WHAT IS A WETLAND ALSO KNOWN AS REED BEDS?

WETLAND
They usually consist of a porous medium of gravel, about 0,5 m deep supporting aquatic plants such as reeds, rushes, sedges etc. Water flows through the porus space between the media and plant roots which provide a large surface area for colonization by communities of pollutant removing microbes.

Although the subsurface environment of a Wetland is largely devoid of oxygen, tiny oxygen rich micro-sites exist adjacent to the macrophyts roots. In traveling through the sequence of oxygen-rich and oxygen-poor environments provided by the Wetland, wastewater is subjected to conditions which enhance the removal of a number of important pollutants such as organic carbon (which depletes dissolved oxygen) in aquatic ecosystems and is hence referred to as Biochemical Oxygen Demand (BOD) and nitrogen (which can cause algal blooms in surface waters).

Other pollutants removal mechanisms in Wetlands include filtration, sedimentation and plant uptake of nutrients. Wetlands also have the capacity to lower the concentrations of disease causing pathogens from wastewater.

SUBSTRATE
The choice of Wetland substrate will depend on the type and quality of influent and the desired quality of effluent.

Gravel of 10 or 20 mm diameter is commonly used, As a rule, media consisting of larger particles will have higher hydraulic conductivities and be less prone to clogging. On the other hand smaller particles will be more prone to clogging but will offer greater reactive surface area and hence provide better treatment. Because clogging is most likely to occur in the entrance zone of the Wetland it is desirable to surround the inlet device with stones of about 100 mm diameter.

When treated effluent is being applied to a soil with low phosphorus sorption capacity (e.g. sandy soils) it can be advantageous to use a Wetland substrate such as crushed brick which has a strong affinity for nitrogen. All commonly used media, including sands, gravels and soils have an initial porosity of between 35% and 45%. This can decrease with time as pore spaces become occupied by macrophyte roots and effluent borne solids.

MACROPHYTES
Most Wetlands built have been planted with Phragmites australis (common reed). Other macrophytes are Schoenaplectus validus (giant club rush) and Typha Orientalis (bull rush) and species from the genus Lomandra and the genus Baumea.