|Authors: ||J. de Haan, J.R. van der Schoot, H. Verstegen, O. Clevering|
|Keywords: ||drain water, purification, dentrification, cost-efficiency, Water Framework Directive|
Vegetable growing leads to high nitrogen emissions.
In the Netherlands, nitrogen emissions can hardly be reduced by reducing fertilization without risks for yield and quality loss.
An alternative measure to reduce emissions is to collect nitrate-rich drain water and remove nitrate from the drain water in constructed wetlands.
This was tested in three different types of constructed wetlands at an experimental farm in the SE of the Netherlands: (1) a surface flow system (SF) planted with Common reed, (2) a horizontal subsurface flow system with Common reed (SSF-reed) and (3) a horizontal subsurface flow system filled with straw (SSF-straw). The water discharge into the wetlands is adjusted to the nitrate removal capacity of the wetlands.
In- and outlet concentrations of nitrogen and other nutrients were measured every two weeks since December 2005. Collected water from pipe drains contained on average 30 mg N L-1. The mean N removal was 58% in SF (1655 kg N ha-1 year-1), 25% in SSF-reed (1447 kg ha-1 year-1) and 63% in SSF-straw (3622 kg N ha-1 year-1). SF and SSF-straw are functioning well.
In SSF-reed, the amount of carbon seems to be insufficient to sustain nitrogen reduction.
Disadvantage of SSF-straw is the negative removal rate of phosphorus (mean 16 kg ha-1). With a removal rate of about 60% within the system, about 20% of the leached nitrogen from the vegetable fields could be removed: about two-third of the leached water is collected in drains and half of the nitrate-rich drain water is collected for purification.
The cost effectiveness (expressed as € per kg N removed) ranged between € 52 and € 104 kg-1 N for SF, between € 29 and 58 kg-1 N for SSF-straw and between € 161 and € 322 kg-1 N for SSF-reed.
Cost reduction is possible by combining with other functions as water storage and nature development.
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