Mechanism Of Nitrogen And Phosphorus Removal By Biological Aerated Filter With Zeolite/Iron-rich Media

Biological Aerated Filter（BAF）is mainly used to remove pollutants in water through biofilm formed on the surface of filter material.It is an efficient sewage treatment technology with solid-liquid separation and biodegradation.BAF is widely used in the field of advanced wastewater treatment and industrial wastewater treatment.However,due to the influence of influent water quality fluctuation and other factors in the actual operation,the simultaneous nitrogen and phosphorus removal effect is not good.According to the current research results on BAF at home and abroad,most of them focus on the improvement of its denitrification and dephosphorization efficiency and the optimization of process parameters,but there are few and not enough in-depth studies on the removal mechanism of pollutants such as nitrogen and phosphorus by BAF.Zeolite,which was used as the main filler in this experiment,was formed into the composite filler system by mixing iron-rich media.And the combination ratio of the two fillers was optimized to achieve the best treatment effect.At the same time,the effect of biofilm on the bioregeneration of zeolite was investigated and the regeneration conditions and mechanism were further studied.Finally,the mechanism of the influence of zeolite on phosphorus removal by controlling the content of iron ion in the reactor through ion exchange was analyzed and studied.In order to explore the possible influence of the combination ratio of zeolite and iron-rich media on the removal effect of BAF,contrast experiment was used to investigate the pollutant removal effect of the experimental group reactor using zeolite and iron-rich media whose volume ratios were 20:1,15:1,10:1 and 5:1 respectively,and the control group reactor using zeolite only during the stable operation period.The experiment results indicated that when the hydraulic load was 0.2 m³/（m²·h）and the gas water ratio was 10:1,the experimental group reactor with a volume ratio of 5:1 had the best denitrification and phosphorus removal effect,whose TN and TP removal efficiency were 11.10%and 24.71%higher than those in the control group.The removal effect of TN and TP in the experimental group reactors was improved to varying degrees compared with that in the control group,and the increase amplitude increased with the increase of the volume ratio of iron-rich media.It shows that mixing a certain proportion of iron-rich media in the zeolite biological aerated filter could promote the denitrification and phosphorus removal effect of the BAF reactor.Both the experimental group reactors and the control group reactor have better removal effects on COD,NH₄⁺-N and SS,and each experimental group reactor has little lifting effect compared with the control group reactor.In order to explore ammonia nitrogen concentration change,the presence of biofilm,and the roles of ion exchange and adding the metal cation within these four factors on the BAF zeolite biological regeneration might affect,the material balance calculation and energy spectrum analysis（EDS）respectively to investigate nitrogen pollutant concentration in each reactor and the nitrogen content in zeolite,and in-depth analysis of regenerative conditions and regeneration mechanism.The experiment results indicated that when the hydraulic load was 0.2 m³/（m²·h）and the gas water ratio was 10:1,the zeolite in Reactor A would undergo biological regeneration and released 5567.08 mg nitrogen during the period of influent water NH₄⁺-N concentration decreased from 50 mg/L to 20 mg/L until stable operation.The zeolite in the upper,middle and lower parts of the reactor packing layer all underwent bioregeneration during the period of influent water NH₄⁺-N concentration decreased from 50mg/L to 20 mg/L until stable operation.The weight percentage of N element decreased from0.87%,0.69%and 0.56%to 0.63%,0.38%and 0.16%,respectively.During the period of influent water NH₄⁺-N concentration decreased from 50 mg/L to 20 mg/L until stable operation,1130.40 mg and 805.99 mg of nitrogen were released by Reactor B which used zeolite without biofilm and Reactor C which used biological gravel without special ion exchange for NH₄⁺-N,respectively,which were far less than the 5567.08 mg nitrogen released by biological zeolite in Reactor A.It indicated that the zeolite without the action of microorganisms only desorbed a small amount of NH₄⁺-N through ion exchange and reached dynamic equilibrium with the concentration of NH₄⁺-N in the influent,and the biological gravel also did not release a large amount of NH₄⁺-N adsorbed by ion exchange due to biological regeneration like biological zeolite.The results indicated that biofilm on packing surface and special selective ion exchange effect of zeolite for NH₄⁺-N are essential conditions for bioregeneration of zeolite fillers in BAF.1666.93 mg nitrogen was released by Reactor A with biological zeolite during the period of influent water adding 200 mg/L NaCl until stable operation,which indicated that the adding a certain concentration of Na⁺could promote the desorption of NH₄⁺by zeolite.And the desorbed NH₄⁺-N was converted into NO₃^--N and NO₂^--N by microbial nitrification,which made zeolite bioregenerate and restore part of its ion exchange adsorption capacity for NH₄⁺-N.In order to explore whether zeolite can control the content of iron ion in the reactor through ion exchange to affect the effect of chemical phosphorus removal,the removal effect of phosphorus pollutants in the experimental group reactor with adsorbed iron ion zeolite and the control group reactor with natural zeolite were investigated through comparative experiments and the mechanism was deeply analyzed.The experiment results indicated that when the hydraulic load was 0.2 m³/（m²·h）,the gas water ratio was 10:1 and the influent TFe concentration is 3～5 mg/L,the natural zeolite filler in BAF had a good removal effect on TFe in a total of 36 days running period.The average concentration of TFe in the effluent of the experimental group reactor was 0.58 mg/L,and the average removal efficiency was as high as84.98%.Therefore,in the composite filler system formed by mixing zeolite and iron-rich media in a certain proportion,natural zeolite will adsorb and store excess Fe³⁺to achieve the effect of controlling BAF effluent TFe.When the influent TP concentration is controlled at about 4 mg/L,the Fe³⁺adsorbed by zeolite in the reactor will be constantly resolved out for chemical phosphorus removal.With the slowing of the Fe³⁺desorption rate in zeolite,the TFe concentration in the water continues to decline,and the chemical phosphorus removal effect in the reactor continues to decline.According to the material balance analysis,110.31 mg phosphorus was removed by the ion exchange desorption of Fe³⁺in the reactor during a total of 12 days of operation.Zeolite filler could not only adsorb a certain amount of Fe³⁺through ion exchange when TFe concentration is higher,but also release part of Fe³⁺through ion exchange to remove phosphorus pollutants in wastewater when chemical phosphorus removal is needed.