Removal of Characteristic Pollutants in Livestock Wastewater by Horizontal Subsurface Flow Constructed Wetlands

To explore the removal efficiency of characteristic pollutants in livestock wastewater by horizontal subsurface flow constructed wetlands (CWs), this study selected tetracycline (TC) and Cu2+, a familiar antibiotic and a typical heavy metal in livestock wastewater, respectively, to build the following four groups of CWs: control (CK group), 1 mg•L-1 TC in influent (TC group), 5 mg•L-1 Cu2+ in influent (Cu group), and both 1 mg•L-1 TC and 5 mg•L-1 Cu2+ in influent ("TC+Cu" group). The average removal rates for control CWs were (84.3±7.2)% for total organic carbon (TOC), (78.6±7.0)% for total nitrogen (TN), (82.1±4.4)% for ammonia nitrogen (NH4+-N), and (88.0±6.0)% for PO43--P in a long-term operation. Compared with that in the CK group, the removal rate of TN in the TC group, Cu group, and "TC+Cu" group decreased by 0.4%-21.7%, 2.8%-25.5%, and 4.3%-27.0%, respectively, and the removal rate of NH4+-N decreased by 1.6%-15.7%, 2.5%-17.8%, and 8.4%-23.0%, respectively. TC or Cu2+ in the influent significantly inhibited the removal of TN and NH4+-N in livestock wastewater by CWs. The removal of TOC, TN, NH4+-N, and PO43--P by the CWs mainly occurred in the front section of the CWs. The removal rates for TC and Cu2+ were above 99.9% and 91.4% in the effluent of both CWs treated with TC, Cu2+ respectively and CWs treated with TC and Cu2+. The results showed that influent had a higher abundance of 11 tet genes than effluent by approximately two to three orders of magnitude through all CWs, suggesting that the CWs may play a dominant role in antibiotic resistance genes (ARGs) and the bacteria removal process. The relative abundances of seven tet genes (tetA, tetC, tetE, tetO, tetQ, tetT, and tetBp) in effluent were lower than those in influent, and seven tet genes were reduced by 43.3% (tetC)-96.3% (tetA) in the CK. Compared to that in the CK, the addition of TC or Cu2+to the influent increased the relative abundance of TRGs in the effluent of CWs. The relative abundances of TRGs in the effluent of the TC group, Cu group, and "TC+Cu" group were 12%-52%, 6.7%-51%, and 24%-82% higher, respectively, than that in the CK. These results suggest that CW is suited for livestock wastewater advanced treatment, as it provides great application prospects in the removal of antibiotics and heavy metals and the alleviation of the future risk of antibiotic resistance genes. © 2021, Science Press. All right reserved.