Study On Endogenous Denitrification Of Low C/N Wastewater By Intermittent Biological Sponge Iron Reactor

With the development of the economy and changes in people’s lifestyles,urban domestic wastewater exhibits characteristics of low concentrations of Chemical Oxygen Demand（COD）,high nitrogen and phosphorus concentrations,and low Carbon-to-Nitrogen（C/N）ratios.The lack of organic carbon sources in low C/N ratio wastewater leads to incomplete denitrification,affecting denitrification and phosphorus removal efficiency and challenging existing process technology.The discharge of substantial amounts of low C/N ratio wastewater into receiving water bodies can lead to eutrophication,posing hazards to the natural environment and human health.Thus,reducing nitrogen and phosphorus discharge and implementing stricter effluent discharge quality standards has become a development trend.Traditional biological nitrogen and phosphorus removal processes are complex,with low removal efficiency,high energy consumption,sludge production,and the need to add alkalinity,carbon sources,flocculants,and other chemicals,among other problems.Therefore,a new nitrogen and phosphorus removal process is urgently needed to deal with the new characteristics of wastewater.Sponge iron,with its large specific surface area,cheap and easy availability,and strong reducing ability,has been widely studied for nitrogen and phosphorus removal and shows potential for effectively treating low C/N ratio wastewater.The research group found that adding iron sponge carriers to the activated sludge system of a Sequencing Batch Reactor（SBR）produces a Biological Sponge Iron System（BSIS）with strong denitrification performance.Furthermore,the addition of sponge iron carriers into the SBR reactor creates distinct and stable anoxic and anaerobic zones,and interactions between the sponge iron and microorganisms lead to the generation of new functional flora.Although the BSIS shows potential in treating domestic wastewater with low C/N ratios,few studies have investigated its performance in removing nitrogen and phosphorus and the underlying mechanisms.Additionally,the high reaction activity and strong magnetic properties of the sponge iron often result in the formation of a passivation layer consisting of iron oxides（Fe（OH）₂,Fe（OH）₃,Fe OOH,Fe₂O₃,Fe₃O₄,and Fe CO₃）and iron-phosphorus compounds（Fe-P）on the sponge iron surface in the liquid phase,inhibiting the dissolution of iron and reducing the phosphorus removal efficiency.Moreover,the continuous corrosion of sponge iron increases the need to extend its service life.Therefore,it is crucial to solve the issue of sponge iron passivation quickly and economically in the same reaction system to restore its effectiveness in pollutant removal when using it as a carrier for nitrogen and phosphorus removal from wastewater.In this study,actual domestic wastewater was used as the experimental influent,and a single iron sponge carrier and an iron sponge+polyurethane composite carrier were added to the activated sludge system to form the 2#and 3#intermittent Biological Sponge Iron Reactors（BBSIR）,respectively,while the 1#reactor served as the control with ordinary sludge system.The nitrogen and phosphorus removal effects of the ordinary SBR reactor and BBSIR were investigated under alternating anaerobic/aerobic（A/O）and anaerobic/aerobic/anoxic（A/O/A）operations,and the mechanisms of nitrogen and phosphorus removal were analyzed.The specific research results are as follows:（1）The results showed that the bio-sponge iron system had higher utilization and conversion in the anaerobic stage for the easily degradable carbon source in raw wastewater than the ordinary activated sludge system.The average COD_insconcentration and rate in reactor 2#during A/O alternating operation were the highest at 17.38 mg/L and 50.94%,respectively.The average COD_insconcentration and rate of reactor 3#during alternating A/O/A operation were the highest at 30.97 mg/L and 72.52%,respectively.The average COD_insconcentration and rate of reactors 1#,2#and 3#operated in A/O/A mode were higher than those in A/O mode.The A/O/A operation significantly improved the carbon storage efficiency in polyglucose microorganisms（GAOs）and polyphosphorus microorganisms（PAOs）of both the normal mud system and the bio-sponge iron system.（2）Simultaneous nitrification and denitrification（SND）of reactor 3#was higher than that of reactor 1#and reactor 2#by 24.61%and 31.24%during the stable operation of A/O and A/O/A,respectively,due to the rich anaerobic and anoxic zones of sponge iron+polyurethane as a carrier filler in the aerobic operation phase.Additionally,the TN removal effect of reactor 3#was the best during both A/O and A/O/A operations,and the average TN removal rate was 65.25%and 76.04%,respectively.The NO₃^--N concentration was always kept at a low level and accumulated mainly in the form of NO₃^--N in each reactor.The average NO₂^--N concentrations in the effluent of reactors 1#,2#,and 3#were 0.79 mg/L,0.42mg/L,and 0.57 mg/L,respectively,during alternating A/O/A operation.The highest Partial Denitrification（PED）rate was found in reactor 2#,And the average PED rates of reactors 1#,2#and 3#were 13.52%,18.92%and 17.63%,respectively.It indicating that the bio-sponge iron system had a better denitrification effect in the anoxic stage.（3）During the alternating A/O operation,the sponge iron system showed a higher TP removal rate than the ordinary activated sludge system,with the highest TP removal effect observed in reactor 2,and the average TP removal rate was 72.60%.The TFe concentration of the mixed solution in the 2#and 3#reactors showed an overall trend of gradual decrease during the alternating A/O/A operation,and the TP removal rate decreased compared with the A/O operation,with the average TP removal rate at 67.35%and 59.72%,respectively.The reduced TP removal rate was mainly due to the passivation layer on the surface of the sponge iron,which made it difficult to dissolve the iron ions,resulting in the weakening of chemical phosphorus removal in the system.（4）Using sponge iron as a carrier filler,anaerobic treatment was found to effectively remove the passivated layer on the surface of the sponge iron in situ and restore the chemical phosphorus removal capability of the system.Under normal influent water conditions,anaerobic treatment of the reactor for 4 days could effectively address the passivation phenomenon on the surface of sponge iron,resulting in the average removal rates of COD,NH₄⁺,and TP reaching 85.67%,99.8%,and 93.98%,respectively,and the efficient removal of phosphorus in the system remained stable for 22-26 days.This study has significant implications for the application of sponge iron in denitrification and phosphorus removal in actual domestic wastewater,providing solutions for economically and effectively addressing the passivation problem and restoring pollutant removal efficiency in situ.