Bioreactors in-depth: Carbon sources, incorporation, maintenance & more
Hello, and welcome back to Advancing Nitrogen Smart, the special podcast series from University of Minnesota Extension. I'm Jack Wilcox in Extension Communications.
Jack Wilcox:Today, we're talking about edge of field practices, and we're gonna focus on one in particular. Here with me are Brad Carlson, Extension educator, and Jeff Strock, soil scientist with the Southwest Research and Outreach Center in Lamberton, Minnesota.
Jack Wilcox:Brad, what are edge of field practices?
Brad Carlson:Well, Jack, when it comes to reducing the amount of nitrate that's ending up in water, there's really, like, three large groups of of practices. One is fertilizer management. Obviously, we spend tons of time talking about that, during the nitrogen smart series. Another has to do with land cover, things such as cover crops or perennials and so forth. The the the other area, though, is what we call edge of field practices.
Brad Carlson:And and so these are things that are installed that's I guess, one way of putting it is is largely their engineering practices. They would be installed or or retrofitted into drainage systems. So I guess one way of thinking about it is if the the nitrate's already gotten into the water. It's a way to sort of treat the water, if you will, for the sake of trying to reduce the amount of nitrate when it's eventually discharged into the surface water. And so there's there's a large number of these things that have been proposed.
Brad Carlson:They kind of they kind of run all the way from you know, we're going to spend our time today talking about bioreactors, but there's there's, practices such as controlled drainage, which is manipulating the water table in the field and reducing the amount of water running out of the drain tile. There's saturated buffers, which is running the water through a a pipe that's, kinda running it back out under the into a a buffer strip so that plants can take the nitrate or nitrogen up out of the water. There's constructed wetlands. There's manipulations that can be done to drainage ditches and so forth. A lot of these practices, they tend to be relatively expensive.
Brad Carlson:And, therefore, in order to make them kinda pencil out and work, you have to have relatively large drainage systems because if they're small drainage systems, then you gotta replicate them all over the place.
Brad Carlson:There is such a thing as them also being too large. I think if you end up with too much water, it can be difficult to treat that all too. So it really kinda requires the right field in a lot of cases. And, of course, it just involves some design work.
Brad Carlson:In a lot of cases, there is cost share available for those. Really glad to have worked on this section with Jeff Strock. Jeff is our our, one of our researchers at one of our research stations. Jeff's at the Southwest Research Station in, Lamberton, and Jeff has done a lot of work with with a number of these different practices. So, Jeff, why don't you, explain to us just what a bioreactor is and how it functions as far as getting the nitrate out of the water?
Jeff Strock:Generally speaking, a bioreactor is a structure essentially in the field. It's an engineering kind of practice, if you will, that uses some source of carbon to help reduce the nitrate that's in the drainage water, hopefully to harmless nitrogen dioxide gas, which will go up into the atmosphere and and basically be harmless. So about, you know, 80% of our our atmosphere is into gas that is up there. So the reason that we call it a bioreactor is that there's a lot of biology involved. And the idea here is is that in most of the first generation bioreactors that that were built over, say, the last twenty years, the the source of carbon and the material that was put into these bioreactors was, you know, like shaved up wood chips.
Jeff Strock:And the wood chips act as a really nice carbon source. They can tend to last quite a while, you know, five to ten years depending on the system and the types of wood chips that you use. The small organisms that live in the water, they colonize on the the wood chips themselves. So they create these little biological communities on the wood chips. They use the wood chips as a source of carbon.
Jeff Strock:And as they grow and their communities get larger, as water that has nitrate flushing through the bioreactor actually flows through it. The little organisms get all excited. They do their job. They start converting nitrate to nitrogen gas, and they essentially go through denitrifying the drainage water that runs through there. Generally speaking, these these things need to be sized.
Jeff Strock:You guys mentioned, you know, there's some some potential challenges with sizing of these different kinds of practices. Bioreactors, you know, can be sized to to work for really fields that are 50, say, to a 100 acres. Of course, larger bioreactors require larger areas to to treat the water. And generally speaking, at least here in Minnesota, the way that we've designed bioreactors is really in a in a linear fashion. So these things really don't take up space in an agricultural field, so we're not taking the land out of production.
Jeff Strock:But we're actually putting these along a field edge in a grassed waterway area or a a grass area along a ditch. And then digging a trench, filling that trench up with wood chips, and then we've got some control structures that are used to manage the water flow through there. So when you talked about cost share, Brad, you know, installation materials and certainly the structures are things that NRCS cost shares on those bioreactors.
Brad Carlson:You know, it's it's it's an interesting concept. We're we're denitrifying the the nitrates in the water. It's actually the exact same thing that's happening when we are losing nitrogen in a farm field where we don't want it to happen, because the soils are saturated. We're actually harnessing the natural process. Basically, once the nitrates in the in the drain tile water, it's gone for the crop.
Brad Carlson:So now, you know, now denitrification can be a good thing because now we're getting the nitrogen out of the water prior to it getting discharged into the surface water somewhere. The thing I like to to compare a bioreactor to, and from a functioning standpoint, they're very different. As far as an appearance standpoint, they're very similar for a lot of farmers, is it's similar to like a a septic system where you're running the water through a drain field. A lot of farmers, you know, mow pretty much every farmer's got one of them on their farm site. They can, you know, visualize the waters running through the tile.
Brad Carlson:It goes into the drain field. Now in those cases, we're trying to get all phosphorus and bacteria out of the water. Generally, nitrogen escapes that water. You know, in this case, we're running the water into something else. But, again, we're trying to treat the water by letting it percolate through a material before it eventually is just free to go into the environment.
Brad Carlson:And and so that's that's one way, I guess, of kinda thinking about these things. Jeff, there's there's been a a few of them. There's not a lot of them, but there are there are some that have that have been built around Minnesota for the last decade or so, really. You know, as far as the ones that you've you've observed, what's been farmers' feelings as far as do they like them? I guess, let's just talk about overall functionality.
Brad Carlson:I mean, is this is this a practice that we think is going to catch on?
Jeff Strock:Great analogy to the, to the the septic system drain field. You know, there there have been limited, adoption of bioreactors here in Minnesota. There's been some, the early ones over in Rice County, And, you know, you mentioned, the drain fields and phosphorus. We we know that as as some of our our bioreactor work has advanced, that we have been able to to observe that bioreactors not only are pretty darn effective in in reducing nitrogen as the the water that's diverted and goes through the bioreactor reduces the the nitrate, but we've also seen some some some pesticide reductions because of the bioreactors as well as some some phosphorus reductions. You know, when we we think about the adoption and and, you know, farmers' likes or dislikes about about bioreactors, We've had, as I said, the early on ones in Rice County.
Jeff Strock:We've had some in Yellow Medicine County. We've had some down in Cottonwood County. And the the the sites that I would say have been installed in Minnesota actually, there's also one down near Albert Lea that's a has a rather large one. But the the the sites that I'm aware of, two of the sites were really focused on being a decent on farm demonstrations to to try to help farmers understand how these work. The other two, the one in Albert Lea and the one in Cottonwood County, those were actually installed with very specific purposes and intent.
Jeff Strock:And the idea there is is that there was ag lands that were draining into some lakes. The the idea was is that there were some tiles that were directly discharging in into a couple of these lakes, rather small lakes. And and so, you know, the the city in the one case or the farmer in the other case had decided to take some action. So because of the fact that they're relatively maintenance free, you know, farmers tend to like them. There there's not a lot of management to have to do to these.
Jeff Strock:Once you put them out there, you just set them and let them run. And and so so from that perspective and, again, as I said earlier too, because they're not taking land out of production, farmers tend to look at these, rather favorably. I I wish I could tell you, you know, why we haven't had, you know, more adoption of of bioreactors with a high level of certainty. One of the challenges that we face with putting bioreactors out there, and it's, you know, we live in America, and and we're a society that, you know, is driven on economics. And so in the very early bioreactors, interestingly enough, the the wood chips that we were using were basically looked at as waste, and so we were getting them for free.
Jeff Strock:And then all of a sudden when the industry started realizing that somebody was willing to pay for wood chips, the price started going up, and it got up, and it went up, and it went up. And so I think sometimes, we see that there might be some some disadvantages to to thinking about, you know, using wood chips just because of cost. We've also been doing some work where, we've looked at the idea of, you know, what's the sustainability of gathering enough wood chips to put a lot of bioreactors in, say, for example, in Minnesota and Iowa. And there can be some challenges. And and in fact, actually, you know, the modeling that I've seen shows that there's could be some some shortages of of wood chips because of the supply and demand issues.
Jeff Strock:So we've also actually been looking at other more sustainable products. Again, it's not necessarily going to be an easy fix, but we've also been looking at corn cobs. And and corn cobs, because we, you know, generate those on an annual basis and they're fairly plentiful, You know, if there was a way that we could harvest those, we have found that, corn cobs can also be used as a source of carbon. So farmers, they're looking at these kinds of things, start to, you know, have positive opinions about things that they can use. And if they can harvest their own corn cobs off their own land and then recharge their bioreactors if that was needed, and they don't have to have some of that extra cost.
Jeff Strock:Types of those types of things can be looked on pretty favorably, Brad.
Brad Carlson:Well, I know the farmers that I talked to, I think the primary concern a lot of them have is, obviously, there's been a huge investment in field tile across Southern Minnesota. You know, the first thing the farmer asks is, is my tail going to still function properly if I put one of these structures in? Bioreactors, with a number of the, other, conservation drainage practices, edge of field practices, have a bypass engineered into them so that if the structure itself is is backing up the water to the point where it's starting to back up into the field and impede the flow of water through the drain tile, it simply runs through the bypass and then discharges directly into the water that way and functions just like as if it wasn't there. A lot of the research has kinda indicated that the performance of bioreactors is probably mostly related to the percentage of water that actually flows through the bioreactor versus that gains up through the through the bypasses. Is that correct?
Jeff Strock:Absolutely, Brad. And, you know, I knew we were gonna get to that. That was a that's a great segue. So what's really, really great about bioreactors is is that there is this sort of protection built into the system. Right?
Jeff Strock:And the protection, I mean, as you just sort of alluded to is is protecting the farm field so that when we do have wet conditions that the bioreactor is not inhibiting flow from the the field tile and so that we can maintain that that drainage, that we need, you know, in our ag fields, especially, you know, early during the growing season so we don't get saturated conditions. The numbers tell us, Brad, that the water that actually passes through the bioreactor can be, you know, upwards of over 90% reductions in nitrate. So so the numbers, the percent reductions can be really quite high. On the other hand, that number can fluctuate, especially, you know, early on in the life of the bioreactor and later on in the life of the bioreactor. We can lose some of that efficiency partly because of the the types of carbon that are available to the microorganisms or the fact that we're populating the the bioreactor with these organisms.
Jeff Strock:So it may not be as efficient early on. But generally speaking, they're very, very efficient and and, like I said, upwards of 90%. The other things that we we are aware of is that under conditions of of high flow conditions. So we're talking, you know, generally, May and June when we tend to get higher flows from our tiles, from year to year. During some of those larger events, we've seen that upwards of 80% or more of the water that comes from the field actually goes through the bypass.
Jeff Strock:So we're treating less than 20% of the water during those high flow periods just because of the fact that, you know, the the the capacity of the bioreactor is exceeded, and we have to shuttle that water out that bypass. So it's a good system. But, again, as we talked a little bit or I mentioned early on in in the in the podcast here that we aren't going to be able to solve our nitrate problems, you know, here in Minnesota or beyond our borders just by putting a bunch of bioreactors out there. They're a tool, and they'll work for some people in some places. But, you know, there are issues, like I said, with this bypass flow during high flow events.
Brad Carlson:Well, and it's worth noting, you know, as our overarching topic is reducing nitrates to water, the goal that has been established nationally is a 45 reduction in in what we would consider our baseline loss of nitrate if you went back, you know, like, to the to the eighties and the early nineties. And so frequently, when we start talking about the percent reduction of nitrate in water, you know, I always keep that 45% number in mind that we've got our state nutrient reduction strategy that's trying to achieve a 45% reduction in the amount of nitrate in water. You know, when you're talking about a 90% reduction of, what comes in versus what goes out, now that's pretty significant. And so, you know, I think, really, with a lot of these practices and bioreactors is is really is no no exception. You know, if we can get these things kind of figured out to, to hit that 45% overall, you know, that's really, that's kinda getting us where we wanna go at least at this point in time.
Brad Carlson:And so I I do think that, if you've got fields that are well suited to these, if if, you can treat enough acres, you know, the solutions are probably going to be varied regionally, and they're going to potentially even be varied across individual farms. But, yeah, I I think this is a very effective, practice that to have in a toolbox.
Jeff Strock:Absolutely, Brad. You know? And and you kind of alluded to, and and I've mentioned it too already that, you know, these are these are gonna be tools in the toolbox. There there are other, you know, edge of field and infield practices or even beyond the field practices that, you know, farmers may have the the opportunity to to be able to install in their in their landscapes.
Brad Carlson:So, Jeff, let's talk a little bit about where the science is headed. I know you've had a chance to do some experimenting on what you call the second generation bioreactors. So if the first generation was just simply, running through the water through some a bed of wood chips, to let it denitrify, and then running out into the stream, tell us what a second generation bioreactor is.
Jeff Strock:Yeah. That's a that's a great question. We we just, we just published an article on this, and the idea with the second generation, bioreactors was really to kind of think about incorporating a couple of different things. We we we know that not only nitrogen, but phosphorus can be an issue in some of our drainage water. And even though the phosphorus levels are pretty low, it doesn't take a lot of phosphorus to to create some water quality problems.
Jeff Strock:So we went in, and we started trying to look at ways to manipulate the bioreactor. And so, basically, we took these these essentially four foot by four foot cubes, if you will, and we basically we built them to layer to essentially make it look like a like a cake. So we put different layers of materials in these bioreactors, and we had nine of them that were all set up side by side. And so our our concept here was just to try to look at both targeting nitrogen and phosphorus, but also to be able to think about the opportunity to be able to go in and create a bioreactor that might be a little bit easier to do maintenance on where we could, you know, essentially have a cube. And if if it's life expectancy for nitrogen and phosphorus removal was, say, two years, that we'd be able to go in and easily replace these cubes, say, for example, on a farm or or, you know, at a at a tile where there was a lake or something like that and try to look at these in terms of sort of adjusting our our ability to to treat nutrients along with some of the maintenance issues that we discovered that we might have to deal with with traditional first generation bioreactors.
Jeff Strock:So in the second generation, we did combine corn cobs, wood chips along with some other materials. The other materials were we selected three other materials. One was crushed concrete. One was looking at steel slag, and then another one was crushed limestone. So those three materials were really ideally selected to help accelerate the phosphorus removal from these bioreactors.
Jeff Strock:So, again, it was the the these these three materials, the crushed concrete, the limestone, and the steel slide were essentially the frosting on the cake. They were on the top of the bioreactor. So we ran drainage water through these, and we found that we were able to, over time, get 50% or more reduction in nitrate, 50% more reduction in phosphorus, and it was it was absolutely just, phenomenal, the the nutrient reductions that we got. We did find that after about three years, we were gonna have to replace the the carbon materials, but we found from from our research that the phosphorus sorbing materials actually would have to be removed on an annual basis. So we would have to figure out a way to be able to go in and and remove, you know, the the limestone, the steel slag, the concrete, and then figure out how to desorb the phosphorus from that and then hopefully recover the phosphorus and then reuse it.
Jeff Strock:Of course, that wasn't part of our our research goal, but, in talking with some of the engineers, at the university that they could see that there was some potential to be able to do those things. So we've tried to make a push in in certain situations, especially in some of these lake areas where, you know, people have concerns about nitrogen and phosphorus that that you could use some technology like this. So, yeah, it it's it's been it's kind of been very, very fruitful for us to to look at this center second generation and some of the opportunities for maintenance that, don't really exist with the first generation.
Brad Carlson:So, Jeff, where do you see this all going? Is is are you gonna continue to, explore the, advancement of the second generation technology? Are we looking at kinda going back to the, first generation stuff, just simply because of its replicability across a wide area? From my standpoint, when I work a lot with with the conservation professionals, they are fairly enthusiastic about these practices. I think, one of the reasons why we haven't seen more of these, really has nothing to do with the the practice itself.
Brad Carlson:It simply has to do with conservation funding and priorities. However, I think with the rewrite of the, state's neutral reduction strategy and the, probably continued availability of funds for conservation practices, we're probably going to see a lot more push for bioreactors here over the next decade. What do you see is is coming down the line as far as any kind of refinements or or advancement, or do you think we've kinda got things where they're they're usable and scalable out in the field right now?
Jeff Strock:Yeah. So let's let's take that last one first, Brad. I I feel like the the technology for bioreactors is is definitely usable and scalable as it is right now. You know, we've been doing this, you know, not only in some small scales in Minnesota, but across the Midwest. And the group of researchers that I work with, you know, we we know a lot about these, and and we feel very, very confident.
Jeff Strock:The other piece, you know, where do we go with first generation versus second generation? I really feel like from a from a general sort of ag application that the first generation with a a couple of little twists is is gonna be, you know, probably the most efficient and cost effective way to go. And and I say that because the the ideas that I have and and others are talking about is in terms of the the carbon sources that we talked of. Right? So, you know, can we can we substitute other carbon sources in there like corn cobs, you know, rather than just wood chips for from a sustainability standpoint.
Jeff Strock:From a more, you know, like, highly focused situation, like the like, two of these lake situations that I've talked about, those probably are gonna be better suited for the second generation bioreactors just because of the fact that they have some additional capacity and additional ability to remove not only nitrogen but also phosphorus. But they're gonna require a little bit more maintenance in terms of of phosphorus absorbing materials. And I I definitely agree with you as as the, you know, the the second version of the Minnesota nutrient reduction strategy is coming out, and, you know, there's an expansion actually within the the nutrient reduction strategy and refinement in in the efficiencies of some of these practices from the first reduction strategy version that came out. We we have a lot of confidence, and I I agree with you that we are going to see, you know, necessarily so, an expansion of of implementation of things like bioreactors on the landscape. And and I I can't agree with you more.
Jeff Strock:I I really wish that, you know, if if we could rub the bottle and have the genie come out or or, you know, get a couple of our wishes granted, it would be that we would have adequate staffing, you know, in our counties, with people that are, you know, CSPs and and and so that they could and engineers actually, to help design some of these things, to help farmers get them implemented. And then also, as like you were saying, you know, to have some investment in in dollars for conservation practices like bioreactors to help offset some of the costs for structures and and, you know, the materials that go into building them. So
Brad Carlson:Well, in Minnesota, this is this is guided largely at the local level by our watershed plans. Some call it our our one water, one plan system. We have watershed plans that are developed on on HUC eight watersheds, a term that a lot of, people struggle with because what does that mean? Basically, you're talking about rivers the size of, say, the Le Sueur River or the Watanwan or or so forth. Most of these watersheds are about one county in size, but they follow watershed boundaries.
Brad Carlson:And they have plans that are written up locally by, the conservation professionals, whether that's soil and water conservation district, NRCS, county water plan, watershed districts, you know, whoever is locally is participating in these things. And so we're going to see a lot of customization as we go across the state. So when we talk about bioreactors being a tool in the toolkit, I think what's really important is that those folks in those places where these things are well suited for need to be familiar with them and then look at using them in the places where they're most appropriate. You know? And then places where they're not gonna work or they're they're not appropriate, then, obviously, they have to select other things, to meet those goals.
Brad Carlson:But I think, ultimately, as as, some of these plans get laid out, farmers should familiarize themselves, with these practices, with bioreactors because there are quite a few people out there who are going to be approached by your soil water conservation district or whoever else about the possibility of putting them in. So it's worth knowing what they what they are and how they function.
Jeff Strock:The other thing too that, you know, obviously, people need to kinda keep in mind as they're considering these things is that, you know, bioreactors obviously aren't a substitute for good nutrient management practices. Right? And so, you know, the university and and extension and people are are are out there trying to help make help farmers make, you know, good prudent decisions on nutrient management. And it's hard from year to year because, you know, the the weather is highly variable at times. And, you know, sometimes the weather doesn't cooperate and it flushes nutrients out, and it's not necessarily the farmer's fault that they lost it.
Jeff Strock:The other thing too that, you know, I know that we as a state and federally have put a massive amount of emphasis and effort into, and and that is is putting out cover crops like rye cover crops to help scavenge nutrients. Again, you know, that's that's a a really good practice. It has some additional benefits, but not unlike bioreactors where we have maybe a few limitations. You know, cover crops also have some limitations that we don't always get them, you know, established well. And and so we we we need to keep the toolbox open that it's not just gonna be a cover crop or nutrient management that are gonna help solve our our nutrient reduction strategy goals, but it's gonna be things like these edge of field practices like bioreactors that are gonna have to be implemented, as you said, where where it's appropriate.
Brad Carlson:Well, thanks, Jeff. That was a really good overview of bioreactors. As I said, they are just one of many conservation drainage practices, edge of field practices, and we'll be covering some of those other practices in future podcasts. So if this is a topic that interests you, just kinda keep watching, and, we'll hit some of those other practices, over the months to come.
Jack Wilcox:Brad Carlson and Jeff Strock, thank you both very much for being here.
Brad Carlson:Thanks, Jack.
Jeff Strock:You bet, Jack.
Jack Wilcox:Do you have a question about something on your farm? Just send us an email here at nutmgmt@umn.edu. Thanks a lot for listening, and we look forward to seeing you next time.
Jack Wilcox:Advancing Nitrogen Smart is proud to be supported by the farm families of Minnesota and their corn check off investment through Minnesota Corn.
