Effect Of Fe²⁺ On The Treatment Of Low C/N Domestic Sewage In Aerobic Granular Sludge System

Low carbon nitrogen ratio and low COD have become the common characteristics of municipal domestic sewage in China,and there is a lack of carbon source.Due to the low content of organic matter in sewage and the lack of carbon source,the use of conventional sewage treatment process can not meet the requirements of nitrogen and phosphorus removal.Divalent iron salt is cheap and easy to obtain.It is often used in the front facilities by sewage treatment plants to remove suspended solids,phosphorus and some cod.Iron oxidizing bacteria will produce capacity and promote biochemical reactions in activated sludge in the process of Fe²⁺being oxidized to Fe³⁺.Studying the ability of adding Fe²⁺to domestic sewage with low carbon nitrogen ratio to promote nitrogen and phosphorus removal can provide new ideas for sewage treatment plants.Aerobic granular sludge technology is a new sewage treatment technology with broad prospects.Compared with the traditional floc activated sludge,because the granular structure of aerobic granular sludge is relatively special,aerobic zone,anoxic zone and anaerobic zone can exist in the interior of aerobic granular sludge at the same time.In this case,microorganisms with different characteristics in sludge have a suitable environment to survive,Thus,the types of microorganisms in the reactor are enriched,and the simultaneous removal of nitrogen,phosphorus and carbon in the same reactor can be realized.Therefore,the research on aerobic granular sludge has become a research hotspot to improve the treatment capacity of municipal domestic sewage.Taking domestic sewage with low carbon nitrogen ratio as the main research object,this thesis studies the effect of Fe²⁺and aerobic granular sludge on the treatment of domestic sewage with low carbon nitrogen ratio.Aerobic granular sludge was cultured by using ordinary flocculent activated sludge as inoculated sludge and self-made SBR experimental device.In the process of cultivation,the appearance,physical properties and pollutant removal effect of aerobic granular sludge were studied.The cultured mature aerobic granular sludge was used to treat the artificially simulated domestic sewage with low carbon nitrogen ratio,and the effect of carbon nitrogen ratio on the treatment of sewage with aerobic granular sludge was explored when the influent C/N=1,3,5 and 7.Then,according to the experimental results,the most appropriate influent carbon nitrogen ratio for domestic sewage treatment with aerobic granular sludge under the condition of low carbon nitrogen ratio is selected.Taking the most appropriate influent carbon nitrogen ratio as the simulated wastewater,the effect of Fe²⁺concentration on aerobic granular sludge treatment of wastewater was studied when the influent Fe²⁺dosage was1mg/L,2mg/L,3mg/L and 4mg/L.The effects of C/N on aerobic granular sludge treatment of domestic sewage under the condition of low carbon nitrogen ratio are as follows.Although the effect of aerobic granular sludge on COD treatment increases with the increase of carbon nitrogen ratio,the concentration of COD in effluent can meet the first-class a standard.With the increase of carbon nitrogen ratio,the removal effect of aerobic granular sludge on ammonia nitrogen is also better and better.Although the denitrification rate at C/N=7 is higher than that at C/N=5,the final result is the same as that at C/N=5,which can completely remove nitrate nitrogen in the reaction unit in the static anaerobic stage.Therefore,it is suggested to control the influent C/N to 5 in actual operation.When the C/N is greater than 5,although the denitrification rate is accelerated,some carbon sources are wasted.The removal efficiency of TP by aerobic granular sludge system increases with the increase of influent C/N ratio.This is because when the influent C/N ratio is low,there is a certain demand for carbon source in the process of biological nitrogen removal and biological phosphorus removal,and there is a competitive effect on carbon source.Therefore,when the C/N decreases,the removal pressure of total phosphorus will increase accordingly due to the lack of carbon source.However,after the biological treatment unit of the sewage plant,the phosphorus in the sewage can still be further removed by adding advanced treatment structures and chemical methods such as adding phosphorus removal agents.Thus,the influent carbon nitrogen ratio under the condition of optimal low carbon nitrogen ratio is obtained when influent C/N=5 in this experiment.When influent C/N=5,the effect of Fe²⁺dosage on aerobic granular sludge treatment of sewage is shown.Fe²⁺can promote the removal of COD in aerobic granular sludge system.With the increase of Fe²⁺dosage,the removal rate of COD in SBR reactor is improved.The optimum dosage of Fe²⁺ion when influent C/N=5 is 2mg/L.In SBR reaction system,the removal effect of nitrite did not change due to the presence of Fe²⁺.When the dosage of Fe²⁺is0mg/L,1mg/L,2mg/L,3mg/L and 4mg/L,the change trend of nitrite concentration of SBR aerobic granular sludge is roughly the same in the typical cycle.When the dosage of Fe²⁺is2mg/L,the duration of nitrate concentration decline is longer than that under other concentrations.There is no obvious difference in the change trend of nitrate concentration in the reactor after adding different concentrations of Fe²⁺.However,when the dosage of Fe²⁺is2mg/L,the nitrate concentration in the reactor is always lower than that of the other four groups.The dosage of Fe²⁺has neither promoting nor inhibiting effect on the treatment of total nitrogen by aerobic granular sludge.Adding Fe²⁺ions into the aerobic granular sludge system can enhance the removal effect of phosphorus,and with the increase of Fe²⁺dosage,the removal effect of total phosphorus is better,but the removal of phosphorus can be removed by chemical means.Comprehensively considered,when the dosage of Fe²⁺is 2mg/L,the treatment effect of aerobic granular sludge on domestic sewage with C/N=5 is the best.