This article presents the design of a constructed wetland (CW) system in an area with limited land availability, resulting in high hydraulic loads. The CWs were constructed to act as a buffering/ polishing step after stabilization ponds for steel industry wastewater post-treatment.
Hybrid pond is a wetland system for Formosa Steel plant wastewater polishing treatment, Vietnam (design flow 36,000 m3/day, total area 10 ha). (1) Waste stabilization pond cells; (2) subsurface flow constructed wetland cell CW1; (3) subsurface flow constructed wetland cell CW2; (4) subsurface flow constructed wetland cell CW3; (5) free water surface flow constructed wetland cell CW4.
An appreciated benefit of the wetland was the ‘green element’ in the industrial landscape.
Full scale wetland design
The full scale constructed wetland system consisted of 4 cells in series, with a total wet area of 4.32 ha, preceded by open ponds. Four full scale wetland cells (CW1, CW2, CW3 and CW4) were constructed using the coarser gravel.
CW1, CW2 and CW3 were subsurface horizontal flow CWs, planted with a mixture (multi-culture) of Phragmites australis Cav., Typha angustifolia L. and Cyperus tegetiformis. The final cell, CW4, was a free water surface wetland planted with P. australis Cav. and T. angustifolia L. P. australis (Cav.), T. angustifolia L. and C. tegetiformis were chosen because they are most tolerant to the local environment conditions, and the plants could be found locally.
Treatment performance of the wetland cells
During the first operation year (September 2017 to July 2018), the pond-CW system has received wastewater already meeting required discharge standards. The effluent from the CWs had consistently lower concentrations of all measured variables, and met the predicted values for BOD5, Tot-N and NH4+-N. Highest removal efficiencies were achieved for NH4+-N (>90%), Mn (>60%) and Fe (45%) with removal efficiencies for Tot-N (14%), BOD5 and COD (around 30%). Concentrations of phenol, CN− and Cr6+ were below 10, 4 and 3 μg/l, respectively in in- and outflows.
Removal efficiencies (concentration reductions) of constructed wetland cells
CW 1–4 at the Formosa Steel plant, Vietnam, during September 2017–July 2018.
Viet Anh Nguyen, Minh Phuong Nguyen, Karin Tonderski, Hai Do Thi, Anh Thi Kim Bui (2019), Design and performance of a coarse media, high hydraulic load polishing wetland for steel industry wastewater, Water Science and Technology 80 (1), pp. 59-66