Research On The Performance Of Hybrid Constructed Wetlands Enhanced With Biochar-ZVI

As one of the 13 most water-scarce countries in the world,China has many issues of water resources such as insufficient water and uneven distribution of water resources.With rapid urbanization and industrialization in China,the demand for water resources and water pollution are becoming serious increasingly.Wastewater treatment plants(WWTPs)are the important unit of urban water circulation,and are important for guaranteeing the quality of water environment.Although the effluent of WWTPs in most cities requires the strict implementation of the ’Discharge standard of pollutants for municipal wastewater treatment plant’(GB 18918-2002)level-A-standard,the discharged tailwater of WWTPs has still a great impact on water quality.To meet’ The major water pollutant discharge standard of urban sewage treatment plant ’(DB33/2169-2018)put forward in Zhejiang,this thesis developed the novel hybrid constructed wetlands system combined with the biochar and Zero-Valent Iron(ZVI),and the pollutants removal performance,nicrobial community and functional genes of biofilm,nitrogen removal pathways and possible mechanism of the wetland systems.The main results are as follows:1.Three groups of the novel hybrid constructed wetlands system was built to study the operation performance under different hydraulic retention time(HRT).The results showed that the HRT under the condition of 1.5 d and surface load of 1.21 gTN/(m2 d)),the removal efficiencies of COD,NH4+-N,TN and TP in the control group(W1)were 94.3±6.0%,68.3±7.1%,52.2±4.5%and 68.1 ±8.0%,respectively,95.43±5.21%,89.11 ± 1.45%,88.08±1.06%and 85.98±2.47%,in the W2 with biochar,and were 96.23±5.3%,88.37±1.47%,88.34±0.65%and 91.59±3.77%in the W3 with ZVI-biochar.The effluent of W2 and W3 could meet ’The major water pollutant discharge standard of urban sewage treatment plant ’(DB33/2169-2018).Analysis of pollutant removal characteristic in three groups of wetland systems along the flow direction showed that,NH4+-N removal efficiency was improved to 89.1±1.5%,and the NH4+-N removal loading of biochar was 3.90 g/(m3 d),which was higher than that in W1 at the same position.At the same time,the removal efficiency of NO3-N in W3 was improved to 81.2±6.3%,in which the removal loading of NO3--N was 6.16 g/(m3·d).2.Analysis of the microbial community of the three wetland systems showed that,the biofilms of W2 and W3 wetland systems were highly enriched with Rhodocyclales,Nitrosomonadales,Rhodobacterales,Nitrospirales and other denitrification functional microorganisms the order level,with the highest abundance of 18.96%,2.88%,2.22%,4.64%,26.31%,2.23%,1.65%,and 1.91%,respectively.Results showed that the biochar can enrich the abundance of functional microorganisms such as Rhodocyclales and Nitrospirales in W2-B and W2-G,and the ZVI-biochar can enrich the abundance of Rhodocyclales and Nitrospirales in the W3-Z and W3-ZB.At the same time,the number of AOB-amoA in the three wetland systems was higher than that of AOA-amoA indicating that the addition of biochar could enhance the enrichment of aerobic ammonia-oxidizing bacteria.The number of nirK and nirS which are the functional genes of denitrification was the highest in W3(2.51×107 copies ng-1),indicating that the addition of ZVI-biochar could enhance the enrichment of denitrifying bacteri.Meanwhile,the number of hszA in the ZVI-biochar layer and gravel layer was the highest in W3,indicating that ZVI could also promote the enrichment of functional gene hszA effectively.3.Results showed that zeolite had a strong adsorption capacity for NN4+-N and TN,and the process met the double constant equation.By analyzing the degradation characteristics of pollutants by biofilms,it was found that the quantity of biofilm attached to the surface of biochar in W2 was the highest(850.494 nmol/g in ATP),resulting in higher NH4+-N removal rate in W2.In comparison,the removal efficiency of NO3--N by biofilm in W3 was improved obviously.Furthermore,the analysis of plant growth in different wetland systems showed that the leaf area index(LAI)(8.9417 and 6.9288)and nitrogen assimilation(28.38±7.83%and 21.99±4.82%)in W2 and W3 were higher than those in W1 indicating that biochar was applied to the growth and nitrogen absorption of wetland plants.The results of principal component analysis(PCA)showed that the copy number of AOB genes was higher than AOA,it was speculated that the nitrification process of wetland system was mainly affected by AOB.In the aspect of NO3--N removal,it was affected by the denitrification functional genes of nosZ,nirS and anaerobic ammonia oxidation functional genes of hszA,while the ZVI addition in wetland system has obvious enrichment potential of nosZ,nirS and hszA.After of all,adding the biochar or ZVI as the wetlands materials could improve nitrogen removal performance in the wetland system,the biochar strengthen the NH4+-N removal and ZVI further strengthen the NO3-N removal.The novel hybrid constructed wetlands system is expected to meet ’The major water pollutant discharge standard of urban sewage treatment plant’(DB33/2169-2018).