Concept
Ecological engineering, in W. J. Mitsch's definition, designs ecosystems for the mutual benefit of human society and nature. Constructed wetlands are a flagship technique: they use natural self-purification instead of mechanical or chemical treatment, so energy and running costs are low.
Types
- Free water surface (FWS): shallow water flows over the soil, closest to natural marshes and best for habitat.
- Horizontal subsurface flow (HSSF): water flows through gravel or sand, with fewer odours and mosquitoes.
- Vertical flow (VF): intermittent dosing from the top improves oxygen supply and nitrification.
How treatment works
Solids settle and are filtered; microbes break down organic matter; nitrogen is removed by nitrification and denitrification; phosphorus is adsorbed by media and taken up by plants. Reeds and cattails supply oxygen through their roots and offer surfaces for microbial biofilms.
Uses
Small community sewage, polishing of treatment-plant effluent, control of agricultural and urban runoff, and restoration of degraded streams and wetlands. Drawbacks are the large land area and lower efficiency in cold seasons.
References
- Kadlec, R. H., & Wallace, S. D. (2009). Treatment Wetlands (2nd ed.). CRC Press.
- Mitsch, W. J., & Jørgensen, S. E. (2004). Ecological Engineering and Ecosystem Restoration. Wiley.
- Vymazal, J. (2010). Constructed wetlands for wastewater treatment. Water, 2(3), 530-549.