Managing dairy farm drainage pollution
NIWA research for Dairy Insight is starting to provide good detail on the effectiveness of constructed wetlands and wood-chip filters in removing pollutants in dairy farm drainage.
Our key finding to date is that, without the addition of treatment supplements, constructed wetlands probably need to be between 2 and 5% of the catchment area to give effective nitrate removal. Constructed wetlands of about this size in our research have achieved 40–50% nitrate removal, but it is too early to say what the long-term average would be.
So far, there are indications that some species of commonly used wetland plants are better than others at removing nutrients. In particular, raupo (Typha orientalis) appears to be more effective than reed-sweet grass (Glyceria maxima) or native sedges. Such findings could help guide the choice of plant species in certain localities.
Wood-chip filters, at loading rates equivalent to 0.6–1.2% of the catchment area, can substantially improve nitrate removal – up to 70%. The filters do, however, need to stay porous and continue to release carbon (as an energy source for bacteria which use the nitrate).
A newly constructed wetland can release phosphorus for a number of years, so we plan to start testing phosphorus-absorbing materials next year in order to devise strategies that prevent such release.