Greenhouse Gas Emissions And Related Effectiveness Of Vegetation-SBR System

CH₄ and N₂O emitted from wastewater treatment facilities are main sources greenhouse gas（GHG）in the atmosphere,which obtained more and more concerns recently.As a landscaping wastewater treatment technique,vegetation-activated sludge process（V-ASP）comprised of conventional activated sludge system and decontamination plants,which has been evidenced to have merits such as stably good effluent quality,small footprint and limited impact onto surrounding environment.These V-ASPs allocated with vegetation are expected to generate relatively lower GHG compared with ASPs.In the present work,the greenhouse gas emission of VASPs wascharacterized,and the effect of operational condition parameters were evaluated.Estimation of GHG emission from V-ASPs by comparison of conventional ASPs were carried out to evaluate its environmental benefit that provide direct scientific baisis to understand V-ASP.V-ASP,constructed wetland and sequencing batch reactor（SBR）system were operated in parallel to study their GHG emission behaviors.It was found that the GHG from V-SBR and SBR were mainly produced at aerobic stage,while the maximum CH₄ and N₂O from the constructed wetland system was mainly from 12:00 noon that gradually decreased from the inlet to the outlet,indicating their clear correlation with surface load rate.As for the same organic removed,the methane emissions from the constructed wetland and SBR were 49 and 1.3 times higher,respectively,in contrary to the V-SBR system.Meanwhile,for the same total nitrogen removed from system,the N₂O emissions from the constructed wetland and SBR system were 21.6 and 1.16 times hgher,respectively compared to V-SBR system.Effect of various operational condition parameters,including influent C/N,temperature,aeration rate,HRT,sludge load rate and coexisting nanoparticles,on GHG emission in V-ASP were studied.The experiments displayed that the CH₄ flux in V-ASP has a positive correlation with influent C/N,while N₂O flux has a negative correlation.CH₄ and N₂O flux increased with the increase of operating temperature,and CH₄ emission was insignificantly affected by the aeration intensity that affected N₂O emission positively.Once the concentrations of nano-silver and nano-zinc oxide in the influent were higher than 1 mg/L and 10 mg/L,respectively,their impact on GHG emission became significant,and a high concentration of nanoparticles in influent resulted in a much more negative impact onto the GHG generation.The main reason was that the accumulation of nanoparticles on the activated sludge surface inhibits the microbial activity,even leads to their death.The effect of HRT on CH₄ emission flux was not significant,but the N₂O emission flux was positively correlated with HRT.CH₄ and N₂O emission fluxes increased with the increasing of sludge load rate and AMO and HAO activity,due mainly to their promotion function to nitrite production and accumulation.PCR-DGGE,Q-PCR and high-throughput sequencing were used to characterize the microbial community structure in V-ASPs.The results showed that the microbial community structure of suspended sludge,suspended sludge and root sludge was different from that of traditional SBR system,althought the dominant bacteria were Proteobacteria.The NOB abundance in suspended sludge of V-SBR system was higher compared to SBR,and the nirS denitrifying bacteria and archaebacteria abundance in plant root sludge were relative higher in contrary to SBR.These high AOB and NOB,and less denitrifying bacteria in SBR produced more N₂O.Meanwhile,the Archaea abudnanc with SBR bulk may also take a certain responsible for its high methane production.Based on the GHG emission results,GHG emission equition in SBR and V-SBR was established according to the carbon and nitrogen balance model.It was calculated that the the annual CH₄ emissions from SBR and V-SBR systems are 2.56 kg/y and 1.92 kg/y,respectively,and the annual N₂O emissions are 0.049 kg/y and 0.045 kg/y,respectively.Compared with the SBR system,the annual emission reductions of CH₄ and N₂O in the V-SBR system were 0.64 kg/y and 0.004 kg/y,respectively.Moreover,the emission of GHG would be further reduced by controlling the operating parameters,influential factors and optimizing the plant configuration.