Source : DemoDAIRY

Nutrient Filter Pond

Sub-surface drainage is a fundamental necessity for efficient farming. Nowhere is this more pertinent than in the high rainfall grazing enterprises that are based in the heavy soil districts of South West Victoria. These grazing systems are inherently “leaky” in that high nutrient inputs and high animal by-product outputs are placed into an environment that has high and intense rainfall frequency. This leads to high rates of surface and sub-surface flows from paddocks to local watercourses.

Be it diffuse flows from overland discharge or point source from tile drain outlets, the fact remains that landholders need to address the fact that the potential exists for excesses of nutrients, chemicals or wastes to be carried to watercourses if no action is taken.

Constructed wetlands (or nutrient filter ponds) are a new technique being promoted to intercept and treat nutrient removal before drainage water reaches a stream or water body. 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 - see ‘Notes Page’ for Background.

Background
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 wetland 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 utilizing reed-bed technology for aerobic/anaerobic water treatment.