Wetlands Will Save Us

I mean, not from Corona Virus, but there are other things that will kill us.

So, for this week’s installment of nerd porn, let us discuss the use and hypothetical development of constructed wetlands!

First, some basics. There are a whole bunch of types of material/processes that can count as a wetland. If you talk to the Army Corps of Engineers, they have a very clear definition of what counts (404 classified), but the rest of us have a more…shall we say…flexible view of the whole thing. For practical purposes, constructed wetlands may be defined loosely as a collection of holistic ecosystems placed with an engineered liner for the purposes of some measure of treatment or water control. These may be further defined into horizontal, vertical, and floating wetland systems.

Horizontal wetlands are the most intuitive. Water flows from a single influent source or diffused influent source and moves linearly through the first two layers of the wetland, with only moderate infiltration down into the deep root structure and subgrade. It’s a ‘flow through’ wetland and tends to be pretty shallow. Over time, the critters in this wetland become specially selected for nutrient uptake at different levels, depending on the distance to/from the influent sources. So, for example, critters that like a certain nutrient in high loads will self-select as a community to co-locate near the influent source. Those plants and animals that need less of the nutrient will move further away. Simple, right?

Vertical wetlands can get a little weird. These use a treatment mechanism that is heavily dependent on geochemistry, making liner selection and non-biotic elements part of the active treatment train. Water can percolate down into deep rooting structures and a much more complex set of ecosystems can develop at deep water levels, shallow water levels, and even subsurface. These wetlands typically need some kind of terracing to allow for multiple influent sources and different biosystems to colonize different depths of water, exposure to sunlight, aerobic v. anaerobic processes, etc.

Floating wetlands are cool. You plant goodies that root in either the subgrade or can just float around on the surface of a pond and do their thing. These tend to be deep water ponds/lakes and tend to have an alkaline pH. There are some pretty neat ideas getting tested out on these bad boys. For example, there’s a Russian scientist using a series of cascading pools with sunflowers planted in Styrofoam, mobile boxes on the surface to treat for alpha, beta, and gamma radiation (using cesium atoms) for nuclear reactor thermal control wastewater treatment. It’s great. Just let the flowers chill on the pond surface, they absorb the radiation. More flowers at the top near the most contaminated pools/water source, fewer flowers in the middle, and fewest flowers at the bottom pool with the addition of lilies and watercress to mitigate pH and trace metals before final discharge. Average residence time from contaminated-nasty-ass-mutagenic-water to pretty-decent-meets-regs-send-it-out-to-the-world water was around 72 hrs. Not too shabby for a few thousand gallons of water a day, right? And you can harvest the sunflowers and sunflower seeds for biofuel, so no nasty radioactive solid waste to deal with.

But that’s just one very cool application. Speaking of harvesting fuel, let’s go back to the idea of a vertical wetland.

One of the major problems with bio-based clean-up methods is the fact that they take up a butt-load of space and you have to deal with landscaping waste. Often, this waste material has nasty bits in it, since the whole purpose of the treatment wetlands is to remove the nasty bits from the water. Well, it has to go somewhere. In the case of most metals, it goes up the roots and gets sequestered in macrophytes’ stalks and leaves. And when those shed or die, you have to do something with them or risk jacking up your overall toxicity. It’s a problem.

Now, there are several saprophytic fungi that genuinely like eating metals and can form symbiotic relationships with macrophytes. These saprophytic fungi have a special enzyme called P-450 monoxygenase that actually can do something called chelation. Why do we care? Ah, my duckies, chelation is a beautiful biological slight-of-hand where toxic metals can be combined with earth metals or nice, stable, non-toxic molecules to form delightful, bio-friendly building blocks that have no toxicity whatsoever. So, back to vertical wetlands and floating wetlands.

Let’s say we have some industrial waste that needs to be managed at, I don’t know, Fukushima. Bad news bears, right? Radioactive, PAH, heavy metal contamination, all kinds of nasty crap is in that water. Enter the vertical wetland.

Now, floating wetlands are a little limited. They can’t treat for quite as many goodies as the vertical, but we are going to capitalize on some of the floating wetland characteristics because they can treat way more water than a straight vertical wetland and Fukushima has a shit-ton of water to clean up and not dump in the ocean. Don’t do that, duckies. Don’t ever do that. We love our oceans.

Anyway, let’s say we need to manage around 3 million gallons of water a day. And let’s say that water has some yummy diesel, gasoline, metals, radioactivity, salts, and some stuff we don’t know about, but pretty sure are organic contaminants. Cool. Surface area is the critical parameter for design selection in wetlands. And we are going to need to bust out our handy plug-flow first order kinetics reaction (which I can’t insert here because evidently my web service doesn’t have an equation editor. Weird, right?).

First, we look at total process volume and how much contaminated nastiness we are going to have to manage. Next, we are going to run an optimization equation which is from our handy, dandy derivative equations to figure out the rate of reaction that will be required to treat contaminants at each stage. How many stages will we need? Let’s solve for surface area.

Alrighty. Now that we know our rate of reaction, our estimated surface area needed, total volume, let’s figure out where we have space to build this bad boy. We may have to get creative and fold space, using a terracing system and run a model to determine flow rates and reaction coefficients over each one of the terraces. Now, because it’s biology, a lot of these numbers are straight bullshit. One of the ways we can mitigate a little of the bullshit is run a field pilot with the critters of interest. So let’s talk about that. Remember I said that some saprophytic fungi can eat metals? We’re going to need to adjust for biological uptake from fungi and phyto to make sure our kinetics are accurate. How do we do that?

Well, someone wicked smart found out that organics (carbon-based at least), tend to intake nutrients at consistent rates that can be described in a couple ways. If it’s a living thing, the logKow is our typical parameter of interest. The higher the logKow, the more crap the critters can hoover up. This is the logarithmic rate (K) of the octanol-water partition. This means, that in a lab, the critter ate so much octanol v. water over time and what that rate looked like. Octanol is a complex carbohydrate we can use as a stand-in for other complex carbs like diesel, gasoline, TCE, and other organics. When it comes to metals and radiation, these numbers break down a little, for obvious reasons, right? I mean, a metal isn’t an organic.

So, for metals and radiation, we have to go old school and measure uptake rates using the pilot test and manually spike the lab sample with our expected contaminant loadings and then test the leaves to see how much of the nasty is trapped in the plant/fungus and make a new ratio (K) value to estimate uptake.

Whew.

Okay, back to the wetland.

Now we know how much nasty water we have to manage, how much space we’ll need, what space we have and how to reconcile surface area needed with surface area available through the use of terracing or vertical, ‘living walls’ (more to follow on this later). We know how much removal our critters can do, so we can start to model our treatment efficacy. If all those numbers add up, we can start the damn thing up and see what happens!

Now, what happens to all the biowaste? And what happens if our critters start making metabolic byproducts that are no good? Some creatures emit cyanide as a waste product, for example, with PAH contamination in contact with iron rich soils.

That’s ok. We can deal with that. Some vertical wetlands are, actually vertical. In tight spaces, they may be constructed so the water cascades down plastic levels, like a giant, controlled water fall. At the bottom, pumps grab the water and send it to the next waterfall for further treatment and on down the line. In other cases, the primary off-gassing happens sub-surface to the water and can be captured by things called leachate collection systems, depending on the density of the gas and the solubility in air/water (Henry’s Law tells us what it’s going to do). Cool. Anyway, this density of off-gassing could allow us to collect metabolic byproducts by gas and run it through a) a straight energy conversion to power the industrial facility or b) pass through a bioreactor to transition it to methane which could be burned as a fuel source to maintain the plant/pumps/industry.

Even better, the biowaste can be composted or recycled in those same reactors to create methane and generate power for the remediation pumps/scrubbers, making the whole process a zero-carbon, emission-free, self-sustaining system.

Go nature.