We have carried out tests with straw filter during spring and summer 2003. The purpose is to reduce the content of nutrients in drainage and waste water. The straw is supposed to be growth site and deliver energy to the microbs. Energy is a limiting factor for the microbs in drainage water, but the access to nutrients is normally good. The straw has an extremely low content of nutrients, but the access to energy in the form of cellulose is good.
When straw is ploughed in the field, you have to compensate for the high C/N-quota in the straw by adding fertilizer to the extent of 30 kg N/ha. Straw, used in a filter, can be ploughed in without adding expensive fertilizer.
The filter have space for a round bale of two meters diameter and 1.5 - 2 meters height. The straw is treated in different ways for the tests. After some time the bale is replaced and plouoghed in. If the straw has collected metals from industries or traffic, it should be burned.
The filter need at least 0.5 m drop in order to force the water through the straw. The straw filter do not need maintenance except changing the bale. The exchange frequency depends on the water flow and the content of nitrogen in the incoming water.
The first test round has been financed by: VA-Forsk, Royal Academy of Forestry and Agriculture, Innovation West, Lidköping City, ViaCon and Agros.
The results from the first research round is available in Swedish with English Summary at: VA-Forsk report no. B 2003-104 pdf-file.
The second test round (December 2003) has demonstrated better efficiency and worked free from disturbances. The third test round started in March 2004.