Evaluation of a hybrid constructed wetland for treating domestic sewage from individual housing units surrounding agricultural villages in South Korea

The treatment efficiency of 2- and 3-stage constructed wetlands (CWs) was evaluated for treating domestic sewage from houses surrounding agricultural villages. The optimum depth of filter media was 90 cm. The optimum volume ratio of vertical flow (VF) and horizontal flow (HF) beds was 1:2, and the optimum filter medium was broken stone in the VF-HF 2-stage hybrid CWs. Based on the above optimum conditions, removal efficiency of biochemical oxygen demand (BOD), chemical oxygen demand (COD), suspended solids (SS), total nitrogen (T-N), and total phosphorus (T-P) were 99, 98, 99, 68, and 72%, respectively. However to utilize constructed wetlands (CWs) for treating domestic sewage for an individual house, would require downsizing of the 2-stage hybrid CWs. In addition, the low removal efficiency of T-N and T-P in 2-stage hybrid CWs would require improvements necessary to meet acceptable water quality discharge standards. Thus, to reduce the CWs’ area and improve the T-N and T-P removal efficiencies, VF-HF 2-stage hybrid CW was modified into VF/HF(I)-HF(II) and VF/HF(I)-HF(III) 3-stage hybrid CW. The optimum reduced size of 3-stage hybrid CW was the VF/HF(I)-HF(II) configuration which also increased T-N removal. Using this system, removal efficiency of BOD, COD, SS, T-N, and T-P were 99, 98, 99, 83, and 75%, respectively. In VF/HF(I)-HF(II) CW, the removal velocity of BOD, COD and SS was rapid on the order of VF (1ˢᵗ stage) ≫ HF(II) (3ʳᵈ stage) ≥ HF(I) (2ⁿᵈ stage), VF (1ˢᵗ stage) ≫ HF(II) (3ʳᵈ stage) > HF(I) (2ⁿᵈ stage) and VF (1ˢᵗ stage) ≫ HF(I) (2ⁿᵈ stage) > HF(II) (3ʳᵈ stage), respectively. The removal velocity of T-N and T-P in VF/HF(I)-HF(II) CWs was rapid on the order of HF(I) (2ⁿᵈ stage) > HF(II) (3ʳᵈ stage) ≥ VF (1ˢᵗ stage) and VF (1ˢᵗ stage) > HF(I) (2ⁿᵈ stage) > HF(II) (3ʳᵈ stage), respectively.