Comparison of Piggery Wastewater Treatment after the Biogas Process by Cyperus alternifolius and Ipomoea aquatica in a Vertical Subsurface Flow Constructed Wetland System in Vietnam

Vertical subsurface flow constructed wetlands (VSF CWs) have been widely applied as an eco-friendly solution for treating livestock wastewater. This study compared the treatment performance of Cyperus alternifolius and Ipomoea aquatica planted in VSF CWs treating piggery wastewater after biogas digestion. Two laboratory-scale VSF CW systems (50 × 50 × 50 cm) with identical media configuration and operational conditions were operated in an intermittent mode with a hydraulic retention time of 3 days over 20 treatment cycles. The VSF CW planted with C. alternifolius consistently exhibited higher removal efficiencies for all monitored parameters compared with the system planted with I. aquatica (p < 0.05). The average removal efficiencies of the papyrus-based system reached 74.2 ± 3.1% for COD, 85.8 ± 3.4% for TSS, 70.9 ± 4.1% for TN, 75.1 ± 5.5% for NH₄⁺–N, and 64.7 ± 7.9% for TP, with most effluent concentrations complying with the Vietnamese discharge standard QCVN 62:2025/BTNMT (Column B). In contrast, the VSF CW planted with water spinach achieved lower treatment efficiencies, with average removal rates of 53.9 ± 2.6% for COD, 80.5 ± 5.6% for TSS, 54.6 ± 5.6% for TN, 57.0 ± 8.6% for NH₄⁺–N, and 44.5 ± 13.1% for TP, and did not consistently meet discharge limits for nitrogen and phosphorus.