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Freshwater Management Series No. 7

Phytotechnologies

III. Examples of Environmental Applications of Phytotechnology >

C. Constructed Wetlands

Recognition of the ability of wetlands to filter, absorb and metabolize suspended and dissolved matter has resulted in the fact that drainage of wetlands has ceased in many countries. In some cases, previously drained wetlands are now being restored. Furthermore, scientists and engineers are now working together to mimic these natural systems in order to contain and/or treat wastewaters and/or agricultural run-off. This has prompted the construction of artificial wetlands to cope with the diffuse pollution originating from agriculture, septic tanks, and other sources. In the U.S., for example, legislation prohibits the drainage of wetlands unless another wetland of the same size is constructed elsewhere.

Construction of artificial wetlands is an attractive and cost-effective phytotechnology that can be used for controlling pollution originating from diffuse sources and for treating various types of wastewater. For example, constructed wetlands can be used to treat dairy farm wastewaters, mine water pollutants, textile wastewater, and pulp mill wastewater. Wetlands are able to cope with the nitrogen and heavy metal pollution from these sources. However, it is essential to properly plan where to place the artificial wetlands, as their effectiveness is dependent on hydrology (i.e., they should be covered by water most of the year and have a sufficient retention time to allow them to treat specific pollutants), and topography (i.e., they should protect the most vulnerable ecosystems, which are often lakes and reservoirs). Furthermore, it is important to ensure that the wetlands themselves are not sources of potential pollutants, such as phosphorus.

Artificial wetlands are usually constructed so tha t water flows primarily over the sediment and through vegetation, or as vegetated submerged bed systems in which water flow is engineered for contact with plant roots. They are excavated with a shallow gradient in soils of low permeability (or lined with an impermeable barrier and then filled with an appropriate soil). They are then either planted or vegetated naturally. They usually comprise several cells that can operate in a series or in parallel, allowing flows to be redistributed for greater control and easier maintenance.

Nutrient retention and processing features that are characteristic of natural wetlands can be exploited in constructing wetlands. For instance, macrophytes can be kept in a rapid growth phase by intentional, programmed disturbances. Maintaining at least a moderate species diversity makes the system more responsive to variations in loading rates of different nutrients. Anaerobic conditions and large areas of vegetation-free sediment surface can maximize retention of both organic matter and nutrients. Manipulation of water turnover time and addition of other electron acceptors beside oxygen can also be performed in constructed wetlands. Linking constructed wetlands (where flows and vegetation can be controlled) with natural wetlands (for supplying soluble iron and aluminum needed for phosphorus removal) is a strategy for optimising the application of this environmentally sound approach to wastewater treatment.

Various emergent macrophyte species can be used in constructed wetlands, including cattails (Typha spp), bulrushes (Scirpus spp), reeds (Phalaris spp), rushes (Juncus spp), papyrus (Cypersus spp), and sedges (Carex spp). Submerged species can be applied in deep-water zones. Species that have been used for this purpose include coon tail or hornwort (Ceratophyllum demersum), redhead grass (Potamogeton perfoliatus), widegeon grass (Ruppia maritima), wild celery (Vallisneria Americana), and water milfoil (Myriophyllum heterophyllum). Artificial wetlands usually require two to four years to obtain sufficient plant coverage and biodiversity to be fully operational. Experience gained from the wetlands constructed to date suggests that the development and application of process models for the construction of artificial wetlands is essential if positive results are to be expected.

Constructed wetlands can serve the same small communities as natural wetlands and can be incorporated into the treatment systems for larger communities as well. Wetlands can also be constructed to treat agricultural runoff or other non-point sources of pollution and are especially well suited for use in surface-mined areas. Ancillary benefits, such as nitrogen and saltwater filtering, supply of water and nutrients, production of food and support of endangered species, can further increase the economic advantages compared to conventional wastewater treatment plants.

The self-purification ability of wetlands has found wide application as a wastewater treatment method in several developing countries such as China, Philippines, Burma, India, and Thailand. An integral part of this is the stocking of fish cultivated in biological sewage stabilization ponds. Through the intermediate activities of bacteria, algae, and other types of plankton, nutrients such as nitrogen and phosphorus are made available and metabolized by the fish.

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