This quick calculator is provided as an aid to developing a simple wetland system for managing nutrient-loaded waste water on the farm. For further information, the guide Managing waste water with a wetland is available.

It is important to note that wetlands are complex systems. In order to simplify construction requirements, the time that water stays in a wetland has been selected as the key design factor. Complete removal of nutrients is unlikely in a constructed wetland system; rather, the system aims to reduce the nutrient levels and to contain the water for later re-use. The required retention period of the system depends on the quality of the water to be treated and the desired outcome.

The most effective way to treat waste water is to combine a couple of different modules into one wetland system; for example, a treatment system might include a sediment trap module plus an artificial wetland module plus a retention pond. A total retention period of at least 7 days for the whole system should be considered.

A wetland waste water treatment system is just one option available in managing nutrient-loaded waste water, but it is a simple, relatively low cost and convenient option.

• Sediment trap module
  The sediment trap needs to hold water for at least 30 minutes. This allows sediments to settle out. The outlet should be screened and/or be situated below the surface to prevent floating debris from being carried out.
   
• Grassed soak area module
  The grassed soak area is a soil filled module which can remove nutrients from waste water. It is planted out with grasses or sedges. A retention period of at least one day should be used.
   
   
• Artificial wetland module
  The artificial wetland is a gravel or sand filled module planted with emergent water plants. It slows down the flow of water and provides an effective ‘polish’ of water, removing nutrients and sediments. A retention period of at least five days is suggested.
   
• Retention pond module
  The retention pond is a dam or tank used to hold water before re-use (or disposal). Some nutrients and sediments will be removed simply by containing the water for a period of time. This module can also provide a storage for rain that falls on the wetland system. A retention period of at least two days is suggested.