How can we clean up pollution from toxic chemicals that have seeped into the groundwater, hundreds of feet below the surface? Lewis Semprini, Distinguished Professor of environmental engineering, discusses strategies for bioremediation, using microorganisms to break down dangerous chemicals into harmless end-products.
Lewis Semprini, Distinguished Professor of environmental engineering, works to develop systems using microorganisms to break down toxic waste in groundwater sources.
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HAUTALA: Let’s talk about water. It’s essential for every living thing on the planet. About 60 percent of your own body weight is made up of water. You couldn’t live more than a few days without it. And yet, more than one-tenth of the Earth’s population lacks reliable access to a source of clean drinking water. Around the globe, water quality is being compromised, as communities struggle with population growth, climate change, industrial and agricultural pollution, and other pressures.
So … What are we doing about it?
NARRATOR: From the College of Engineering at Oregon State University, this is Engineering Out Loud.
HAUTALA: Welcome to a new season of “Engineering Out Loud.” For the next six episodes, we’ll be looking at some of the issues surrounding clean water – and at some of the technologies being developed right here to help ensure a safe supply of this precious resource, from Cambodia to Corvallis.
Our local listeners probably remember a water contamination scare that happened recently in Detroit Lake, near Salem, Oregon. Microorganisms called cyanobacteria caused a spike in levels of naturally occurring substances called cyanotoxins in the drinking water. This contaminated water can make people sick if they drink enough of it. I live in Salem, and I’m happy to say I survived the great cyanotoxin scare of 2018. For my wife and me, and our two dogs, it wasn’t really that much of a disruption in our routine. We just drank bottled water for a couple of weeks to be on the safe side. The dogs too. But now, every time I head to the tap, I pause for a moment and think about just how much we take clean water for granted.
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This past spring, thanks to a generous gift from Jon and Stephanie DeVaan, Oregon State University launched the Clean and Sustainable Water Technology Initiative. Heading up that effort is our very own Professor Lewis Semprini.
SEMPRINI: I’m Lew Semprini, I’m a distinguished professor of environmental engineering.
HAUTALA: I talked to Professor Semprini this past summer about the initiative and the work that the DeVaans’ gift has enabled.
SEMPRINI: It’s just getting started, but part of that is allowing us to develop some fellowships for recruiting graduate students to OSU. This summer, we’re starting the first undergraduate research program. We’re going to have students, undergraduates, coming, working with faculty members, in civil engineering and in environmental engineering, on various research projects that are ongoing. And then we're starting to develop a workshop in the summer to start talking about the issues of clean water technology, also a venue for students to present their research work.
HAUTALA: One goal of the Clean Water Initiative is to help Oregon State become a national leader in designing, building, and testing systems-level approaches to the problem of access to clean water. Lew Semprini has been at work on this problem for a long time.
SEMPRINI: Right. So, I started, I was at Stanford as a research associate before I came to OSU. And so, around 1986, I got started looking into in situ bioremediation, part of that position I had at Stanford.
HAUTALA: Bioremediation. That’s a broad term for any type of technology that uses living microorganisms to clean up pollution in the environment. Lew works with specialized strains of bacteria that have the superhero-like ability to break down some particularly nasty industrial pollutants known as chlorinated solvents.
SEMPRINI: So, these chlorinated solvents, one that was used quite a bit was trichloroethylene, and they are degreasing compounds. So, they were very good and used in manufacturing, cleaning. The compound perchloroethylene was used as a dry-cleaning fluid, so they’re very good degreasers.
HAUTALA: Ironically, chlorinated solvents came into use largely because of safety concerns.
SEMPRINI: Their use came about in coming up with compounds that were also not flammable. So, before that, we used a lot of chemical degreasers that were flammable in nature. So, these compounds had safety factors related to them not being flammable, and actually having excellent properties for degreasing.
HAUTALA: These chemicals did such a great job at what they were supposed to do that they were used everywhere, at thousands of sites throughout the country.
SEMPRINI: There are many sites. And, again, it’s not just a problem in the United States; it’s a problem wherever industrially these chemicals were used.
HAUTALA: The problem comes when these chlorinated solvents start to seep into groundwater after they were disposed of improperly, dumped into landfills, or stored in leaky underground tanks. If you’re not a geologist, you might remember this from an earth science class -- maybe it’s been a little while -- but there is a hundred times more water in the ground than is in all the world's rivers and lakes. It’s everywhere, filling in all the little cracks in the soil and rocks beneath the surface, and flowing through porous rock formations such as sandstone. If you went out into your backyard right now and started digging -- I wouldn’t recommend doing this without a permit -- but if you kept digging down, say 100 to 500 feet, eventually, you’d hit water.
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SEMPRINI: The other property of these chemicals is they’re denser than water, so they want to sink. So, when they leaked out of the tanks they kind of went down and migrated down through the subsurfaces, and they could go long distances until they hit something that confined their transport.
HAUTALA: When he says something that confined their transport, he means something solid like a layer of rock.
SEMPRINI: So, you could get large amounts of these solvents strung out in the subsurface, and then slowly dissolving to create these groundwater plumes that could be large in extent and had been generated over long periods of time.
HAUTALA: These plumes -- where the chlorinated solvents dissolve and spread out in the groundwater -- can cover areas measured in square miles, and they can stretch down hundreds of feet. Now, groundwater supplies about half of the drinking water in the United States. In rural areas it’s closer to 99 percent. The thing about chlorinated solvents is, while they may be great degreasers and dry-cleaning chemicals, you really don’t want to drink a big glass full of them.
SEMPRINI: Right. So, one of the big problems is, many of these compounds, they’re either known or suspected carcinogens. The EPA has put a maximum concentration levels in drinking water; like, the standard for trichloroethylene is 5 micrograms per liter.
HAUTALA: That’s 5 parts per billion, with a B. That’s like a couple of teaspoons’ worth in an Olympic size swimming pool.
SEMPRINI: But the solubility of TCE in water is 1100 milligrams per liter, so the drinking water standard is many orders of magnitude lower than the maximum amount that TCE can dissolve in water. We have very low concentrations that are needed to meet a drinking water standard but potential for plumes being much higher concentrations in the subsurface.
HAUTALA: What that means is that contaminated groundwater could potentially contain about 220,000 times the level of trichlorethylene that the EPA allows in drinking water. Are you feeling queasy yet? Grab yourself a glass of water. It gets worse.
SEMPRINI: And many of these compounds also can be transformed in the subsurface. So sometimes you can create a compound that is more toxic or has a lower drinking water standard than the compound you started out with originally. So, transformations occur – one known with trichloroethylene is the production of vinyl chloride, which is part of its transformation process. And vinyl chloride is a known carcinogen has even a lower drinking water standard than trichloroethylene.
HAUTALA: Meaning you want even LESS of it coming out of your tap. The traditional strategy for cleaning up this mess has involved pumping contaminated water out of the ground, treating it to remove the chlorinated solvents, and then sending it back underground or off to a wastewater facility for further treatment. All of that pumping uses a lot of energy, and so it’s very expensive. The strategy Lew Semprini is working on, bioremediation, has one huge advantage in that it enables treatment of the groundwater in situ, which is just a fancy Latin term meaning “on site.”
SEMPRINI: So we try and basically stimulate microorganisms in the subsurface to enhance the remediation process and try and make reactors in the subsurface to actually intercept these plumes and treat them in situ. And in this case, in enhanced or engineered bioremediation, you have to figure out how to stimulate that process in situ -- you know how to go about either taking advantage of the native microorganisms that exist there or using a process called bioaugmentation, to actually add specific microbial cultures to the subsurface, in order to enhance the remediation.
HAUTALA: Some of these specific microbial cultures come from exotic, faraway places with names like … Corvallis, Oregon.
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SEMPRINI: One of the cultures we actually work with in the lab we got from, it was called the Evanite site, in Corvallis, where they used trichloroethylene. And years ago, a graduate student went down in the sediments there along the banks along the Willamette River we got some material and brought in into the lab, and we started to culture and isolate these dehalogenating bacteria from the subsurface there. We call that the Evanite culture. But many of these cultures have a common microorganism in them -- it's been called Dehalococcoides mccartyi.
HAUTALA: In environmental engineering circles, that’s how you know when you’ve become a really big deal: They name a species of bacteria after you.
SEMPRINI: It actually was named after a professor I worked with at Stanford, Perry McCarty -- the microorganism was eventually named after him. But there’s different strains of this microorganism that do different steps in this transformation process. They look a lot alike genetically, but there’s little different variations that they do various different steps in the process.
HAUTALA: The process he’s talking about is called dehalogenation, which involves breaking the chemical bonds between atoms of carbon and atoms of another type of element, called a halogen. In the case of chlorinated solvents, the halogen is chlorine. Breaking those carbon-chlorine bonds is essential in converting these solvents into less harmful molecules.
SEMPRINI: The ultimate goal is to transform them to nontoxic products. So, something like trichloroethylene, you could see if you could transform it to carbon dioxide and chloride ion, which are benign, then you've completely broken down that compound. And in other mechanisms, we’ve transformed them to compounds like ethene, which are also non-toxic. So there's different routes to try and transform these chlorinated solvents.
HAUTALA: One of these routes, of particular interest to Lew Semprini, is called organo-halorespiration. I’ll let Lew go into that a little bit.
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SEMPRINI: This is where we take advantage of anaerobic conditions in the subsurface. So, lack of oxygen. There are some strict anaerobes. This process was discovered when we first started working at contaminated sites; we started to get some observations that trichloroethylene was the contaminant that was disposed of, but then these other contaminants start to be observed. Things like cis-dichloroethylene, vinyl chloride, even ethene. And so observations started to come in that, you know, transformations might be occurring naturally in the subsurface.
HAUTALA: So, as it turns out, there was a whole series of transformations occurring, sort of like a microscopic biochemical assembly line. Or, more accurately, a disassembly line.
SEMPRINI: And then in the lab, people began to investigate to find out that this reductive dehalogenation process was going on -- you could actually track the parent compound being degraded to daughter compounds in a successive series of transformation processes occurring. And originally we thought it was just fortuitous that other microorganisms that were anaerobes were kind of doing this. And then with time, we found out there was a specific groups of microorganisms that could actually grow on chlorinated solvents, and it became known as halorespiration. And they were actually breathing chlorinated solvents like we breathe oxygen.
HAUTALA: You heard that right. The microorganisms that grow on these chlorinated solvents and break them down technically aren’t eating them; they’re breathing them. (Their “food,” such as it is, is actually hydrogen. I know that probably sounds strange, but we’ll get back to that in a minute.) The important thing to understand here is that the microorganisms use the solvents exactly the same way the cells in our bodies use oxygen to get energy out of the food we eat.
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SEMPRINI: So, trichloroethylene, they could respire that and make the daughter products. And what would happen here is they would take a chlorine ion off the molecule -- trichloroethylene is a two carbons, a double bond, and three chlorides on it, and a hydrogen -- they would take one chloride off and put a hydrogen on, and make cis-dichloroethylene, and they would be able to get energy out of that process. And it’s called a reduction because TCE is being reduced to a lower oxidation-state compound when you do that. And it keeps on going, and you could keep taking chlorides off one at a time until you get to ethylene, which has no chlorides on it, and four hydrogens. And this is a nontoxic end product.
HAUTALA: If you’re not quite able to follow all of that chemistry, the important thing to understand here is that these microorganisms can break down the chlorinated solvents into smaller and smaller molecules, until there’s nothing left but harmless, non-toxic end products. Pretty cool trick, huh? And nobody’s quite certain how they learned how to do it. At least not yet.
SEMPRINI: So, it’s been interesting in that when we look for them, we find them in a lot of places, and that's been a big question -- how did they evolve? These chlorinated solvents have not been around forever, but we have these microorganisms that could do this halorespiration and grow and get energy out of these compounds.
So, in my work, a part was the kinetics. How does this work, when you get these cultures and you're able to enrich them in the lab, looking at the various steps in the process. And by kinetics, we mean rates, the rates at which these compounds would get transformed. So I’ve done a lot or work of actually studying the kinetics of this process and figuring out when you have these multiple steps going in, how do we kinetically describe these rates of reaction?
HAUTALA: One key factor in this process is the addition of a substrate, a food source for the microorganisms to munch on while they’re busy dehalogenating solvents. As I mentioned before, nothing hits the spot quite like good old yummy hydrogen.
SEMPRINI: To generate the hydrogen that’s needed to drive this reaction, often fermenting substrates are added. And by a fermenting substrate, it’d be something like lactate, some food-grade compound, that in the ground, it ferments. And when it ferments, one of the products that’s formed is hydrogen. Okay. What happens there, though, if you add too much lactate, you could start getting into methanogenic conditions, where methane, okay, is formed in the subsurface. So, some of the energy you put in through lactate is diverted to making methane. Making lots of methane in the subsurface may not be a good thing.
HAUTALA: Spoiler alert: It’s not.
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SEMPRINI: So, there’s side reactions that go on when you carry out this process. So, some of our activities have been in the lab -- well, how do create the conditions, say, that you don’t make methane? How do you come up with very efficient systems that you take advantage and try and get the reactions that you want to happen, happen?
HAUTALA: This particular type of anaerobic bioremediation process works best when concentrations of contaminants are high. As the concentrations get lower, other types of bioremediation start to look more feasible.
SEMPRINI: When we get into low concentrations, you often wind up to do things anaerobically, you’re adding a lot of substrates, but you’re only transforming a small amount of the contaminants when you do that. So, there, oftentimes you get in that, that type of bioremediation may not make sense. So then, the other processes I’ve been studying are processes -- cometabolism. And cometabolism is an aerobic process -- we use aerobic microorganisms. And there, we try and develop the enzymes to do the transformation, by feeding a food substrate that stimulates a particular enzyme that promotes the transformation.
HAUTALA: In this approach, called aerobic cometabolism, the microorganisms aren’t really eating or breathing the solvents. In fact, they don’t really need the solvents at all. But when you feed these microorganisms the right kinds of stuff, they make these enzymes that do all the hard work of dehalogenation, really just as a kind of side job.
SEMPRINI: And these often are enzymes called monooxygenases, or dioxygenases, that are very good at oxidizing compounds. So they actually initiate the oxidation of compounds that are hard to oxidize. One micro -- groups I work with, methanotrophs, oxidize methane, and there’s an enzyme called methane monooxygenase, that initiates the oxidation of methane. But that methane monooxygenase also fortuitously oxidizes TCE, and other compounds. So in aerobic cometabolism, we try and bring up the enzymes by adding a food source that stimulates a certain enzyme system, and then you get fortuitous degradation of these compounds.
HAUTALA: “Come for the methane, stay for the TCE.” It has a nice ring to it, no?
SEMPRINI: The beauty of it is you could do very low concentrations. So, it actually, because you’re driving this with an enzyme in a food source, you could drive concentrations much below these MCL --
HAUTALA: MCL -- the maximum concentration levels.
SEMPRINI: -- in drinking water standards.
HAUTALA: Since this interview was recorded, the Clean Water workshop Lew mentioned at the start of the program has taken place, with Oregon State faculty members, graduate students, and water experts coming together right here on our campus to talk about how to preserve and expand access to clean water, and the technologies that will make that possible. For more information about the workshop and the Clean Water initiative, please follow the links on our website.
This episode was produced by me, Keith Hautala, with additional audio help from Molly Aton. Our intro music is “The Ether Bunny” by Eyes Closed Audio on Soundcloud, used with permission under a Creative Commons Attribution license. Other music and sound effects were also used with appropriate licenses. You can find the details in our show notes, which are online, along with other episodes, at engineeringoutloud.oregonstate.edu. Subscribe on our website, or by searching for “Engineering Out Loud” on your favorite podcast app.
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