Constructed wetlands and vegetation filters: an ecological approach to wastewater treatment

Constructed wetlands (CWs) are man made vegetation filter systems, which simulate the ability of natural wetlands to remove pollutants from water. Eco-technical treatment can facilitate the re-use of process waters, drainage waters and effluents. Aquatic microbial communities, in association with plant roots and a supporting mineral matrix, are effective at removing pollutants, such as suspended solids, dissolved and particulate organic matter, nitrogen, phosphorus, metals and pathogenic organisms from effluent streams. Treatment of polluted waters in a small, self-contained CW bed is relatively inexpensive and involves low technology with a "green" image. CWs can enable contaminated waters to be re-used productively, for example in agriculture, horticulture and energy forestry. Applications for CWs include wastewater (sewage) treatment at secondary or tertiary stages, sludge drying, surface runoff (commercial, industrial), groundwater treatment, industrial and agricultural process water treatment, and for ecological habitat creation. CW systems vary in design, including surface flow vegetated channels and sub-surface soil/vegetation filters. The latter may employ horizontal flow (HF), vertical flow (VF) or tidal flow (TF) hydraulic regimes and these may be combined in hybrid systems to optimise pollutant removal. Macrophyte species planted in the bed include reeds (Phragmites), cattails (Typha), rushes (Juncus) and willow (Salix). The porosity of the bed fill material is critical for the hydraulic loading rate and retention time of effluent passing through it, which in turn determines the efficiency of water treatment. The microbial community in the bed is responsible for the processes of degradation and chemical transformation, which result in pollutant removal. Both aerobic and anaerobic processes are involved, but degradation of carbonaceous matter (BOD) to CO2 and transformation of ammonia to nitrate require biological oxidation, and hence a continual supply of oxygen. This is achieved most efficiently in compact VF systems; as the surface is flooded, air is forced into the bed, while effluent percolates downwards through the matrix. Horizontal flow (HF) beds typically achieve lower oxygen transfer rates but, with largely anaerobic conditions, they are effective in removing nitrogen to atmosphere via de-nitrification. The role of plant roots in providing an oxygen source in the subsurface environment, according to the Root-Zone Model, and in creating reduced/ oxidised micro-environments in the matrix, is considered. Evidence for the release of oxygen by plant roots and the presence of aerobic/anaerobic micro-gradients is discussed, with reference to measurements in laboratory microcosms and in-situ field systems. The importance of plant uptake in removing N and P from effluent is assessed. CWs are effective in treating polluted waters arising from a wide range of domestic, industrial and agricultural operations and are particularly appropriate for isolated rural situations, enabling water of acceptable quality to be discharged to environment, or to be re-used locally as fertilizer. CW technology offers a cost-effective means of protecting water resources from contamination, whilst providing local habitat diversification