Constructed wetlands for this purpose can be located within surface drains (i.e. in-stream wetlands) or at the outlets of the drainage network of either surface or, more commonly, of sub-surface drains.
The use of low nutrient carbon-rich supplement such as sawdust, woodchips or cereal straw can markedly increase the nitrate removal.
Woodchip filters can remove and immobilise a proportion of the nitrogen passing through them, and provide a slow release of organic matter that promotes bacterial conversion of nitrogen to nitrogen gas.
These carbon rich anaerobic environments are excellent at converting a high proportion of nitrogen to inert N2 rather than the greenhouse gas N2O.
Constructed wetlands are most efficient where flows variations are not extreme and nitrate loadings are not excessive, so wetlands should be seen as a final buffer and not replace sound management of grazing, nutrient and effluent application on drained land.
DemoDAIRY is monitoring the installation of a woodchip filter and constructed for the nutrient removal of a recently tile drained 20 hectare parcel of land.
The system comprises a 40 cubic metre wood chip filter leading to a 200 square metre wetland utilising reed-bed technology for aerobic/anaerobic water treatment.
In 2009 Drain Tech completed the installation of the wood filter and constriction of the Nutrient Filter Ponds. DemoDAIRY received funding from Corangamite Shire and Glenelg Hopkins Catchment Management Authority to begin the planting of the reeds to uptake the residual nutrients.
With the help from Conservation Volunteers Australia. Carex tereticaulis, Carex appressa and Carex tasmanica were planted in the ponds, with Lomandra longifolia and Leptospermum langierum planted on the higher ground. Drain Tech also donated Phragmites australis which have also been planted at the site.
The first water quality testing has been completed with the results confirming that the system is filtering a level of nutrients from the water.