Enhanced Nitrogen Removal Effect Of Constructed Wetland Using Alkali-Modified Corncob As Carbon Source

Water plays a very important role in the development of human society,among which nitrogen element pollution is one of the common pollution in water bodies.In recent years,as an environmentally friendly ecological project,constructed wetlands（CWs）have been widely used in the treatment of various polluted wastewater.However,the problem of low efficiency of nitrogen generally exists in CWs in the treatment of nitrogen pollution,especially some low ratio of carbon to nitrogen（C/N）sewage.In recent years,using cheap,easily available plant biomass with high organic matter content as an external carbon source to add to the CWs can effectively improve the system’s denitrification effect,and the modification of plant biomass can solve the problems of large amount of N,P and organic matter release in the early stage,and insufficient carbon release in the later stage.This study first explored the alkali modification method of corncob,and clarified the best alkali modification method of corncob through orthogonal experiment design.The surface morphology of alkali-modified corncob（AM-corncob）was analyzed by scanning electron microscope,and the carbon release performance of AM-corncob was analyzed by static release experiment.Secondly,a vertical flow CWs was constructed with AM-corncob as carbon source.The CWs with blank and corncob as the carbon source were used as the control group.Through continuous monitoring,the effect of the CWs with AM-corncob as the carbon source on COD,NH₄⁺-N,NO₃^—-N,NO₂^—-N,TN and TP was explored,and the influence of intermittent aeration and influent pollution load on the removal of pollutants in the CWs were analyzed.When the constructed wetland system was stable,the variation of pollutants in the CWs was monitored in a typical cycle.The main conclusions are as follows:（1）The carbon release process of corncobs obtained by different alkali modification treatments all conformed to the second-order kinetic equation,and the influence degree of alkali concentration,treatment time,and treatment temperature on corncob carbon release was alkali concentration>treatment time>treatment temperature.The best alkali treatment method was 2%（alkali concentration）,12 h（treatment time）,and 20°C（treatment temperature）.The surface roughness of the AM-corncob was increased,and the pores were increased,which was conducive to the release of carbon sources and the growth of microbes.Compared with the original corncob,the AM-corncob had better carbon release.The average COD release amount and release rate were 24.26 mg/g and 2.19 mg/g/d,respectively.The C/N ratio of the extract had increased from 38.56 to 128.69,indicating that the AM-corncob is suitable as an external carbon source for CWs（2）During the whole experiment period,the CWs with AM-corncob as carbon source had better NO3^—-N removal rates,which were 98.92%,98.38%and 91.39%,respectively,significantly higher than the blank control group,indicating that AM-corncobs can effectively enhance the denitrification of CWs.Intermittent aeration can significantly improve the removal rate of COD,NH4⁺-N and TP in CWs.The average concentration of COD in the effluent of each group of CWs was lower than 50 mg/L,which meets the first-level A standard in Pollutant Discharge Standard for Urban Sewage Treatment Plants（CB18918-20002）.The average removal rate of NH4⁺-N and TP in CWs with AM-corncob as carbon source was 60.93%and 89.50%,respectively.In stages 1 and 2,compared with the CWs using the original corncob as the carbon source,the CWs using the AM-corncob as carbon source had a higher NH4⁺-N and TN removal rate and lower NO2^—-N accumulation.When the influent pollution load doubles,the concentration of pollutants in the effluent of each group of CWs would increase,and NO2^—-N accumulation would occur.（3）In a typical cycle,pollutants in CWs had the same changing trend at different stages.The change of COD and NH4⁺-N concentration in CWs with AM-corncob as carbon source was similar,that is,it dropped rapidly within 2 h and then stabilized,indicating that aerobic microorganisms can use DO to degrade organic matter and NH4⁺-N.The NO3^—-N concentration in CWs with AM-corncob as the carbon source decreased before 48 hours and then stabilized,which was 8 hours higher than that in CWs with the original corncob as the carbon source.However,the concentration of NO3^—-N in blank group increased.The TN concentration in the effluent of the CWs with AM-corncob as the carbon source was similar to the NO3^—-N concentration,indicating that nitrification was the main factor affecting the denitrification of the CWs with AM-corncob as the carbon source.The TP concentration of CWs with external carbon source was lower than that of the blank group.It decreased within 8 hours and then stabilized.When the pollution load doubled,the TP concentration rapid decrease before 24 hours and then slowly decreased until becoming steady.