogen removal rate of 29.3 g·?m³·d?^-1 was achieved by internal reflux upflow reactor;The hydraulic retention time is 12 hours,the nitrogen removal rate of 29.3g·?m³·d?^-1 was achieved by with string upflow reactor.But the mixing process requires the zero valent iron to float,excessive mixing intensity results in sludge loss,later the reaction rate decreased,the rate of nitrogen removal in unstable stage was12.7 g·?m³·d?^-1,so the upflow reactor with external reflux is easy to operate,hydraulic hybrid power is light,not causing sludge loss,the ferric iron in the upper return water reacts with the bottom zero valent iron to form two valent iron,providing for the central suspension of iron autotrophic denitrifying sludge utilization.Therefore,an upstream flow reactor with reflux for ferrous iron donors was selected.The internal reflux upflow reactor has a lower capacity for the removal of ferric compounds,much ferric hydroxide is detained in the reactor,adsorpting sludge to make it mineralized.In order to solve the mineralization problem,attempts were made to change the reflux method,an effluent water return tank is arranged outside the reactor,substance of ferric hydroxide is precipited at the bottom of the effluent recycle pool,reflux will utilize the supernatant and the ferric ion it contains,the pool will separate the ferric hydroxide external reflux and return water,remove its influence on sludge,the ferric iron in the supernatant reacts with the bottom zero valent iron to form two valent iron,the way also provides muddy water mixing power.In the study of measures for solving mineralization problems,a method for directly adding flow of fresh activated sludge in an internal reflux upflow reactor has been carried out,but the result shows that the method of changing reflux is better than sludge flow adding method in solving the problem of sludge mineralization,and It is difficult to use sludge in the project.The reflux tank installed outside reactor can solve the problem of sludge mineralization for long-term,compared to 9.9 g·?m³·d?^-1 of mineralization,the nitrogen removal capacity was recovered to 33.4 g·?m³·d?^-1,the rate of nitrate nitrogen conversion and nitrogen removal rate are consistent,the rate of nitrate nitrogen transformation was 34.3 g·?m³·d?^-1,when mineralized,the effluent ammonia nitrogen content is 12.38 mg·L^-1,however,the rate of ammonia nitrogen and nitrite nitrogen production is very small at this time,0.75 mg·L^-1,0.22 mg·L^-1respectively.?3?In order to make the design principle of the reactor better realize and improve the reaction rate of the reaction process,the operating parameters and conditions of upflow reactor with external reflux were determined.For this kind of reflux,the retention of iron compounds in the system can be well realized and a certain amount of ferric iron in the reactor can be maintained,to ensure sufficient concentration of ferric iron in the recirculating water,reacted with the bottom zero valent iron,the amount of iron consumption and the amount of discharged sediment at the rising velocity of 3.49 m·h^-1 and 5.24 m·h^-1 were compared,it was found that when the rising velocity is 3.49 m·h^-1,the external reflux pool can deposit 58%of the iron consumed by nitrate conversion,It is shown that 42%of the ferric iron is in the reflux cycle,a certain concentration of iron ion was guaranteed,and at v=5.24 m·h^-1,the total iron content of the external reflux pool is 3⁴ times the amount of iron consumed by the nitrate conversion.Over time,ferric iron in circulation water will be reduced,make against the formation of two valent iron substrate,showing that the import of nitrate nitrogen concentration has a declining trend with the velocity rising(change from 3.49 m·h^-1 to 5.24 m·h^-1).With Higher,it is easy to bring out ferric iron substances and a small amount of sludge,the value should be less than 5.24 m·h^-1,sludge suspension is poor at lower levels,the value should be near 3.49 m·h^-1.Therefore,the proposed range of value is between 3.0⁴.0 m·h^-1.?4?To ensure that contaminants are removed and the volumetric load of the reactor is reasonably obtained,considering the influence of process temperature and iron powder interval in iron autotrophic denitrification process.HRT is at 26h,through long-term observation,the nitrate removal load was obtained during the stabilization period.When the average temperature is 22?,the volume load of the reactor is 22.39 g·?m³·d?^-1;When the temperature is 30?,the volume load of the reactor is 38.29 g·?m³·d?^-1.Therefore,the reaction temperature is the important parameter control of the reactor operation,which affects the volume load of the reactor.Study on influencing factors of temperature in batch reaction bottle,a batch reactor experiment was designed at 15?,25?,35?,respectively,the activity of iron autotrophic denitrifying bacteria was the strongest at 35?,Nitrate reduction occurs in the biological response.25?is later,the nitrate reduction reaction,chemical process and biological process,both occur in the bottle.At 15?,the activities of biological enzymes of iron autotrophic denitrifying bacteria decreased,the chemical reaction between iron and nitrate occurs mainly in the bottle,Therefore,in order to guarantee the reaction product and reaction rate,the reaction temperature should be kept at 30?or above,the volumetric nitrogen removal rate of reactor was 38.29 g·?m³·d?^-1.?5?Due to reduced iron powder interval addition,during the run,the transformation change of nitrate is wavy,the interval needs to be optimized,under the above volume load,according to the conservation of electron gain and loss in redox reaction,the dosage was calculated.In the early stage,10 g of reduced iron powder was added every 10 day.To ensure stable substrate supply,maintain a steady rate of reaction,then excess iron should be maintained,then select a cycle,the amount of nitrate transformation,total iron of effluent and effluent iron were selected as observation quantity amount.On a certain day,if these observations occur at the same time occur a larger decline,make sure the next day of the day is the optimized cycle time point.Accordingly,the dosage and interval are optimized,the result is that add10 g of iron powder every 6 day.?6?Through Determination of pH value in batch test process,zero valent iron autotrophic denitrification is an acid consuming process,but the process of pH will not rise continuously,unlike chemical reactions that it rises to 10.Instead,it focus on7.5⁷.8,so this experiment can not show the difference of pH on iron autotrophic denitrification,therefore,a continuous flow fermentor was used to study the process.Comparison of denitrification rates in different pH continuous flow experiments at 6,6.5,7,7.5 and 8,the results show that denitrification rate is the highest at 6.5,the rate of nitrogen removal reached 32.4 g·?m³·d?^-1,therefore,the reactor pH value operating conditions should be controlled at around 6.5.?7?Anoxic environment(0.2⁰.4 mg·L^-1)is beneficial to denitrifying reaction of iron autotrophic denitrifying bacteria,in a temperature controlled 25?shaker,an absolute anaerobic reactor that were used to remove 30 min by nitrogen and an oxygen free reaction bottle(DO 0.34 mg·L^-1)are comparied.The ammonia concentration in the deoxidization bottle accumulates continuously,no accumulation of ammonia occurred in the deoxygenated environment,18.3%of the nitrates were converted to nitrogen and lost.The absolute anaerobic environment is not conducive to the autotrophic denitrification of zero valent iron,because the reaction initially requires proper oxygen to corrode the elemental iron for ferrous iron,and some iron autotrophic denitrifying bacterias belong to facultative growth bacteria.Therefore,the anoxic environment is favorable for the operation of an iron autotrophic denitrifying reactor.?8?The theory of"co-substrate"for organic and ferrous metals based is from Weber,the theory suggests that the presence of organic compounds is beneficial to the growth and activity of iron autotrophic denitrifying bacteria,the stimulatory effect of organic carbon source on iron autotrophic denitrification was studied,the results show that less organic compounds can play a greater role.The temperature is 25?,the average nitrogen removal rate was 47.32 g·?m³·d?^-1when the ratio of carbon to nitrogen was 0.3;when 1,69.54 g·?m³·d?^-1 was achieved.Compared with the inorganic carbon source in upflow reactor,the rate of nitrogen removal was 22.39g·?m³·d?^-1at 22?.In low organic matter,the reaction rate increased greatly.Therefore,the application of iron autotrophic denitrification for municipal wastewater for advanced treatment with low organic carbon and black and odorous surface water containing nitrogen nutrients has the research value.