Removal Of Characteristic Pollutants In Livestock Wastewater By Horizontal Subsurface Flow Constructed Wetland

Livestock wastewater contains high concentrations of organic pollutant,ammonia nitrogen,phosphorus and trace heavy metal,veterinary antibiotics and become more and more harmful to the environment with the development of livestock industry.Antibiotic and Antibiotic resistance genes（ARGs）raised by abusing antibiotic has raised significant concern and posed great threat to the environemnt.Livestock wastewater has become the main source of transmission of antibiotcs and ARGs into the environment due to the widely applying of veterinary antibiotics in livestock industry.In order to investigate the removal efficiencies of carbon,nitrogen,phosphorous,TC,Cu and TRGs,The research constructed four horizontal subsurface flow constructed wetlands（CWs）:CK group,TC（Tetracycline,TC）group(1 mg·L^-1 TC in influent),Cu group(5 mg·L^-1 Cu²⁺in influent),TC+Cu group(1mg·L^-1TC and 5 mg·L^-1Cu²⁺in influent).The conclusions are as follows:TOC（Total Organic Carbon,TOC）,TN（Total Nitrogen,TN）,NH4+-N,PO₄^3--P were reduced by（84.3±7.2）%,（78.6±7.0）%,（82.1±4.4）%,（88.0±6.0）%in CK;Compared with CK,the removal rate of TN in TC group,Cu group and TC+Cu group decreased by1.6%～15.7%,2.5%～17.8%and 8.4%～23.0%,and removal rate of NH₄⁺-N decreased by1.6%～15.7%,2.5%～17.8%and 8.4%～23.0%.TC or Cu²⁺had significantly inhibiting effect on the removal of TN and NH₄⁺-N,The aerobic-anaerobic microenvironment formed by plants played an important role in removing nitrogen from wetlands.Removal of TOC,TN,NH₄⁺-N and PO₄^3--P by CWs mainly occurred in the front of CWs.The removal rates of TC and Cu²⁺by three CWs were above 99.9%and 91.4%.The absolute abundance of 16Sr RNA gene and 11 tet genes in effluent were 2～3 orders of magnitude lower than that in influent,the removal rates of 11 tet genes by CK group,TC group,Cu group,TC+Cu group were 0.66～3.71 log,0.96～3.64 log,0.84～3.60 log and0.68～3.47 log;the removal rate of 16Sr RNA gene by CK group,TC group,Cu group,TC+Cu group.were 1.2～3.19 log,1.14～3.16 log,1.10～3.15 log and 1.07～3.20 log.The relative abundances of 7 tet genes（tet A,tet C,tet E,tet O,tet Q,tet T,tet Bp）in effluent were lower than that in influent,7 tet genes were reduce by 43.3%（tet C）～96.3%（tet A）in CK,adding TC or Cu²⁺to the influent increased the relative abundance of TRGs in effluent compared with CK,the relative abundance of TRGs in effluent of TC group,Cu group and TC+Cu group were 12%（tet Q）～52%（tet C）,6.7%（tet Bp）～51%（tet E）,24%（tet Bp）～82%（tet A）higher than that in CK,respectively.This finding suggested CW is an advanced and proper treatment technology for livestock wastewater.TC and Cu²⁺could increase the absolute abundance of 11 tet genes in soil,but have a certain difference in increasing the absolute abundance and relative abundance of TRGs in different regions of CWs.The enhancement to TRGs are more obvious in the front area and in the rhizosphere area of wetland than other areas of CWs by TC and Cu²⁺;Compared with TC+Cu treatment,TC treatment and Cu²⁺treatment had more obvious effect on increasing the relative abundance of TRGs,This may be due to the fact that TC and Cu²⁺could exert selective pressure on microorganisms,increase the absolute abundance of TRGs and reduce the number of background microorganisms in soil.The absolute abundance of 11 tet genes gradually decreased along the direction of the wetland.The absolute abundance of TRGs in the front of the CK group,TC group,Cu group and TC+Cu group are 2.29～6.72 times,2.89～8.16 times,2.07～6.18 times and2.66～8.43 times higher than that in the end,respectively;The absolute abundance of TRGs in rhizosphere soil of CK group,TC group,Cu group and TC+Cu group were 1.34～5.5times,2.05～15.82 times,2.52～8.13 times and 1.6～2.78 times higher than that in non-rhizosphere soil,respectively.The relative abundance of TRGs of in rhizosphere soil of CK group,TC group,Cu group and TC+Cu group were 1.53～3.77 times,1.03～6.42times,2.17～7.14 times and 1.55～2.6 times higher than that in non-rhizosphere soil.the rhizosphere environment was conducive to the amplification of TRGs.Constructed wetland had good removal effect on carbon,nitrogen,phosphorus,TC,Cu²⁺and TRGs in livestock wastewater.Adding TC and Cu²⁺in the influent would inhibit the denitrification efficiency of the constructed wetland and decrease the removal rate of TRGs;both TC and Cu²⁺could increase the absolute abundance and relative abundance of TRGs in soil.The results showed that CWs are suitable and feasible for treating livestock wastewater.