| The secondary effluent from municipal wastewater treatment plant has the characteristics of low biodegradable organic matter concentration and high ammonia nitrogen and TN concentration.The advanced denitrification treatment of the secondary effluent can reduce the pollution load of the receiving water and supplement the fresh water resources.In this paper,the secondary effluent from municipal wastewater treatment plant in Changsha County,Hunan Province was used as the research object.Based on the bio-zeolite rapid adsorption ammonia nitrogen-regeneration dynamic balance technology,a bio-zeolite constructed wetland pilot model was built.The effect of bio-zeolite constructed wetland on enhanced nitrification ammonia nitrogen of secondary effluent from municipal wastewater treatment plant was studied The influence of temperature on ammonia nitrogen nitrification rate was researched,and the main removal pathway of nitrogen was quantitatively analyzed from three aspects: the decrease of ammonia nitrogen,the increase of nitrate nitrogen and the amount of TN in inflow and outflow.Combining the nitrification potential of zeolite matrix and the spatial and temporal distribution characteristics of ORP in the bio-zeolite constructed wetland,the mechanism of strengthening nitrification is analyzed.According to the outflow concentration of nitrate in the bio-zeolite constructed wetland,three groups of denitrification bed with different carbon rate feeding with synthetic wastewater was built in the laboratory.The nitrate removal efficiency,the nitrate concentration along bed,the effluent concentration of organic matter,ammonia nitrogen,nitrate nitrogen,and nitrite nitrogen of three denitrification beds were studied.The optimal volume of sawdust was determined.The influences of temperature and hydraulic retention time on nitrate removal efficiency also were investigated.The main research results were as follows:1.The effect of bio-zeolite constructed wetland to enhance the nitrification of ammonia nitrogen in secondary effluent is significant.In the normal operation stage,the average removal rate of ammonia nitrogen is 90.5%,and the concentration of ammonia nitrogen in effluent is less than 0.5mg/L.2.The influence of temperature on the ammonia nitrogen nitrification rate of bio-zeolite constructed wetland was significant.The average nitrification rate of ammonia nitrogen in bio-zeolite constructed wetland was 50.1 % at the low temperature stage(7 ?~13 ?).In the normal operation stage(18 ?~34 ?),the average nitrification rate of ammonia nitrogen in bio-zeolite constructed wetland was 134.9 %.3.During the 377 d,the decrease of ammonia nitrogen and the increase of NO3-N in influent and effluent were basically equal.Ammonia nitrogen is mainly converted into nitrate nitrogen by microbial nitrification in the constructed wetland.4.The ORP values of different parts of bio-zeolite constructed wetland in different operation time are above 300 mV,and the average ORP value is 456 mV.This indicates that there are aerobic environment and strong nitrification ability in bio-zeolite constructed wetland.The average ORP values of constructed wetlands were 436.2 mV,457.6 mV,476.9 mV and 452 mV at inlet 2 h,outlet 0 h,3 h and 7 h,respectively.The oxygen of water and atmospheric intake can meet the bio-zeolite constructed wetland to occur nitrification and organic matter degradation reaction.At the depth of 15 cm,25 cm and 35 cm,the average ORP values of constructed wetlands are 538.7 mV,424.1 mV and 404.2 mV,respectively.The oxygen of bio-zeolite constructed wetlands mainly comes from atmospheric diffusion and dissolved oxygen of influent itself.With the increase of the depth of bio-zeolite constructed wetlands,the atmospheric diffusion is weakened,so the ORP gradually decreases along the depth.5.The average nitrification potential and average denitrification potential of bio-zeolite constructed wetland were 1.30 mg·kg-1·h-1,11.19 mg·kg-1·h-1,respectively.The nitrification potential and denitrification potential in bio-zeolite constructed wetland system showed obvious stratification.The average nitrification potential of surface layer,middle layer and bottom layer were1.86 mg·kg-1·h-1?1.06 mg·kg-1·h-1?0.98 mg·kg-1·h-1,respectively.The average denitrification potential of surface layer,middle layer and bottom layer were 12.53 mg·kg-1·h-1?11.09 mg·kg-1·h-1?9.94 mg·kg-1·h-1,respectively.Along the direction of water flow,the nitrification potential decreased gradually,and the denitrification potential increased gradually.6.In the normal operation stage,the average influent concentration of nitrate was 9.9 mg/L,and the average nitrate removal rates of No.1(the volume ratio of sawdust was 50 %),No.2(the volume ratio of sawdust was 30 %),and No.3(the volume ratio of sawdust was 15 %)denitrification bed were 63.4 %,50 %,and 33.9 %,respectively.The optimal volume ratio of sawdust was 50 %.7.The concentration of nitrate nitrogen in the denitrification bed decreases along the distance,and the decreasing range of nitrate nitrogen are positively related to the amount of sawdust filled.The more the amount of wood filled,the more electron donors,the greater the decreasing ranges of nitrate nitrogen.From 0 cm to 20 cm,20 cm to 40 cm and 40 cm to 60 cm,the decrease of nitrate nitrogen in per unit distance is not consistent,and it indicates that nitrate nitrogen is not removed in denitrification bed by a uniform rate.8.In first and second days of the start-up phase,The effluent COD,ammonia nitrogen,nitrite nitrogen concentration of No.1,No.2 and No.3 denitrification bed were very high,the highest were 463.3,397.6 and 200.3 mgCOD/L,14.72,8.56 and 3.59 mgNH4+-N/L,1.61,1.27 and 1.20 mgNO2--N/L.They were mainly from leaching of sawdust.On the third day,the effluent concentrations of COD,ammonia nitrogen and nitrite nitrogen decreased rapidly in the denitrification beds,and they decreased slowly and stabilized at the later stage.9.In the normal operation stage,the average effluent concentrations of COD,ammonia nitrogen and nitrite nitrogen of No.1,No.2,and No.3 denitrification bed were 17.1,13.9,11 mg COD/L,0.31,0.25,0.20 mg NH4+-N / L,0.09,0.18,0.10 mg NO2--N/L.In the normal operation stage,the release rate and utilization rate of carbon and ammonia nitrogen were balanced,and the concentration of effluent COD and ammonia nitrogen was basically stable.Nitrite nitrogen was mainly from the denitrification intermediate.10.Temperature and hydraulic retention time have effect on the removal of nitrate in denitrification bed.The temperature coefficients Q10 of No.1,No.2,and No.3 denitrification bed were 1.19,1.15 and 1.10,respectively.When the water temperature range was 10 ?~36 ?,the nitrate removal efficiency of the three groups of denitrification bed increases with the increase of the water temperature.In the appropriate temperature range(25 ?~40 ?),when the hydraulic retention time of No.1,No.2,and No.3 denitrification bed double increased from2.9d to 5.9d,the nitrate removal rate increased from 63.3%,45.9% 32.8% to 85.6%,68.9%,50.9%,respectively. |
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