Studies On Influencing Factors And Reduction Measure Of Greenhouse Gas Emissions In Constructed Wetland Wastewater Treatment Systems

Wastewater production is increasing with global population growth and economic development,a large amount of industrial,agricultural and domestic wastewater has been produced,which threatens the water quality and even human health.Constructed wetlands(CWs),as a green wastewater treatment technology are used to treat a variety of wastewaters(e.g.,domestic sewage,industrial drainage,agricultural wastewater and leachates,etc.)due to its pollutant removal performance,minimal operating costs,stable running efficiency,eco-friendly characteristics and aesthetic values.Among all kinds of wastewater treated by CWs,nitrogen,phosphorus and organic pollutants are the most widespread.However,the negative environmental impacts,especially the emissions of greenhouse gas(GHG),occurring during their operation may offset some of the environmental and ecological benefits of CWs.Although the total amount of GHG emissions from CWs worldwide are lower than that from other sources such as natural wetlands,it is necessary to identify the factors affecting GHG emissions from CWs and propose appropriate and effective measures to mitigate GHG emissions from CWs,as both the number and coverage of CWs have been rapidly increasing worldwide.Therefore,in this research,we took synthestic wastewater(based on the Grade I treatment standard of municipal sewage treatment plants in China)as the research objective to carry out the removal efficiencies of tyical pollutants(i.e.,nutrients including nitrogen and phosphorus and organics)and GHG emissions in CWs.The appropriate wetland plant species was selected,the optimum configuration of CWs was determined,the optimal influent chemical oxygen demand(COD)/nitrogen was obtained,and the possibility of adding biochar into CWs as a tenical measure to enhance pollutants removal efficiency and reduce GHG emissions was explored and verified.The main conclusions are as follows:(1)The three tested wetland plants,i.e.,Canna indica(C.indica),Cyperus alternifolius(C.alternifolius),Phragmites australis(P.australis),showed different performance in water purification,pollutant absorption,total and unit dry matter GHG emissions.The planted CWs enhanced the removal efficiencies of pollutants and the CWs palnted with C.indica outperformed the other two tested plant species.The highest carbon dioxide(CO₂)flux(582.01 mg/m²/h),methane(CH₄)flux(21.88ug/m²/h)and nitrous oxide(N₂O)flux(37.27 ug/m²/h)were observed in CWs planted with C.indica,P.australis and C.alternifolius,respectively.Meanwhile,the lowest global warming potential(GWP)was obtained in CWs planted with C.indica.Based on the q-PCR results,the N₂O fluxes were regulated by the nir K and nir S genes abundance.In conclusion,the C.indica was selected as potential plant species in CWs to obtain optimal wastewater treatment efficiency and produce the minimum emissions of GHG.(2)The subsurface flow CWs(SSFCWs)generally exhibited a better performance as compared to surface flow CWs(SFCWs).The highest removal percentages of COD(85.56%),nitrate(NO₃^--N)(85.06%),total nitrogen(TN)(90.14%)and total phosphorus(TP)(99.72%)were all obtained in SSFCWs.There was no significant difference between SSFCWs and SFCWs in CO₂ flux.However,in SSFCWs,the CH₄ flux(0.003 mg/m²/h)was significantly(p<0.05)higher than that in SFCWs(-0.029 mg/m²/h),while the N₂O flux(0.041 mg/m²/h)and GWP(12.30mg/m²/h)were significantly(p<0.05)lower than that in SFCWs(0.090 and 26.01mg/m²/h,respectively).As a result,the comprehensive environmental benefit of SSFCWs was better than that of SFCWs,which can be used as the preferred configuration for subsequent construction of CWs.(3)Influent COD/N plays a crucial role in the biogeochemical cycle of carbon and nitrogen in CWs.The pollutants removal efficiencies and GHG emissions were markedly affected by influent COD/N ratios.The effluent concentration of NO₃^--N decreased with the increase of influent COD/N ratios,while the trend was completely reversed for COD.In CWs with COD/N of 10,the removal efficiencies of COD,ammonia nireogen(NH₄⁺-N),NO₃^--N,TN and TP were 89.79%,96.58%,93.08%,93.49%and 75.84%,respectively,which were all better than the other four treatment groups(i.e.,influent COD/N were 0,5,15 and 20).For GHG emissions,the CO₂fluxes increased with increasing influent COD/N ratios.Meanwhile,the lowest CH₄(-42.49?g/m²/h),N₂O fluxes(31.77?g/m²/h)and GWP(52.89 mg/m²/h)were all observed under a COD/N ratio of 10.The highest ratio of nos Z/(nir S+nir K)was also observed at a COD/N ratio of 10 suggested the dominance of nos Z-harboring denitrifiers communities.Overall,in SSFCWs,the influent COD/N ratio of 10 was determined to be optimal for simultaneously achieving relatively higher pollutant removal efficiency and lower GHG emissions.(4)The amendment of biochar improved the efficacy of CWs for removing pollutants and mitigating GHG emissions.The highest removal percentages of COD(89.6%),NO₃^--N(89.2%)and TN(92.5%)were obtained in biochar-amended SSFCWs.The effect of biochar on GHG emissions is heterogeneous:biochar has no significant effect on CO₂ emissions,but significantly(p<0.05)reduces N₂O fluxes,while significantly(p<0.05)promotes CH₄ fluxes.The q-PCR results indicate that the reduced N₂O fluxes in biochar-amended CWs were driven by the enhanced transcription of the nos Z gene and the ratio of nos Z/(nir S+nir K).However,the effect of biochar on CH₄ emissions is bidirectional regulated by pmo A gene abundance and biochar's own physical and chemical characteristics(e.g.,high O/C ratio).The lowest GWP(5.25 mg/m²/h)was simultaneously obtained in biochar-amended SSFCWs,and the addition of biochar reduced GWP by 57.3 and 3.0%for SSFCWs and SFCWs,respectively.Therefore,adding biochar into CWs can not only improve water purification performance,but also reduce GHG emissions.