Effects Of Ferric-Carbon Micro-Electrolysis And Zeolite On Water Purification And Greenhouse Gas Emissions Of Aeration Constructed Wetland

With the rapid economic development,the discharge of a large amount of domestic and industrial sewage gradually affects people’s production and life and the global ecological balance.The constructed wetland（CW）systems achieve efficient removal of pollutants in sewage through the physicochemical,chemical and biological processes,which is one of the vigorously promoted sewage treatment technologies in the world.Intermittent aeration solves the shortcomings of insufficient dissolved oxygen in traditional CW,and greatly improves the removal effect of organic matter and total nitrogen in CW.Therefore,intermittent aeration constructed wetlands have been widely used in various kinds of sewage treatment.However,the aerobic-anoxic environment formed during the operation of intermittent aeration in CW lead to a large amount of greenhouse gas（GHG）emissions,and the greenhouse effect caused by GHG has become a global environmental problem.From a long-term perspective,it is necessary to take active measures as soon as possible to control GHG emissions in order to ensure the comprehensive environmental benefits of intermittent aeration CW.For the purpose of exploring the effects of ferric-carbon micro-electrolysis filler and zeolite on wastewater treatment and GHG emissions in intermittent aeration CW,four kinds of intermittent aeration CW were constructed,which were ferric-carbon micro-electrolysis filler+gravel（CW-A）,ferric-carbon micro-electrolysis filler+zeolite（CW-B）,zeolite（CW-C）,and gravel（CW-D）.And the concentration of pollutants（COD,TN,NH₄⁺-N,NO₃^--N,NO₂^--N）in the effluent and the emissions of GHG（CH₄and N₂O）of the intermittent aeration CW were measured during the entire experiment period.In addition,the impact mechanism was analyzed from the perspective of microbial community plants.The main results are as follows:（1）Compared with CW-D,the ferric-carbon micro-electrolysis filler significantly increased the p H（P＜0.05）and DO（P＜0.05）of the CW effluent.The removal rate of COD of the four groups of CW systems was more than 95%,but there was no significant difference between different constructed wetlands（P>0.05）.The mean removal rate of NH₄⁺-N in CW-A,CW-B,and CW-C were significantly higher than that of CW-D（P<0.05）.NO₃^--N can hardly be detected in CW-A and CW-B,and the removal rate were over 99%,while the removal rate of NO₃^--N in CW-C and CW-D were only 93%to 94%.It can be seen that the ferric-carbon micro-electrolysis filler had an excellent effect on the remove NO₃^--N in constructed wetlands.Overall,both ferric-carbon micro-electrolysis fillers and zeolite can significantly improve the denitrification effect of constructed wetlands.The removal rates of TN in CW-A,CW-B and CW-C were significantly higher than that of CW-D（P<0.05）,and the combined use of ferric-carbon micro-electrolysis filler and zeolite（CW-B）had the best nitrogen removal efficiency.（2）Compared with CW-D,the mean CH₄fluxes in CW-A,CW-B and CW-C were reduced by 31.93%（P<0.05）,51.08%（P<0.05）and 53.69%（P<0.05）,respectively.Moreover,zeolite had a more obvious reduction effect on CH₄.The dissolved CH₄were blown off to the atmosphere during aeration period,which led to the appearance of CH₄emission peaks（P<0.05）.The CH₄emission peaks of the aeration period in CW-A,CW-B and CW-C were significantly lower than that in CW-D,indicating that the ferric-carbon micro-electrolysis filler and zeolite have the effect of reducing CH₄emissions in the intermittent aeration CW.Compared with CW-D,CW-A and CW-B reduced the N₂O emissions by 48.53%（P<0.05）and 58.20%（P<0.05）,respectively,and significantly decreased N₂O emissions in the aeration period.These results showed that ferric-carbon micro-electrolysis fillers can significantly reduce N₂O emissions from CW.Compared with CW-D,the integrated GWP were significantly reduced by CW-A（P<0.05）,CW-B（P<0.05）and CW-C（P<0.05）.Overall,the results showed that the combined addition of ferric-carbon micro-electrolysis filler and zeolite（CW-B）had the best effect on alleviating global warming of intermittent aeration CW.（3）The abundance and microbial diversity of microbes of CW-A,CW-B and CW-C were significantly higher than those of CW-D（P<0.05）.Both of the ferric-carbon micro-electrolysis filler and zeolite changed the microbial community in the CW.From the perspective of the genus,the relative abundances of Nitrospira（which was involved in nitrification）,Thauera,Denitratisoma and Thiobacillus（which were involved in denitrification）in CW-A,CW-B and CW-C were higher than those in CW-D（P<0.05）,indicating that the ferric-carbon micro-electrolysis filler and zeolite promoted the progress of nitrification and denitrification,thereby improving the nitrogen removal efficiency of the CW.The abundance of pmo A gene（methanotrophic functional gene）in CW-A and CW-B was significantly higher than that of CW-D,while the abundance of mcr A gene（methanotrophic functional gene）was significantly lower than that in CW-D.These results showed that the decrease of CH₄production and the increase of CH₄oxidation may be the main reasons for the reduction of CH₄emissions from ferric-carbon microelectrolysis fillers.With the comparison of CW-D,CW-A and CW-B significantly increased the abundance of AOA（ammonia oxidizing archaea functional gene）and AOB（ammonia oxidizing bacteria functional gene）,indicating that the ferric-carbon micro-electrolysis filler improved nitrification process in CW.CW-A,CW-B and CW-C had significantly improved the abundance of nir K,nir S（nitrite reductase functional genes）and nos Z（nitrous oxide reductase functional genes）,and the ratio of nos Z/（nir K/nir S）was higher,which showed that both the ferric-carbon micro-electrolysis filler and zeolite promoted the complete denitrification progress,thereby reducing the emission of N₂O.（4）Compared with CW-D,CW-A,CW-B,and CW-C significantly increased the aboveground and underground biomass of calamus（P<0.05）.And the highest biomass was observed in CW-B,indicating that the ferric-carbon micro-electrolysis filler had a positive effect on the growth of plants and promoted the fixation of nitrogen by the constructed wetland plants.