Effects of Hematite and Biochar Addition on Wastewater Treatment Efficiency, Greenhouse Gas Emission, and Microbial Community in Subsurface Flow Constructed Wetland

Abstract

The type and structure of the substrate in constructed wetland affects the diversity and abundance of microorganisms, thereby influencing the effect of sewage treatment. In this study, four groups of wetlands were constructed in the greenhouse:blank-constructed wetland (CW0), hematite-constructed wetland (CW1), biochar-constructed wetland (CW2), and hematite+biochar-constructed wetland (CW3), to study the differences in sewage treatment effects, greenhouse gas emissions, and microbial community structures of constructed wetland systems under different filler substrates. The results showed that the addition of hematite or biochar increased the COD removal rate of -0.12% to 1.7%. The addition of biochar increased the removal rate of NH4+-N by 22.48% and NO3--N by 6.82% and reduced the emission flux of CH4 by 83.91% and N2O by 30.81%. The addition of hematite reduced the removal rate of NH4+-N by 1.12%, increased the removal rate of NO3--N by 3.98%, and reduced the emission flux of CH4 by 33.29% and N2O by 25.2%. Adding biochar or hematite increased the relative abundances of Actinobacteria and Proteobacteria, which was beneficial to the removal of COD. The Ace, Chao, Sobs, and Shannon indexes in the substrate treated with biochar were the largest, and the Simpson index was the smallest. The treatment with hematite was the opposite, indicating that the richness and diversity of microbial communities in the treatment system with biochar was the largest. Adding hematite reduced the richness and diversity of the microbial community in the constructed wetland system. Adding biochar or hematite increased the relative abundances of Dechloromonas, Thaurea, Saccharimonadales, and other denitrifying bacteria, which was beneficial to wetland denitrification. The addition of biochar increased the abundances of nosZ, nirS, and nirK functional genes, which were conducive to the occurrence of denitrification. The addition of biochar increased the abundances of pmoA functional genes, reduced the abundance of mcrA functional genes, and inhibited the production of CH4. It also increased the abundance of methanotrophic bacteria and promoted the occurrence of the CH4 oxidation process. Although the addition of hematite increased the abundance of mcrA functional genes, Fe3+ competed with methanogens for electron donors and inhibited the production of CH4.