Last week, Dr. Laura Christianson joined us for our monthly Iowa Learning Farms webinar. Christianson has nine years of experience focused on agricultural drainage water quality and denitrification bioreactors for point and nonpoint nitrogen treatment.
Bioreactors: What We Know
Laura’s experience with bioreactors over the past nine years has led her to study bioreactors with many shapes, sizes and designs. She authored in a meta-analysis on bioreactors that synthesized existing research.
For bioreactor basics, the meta-analysis found that bioreactors can remove an average of 25-45% of the annual N load leaving a field, although a range of 10% to 100% of N load reduction has been seen. The cost of a bioreactor that treats 50 acres was approximately $10,000. The analysis also found that woodchips in a bioreactor can last approximately 10 years, but a range of 7-15 years has been seen. The woodchips need to be changed not because the bioreactor is running out of a carbon source, but because woodchip degradation causes hydraulic performance decline within the bioreactor. The meta-analysis also investigated different factors that impact nitrate removal effectiveness in a bioreactor. Some of these factor are:
Hydraulic retention time
Water should be retained within a bioreactor for a minimum of six hours. Great visuals and an explanation are covered in the webinar.
Water temperature and age of bioreactor
Water lower than 43 degrees F affect nitrate load reduction potential. In the first year, bioreactors are incredibly efficient at N load reduction as microbes begin to feast on available carbon and dissolved oxygen in the water. However, Laura says, “Once your bioreactor is more than a year old, that’s when you really settle in to know what your long-term nitrate removal performance is going to be.”
Porosity of the woodchips
There was no significant difference in N removal when using different types of wood (hardwood vs. softwood) in a bioreactor. However, the physical properties of the wood matter. Use chips with particles size between ½ inch and two inches instead of shredded or mulched wood.
Bioreactors: The Future
Future research on bioreactors is moving us beyond the first generation of bioreactors.
Bioreactors with baffles
Plastic baffles in the bioreactor route the water through the woodchips so all woodchips are utilized in the denitrification process.
Two bioreactors are installed side-by-side. One serves as the primary bioreactor, and bypass water from the primary bioreactor is routed to a second bioreactor to continue N removal on bypass water that would normally not be treated.
For areas with ditch drainage, the bottom of the ditch is excavated, woodchips are placed, and wooden check bands are installed incrementally throughout the length of the treated ditch bed.
Bioreactors paired with
Water is routed through a phosphorous-absorbing filter prior to its entry into the bioreactor.
If you would like to brush up on your bioreactor knowledge, don’t miss this webinar!