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Bioreactor gets recharged

By Staff | Nov 24, 2017

Photo of field day attendees who watched the excavator spread new woodchips in the bioreactor, as well as an image of old woodchips that will be sent to the lab for several tests.



JEFFERSON – Bioreactors have become a more common edge of field conservation practice for almost a decade now.

Most recently the first ever bioreactor received a recharge.

According to Keegan Kult, environmental scientist with the Iowa Soybean Association, the woodchips placed inside the bioreactor should last 10-15 years, but due to some concerns that come up while testing and also the fact there was funding available, they decided to recharge the bioreactor after just nine years.

The bioreactor, located on Mike Bravard’s farm near Jefferson in Greene County, was installed in August of 2008 with funding provided in part by the Agriculture Clean Water Alliance (ACWA), Kult said.

“There were bioreactor demonstration projects back then and they had been doing them on more of a plot scale, and they decided to come together and look at bioreactors on a field scale,” said Kult. “So this was the first one installed in an actual field setting.”

Kult said data collected from this particular bioreactor helped to write the NRCS’ practice standard. The data was also included in the Iowa Nutrient Reduction Strategy Science Assessment for performance of a bioreactor.

As far as performance on this bioreactor, Kult said it was monitored closely for the last seven years and within those seven years, said results showed 1,500 pounds of nitrogen had been removed from the system due to the bioreactor.

“We thought that was pretty good performance,” he said.

Concentration reduction within this bioreactor, he added, on average showed 51 percent concentration reduction.

Kult admits had the bioreactor been closely monitored these last few years, the results would most likely had proven better.

How does someone know if their bioreactor needs recharged?

“Monitoring the bioreactor is key,” said Kult. “If you’re not seeing as high as a nitrate reduction as we are used to, or if the water is flowing through them slower than before, that is one way to tell.”

Visually checking the bioreactor is another way to tell how it is performing, Kult said.

Some subsiding of the bioreactor happens naturally as the woodchips settle down.

If this seems to happen a lot, he said, you may want to pull a few water samples to check the bioreactor’s performance.

With this particular bioreactor, Kult said they noticed the concentration of the water coming through wasn’t being reduced as much as it previously had, so they put more stop logs in the control structure – which controls the outflow of the bioreactor to slow the water down.

By slowing the water down coming through a bioreactor, Kult said this causes water to eventually by-pass the system.

“To get a little more technical, we also did some alkalinity studies, which is looking at the inorganic carbon that is generated, so we know how much nitrogen was being removed, and the way that works is through chemistry we know we should generate an x amount of inorganic carbon coming out of it and we noticed those ratios were starting to drop,” he said.

Recharging process

Kult said they believe the expense of recharging a bioreactor should be at half of the cost of the initial installation.

First a contractor comes in a removes the soil cap, which is about the two top feet of soil. Then the geofabric layer is removed, as are all of the existing woodchips.

In this particular bioreactor, Kult said, since it was their first one, it features a different design.

“This one has about a one foot trench below the depth of the tile and we don’t do that anymore because we want to have all of the water to be able to flow out of the system,” he said. “So, we left a little over a foot of the woodchips in to bring it up to the flow of the line of tile, covered that with plastic to help keep water from going down in to it and put fresh chips over that.”

This process, and a different formulation of woodchips, should give this particular bioreactor more of the 10-15 year window life span.

This recharge, Kult said, is going to allow for some more insight into what happens inside a bioreactor.

“We’re going to take a look at the spent woodchips,” he said. “There are a lot of research questions out there what carbon is left in it, if there’s more nitrogen in those chips.”

Some of the samples are also going to be sent up to Minnesota for further research through hydraulic conductivity tests.

“Right now, we are designing bioreactors with the idea of how well water flows through when they are fresh, new woodchips,” he said. “But, we don’t know the properties once they break down very well we will take a look at that. We should get some good data from that to help inform us if we need to tweak the design a little bit to compensate for how the woodchips break down in the design.”

What is a bioreactor?

Kult described a bioreactor as an edge of field conservation practice, which is kind of in the same realm as saturated buffers.

“We are looking at treating tile drainage with these edge of field practices, so we tap into the tile line to divert a portion of the water through an underground bed of wood chips,” Kult said. “And we design to these to treat 15 percent of the peak flow from the field tile with a four hour retention time for the water to be within the bioreactor.”

Kult said they are seeing a big impact on these edge of field practices with it comes to terms of looking at the Iowa Nutrient Reduction Strategy.

“We are seeing these edge of field practices being effective in terms of cost of pounds of nitrogen removed,” he said. “Especially because how cost effective they can be.”

Kult encourages those that are interested in having a bioreactor installed on their farm to contact their county’s NRCS office to inquire about what programs are available or also the ISA. Currently the ISA has a contract with IDALS to help facilitate the design of these practices.

“We are able to come out and do a consult on your farm to see what practices will fit,” he said.

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