A circular economy for salt that keeps rivers clean | Tina Arrowood

Growing up in northern Wisconsin,

I've naturally developed a connection to the Mississippi River.

When I was little,

my sister and I would have contests to see who could spell

"M-i-s-s-i-s-s-i-p-p-i" the fastest.

When I was in elementary school,

I got to learn about the early explorers and their expeditions,

Marquette and Joliet, and how they used the Great Lakes and the Mississippi River

and its tributaries to discover the Midwest,

and to map a trade route to the Gulf of Mexico.

In graduate school,

I was fortunate to have the Mississippi River

outside my research laboratory window

at the University of Minnesota.

During that five-year period, I got to know the Mississippi River.

I got to know its temperamental nature

and where it would flood its banks at one moment,

and then shortly thereafter,

you would see its dry shorelines.

Today, as a physical organic chemist,

I'm committed to use my training

to help protect rivers, like the Mississippi,

from excessive salt that can come from human activity.

Because, you know,

salt is something that can contaminate freshwater rivers.

And freshwater rivers, they have only salt levels of .05 percent.

And at this level, it's safe to drink.

But the majority of the water on our planet is housed in our oceans,

and ocean water has a salinity level of more than three percent.

And if you drank that, you'd be sick very quick.

So, if we are to compare the relative volume of ocean water

to that of the river water that's on our planet,

and let's say we are able to put the ocean water

into an Olympic-size swimming pool,

then our planet's river water would fit in a one-gallon jug.

So you can see it's a precious resource.

But do we treat it like a precious resource?

Or rather, do we treat it like that old rug

that you put in your front doorway and wipe your feet off on it?

Treating rivers like that old rug has severe consequences.

Let's take a look.

Let's see what just one teaspoon of salt can do.

If we add one teaspoon of salt

to this Olympic-size swimming pool of ocean water,

the ocean water stays ocean water.

But if we add that same one teaspoon of salt

to this one-gallon jug of fresh river water,

suddenly, it becomes too salty to drink.

So the point here is,

because rivers, the volume is so small compared to the oceans,

it is especially vulnerable to human activity,

and we need to take care to protect them.

So recently, I surveyed the literature

to look at the river health around the world.

And I fully expected to see ailing river health

in regions of water scarcity and heavy industrial development.

And I saw that in northern China and in India.

But I was surprised when I read a 2018 article

where there's 232 river-sampling sites

sampled across the United States.

And of those sites,

37 percent had increasing salinity levels.

What was more surprising

is that the ones with the highest increases

were found on the east part of the United States,

and not the arid southwest.

The authors of this paper postulate

that this could be due to using salt to deice roads.

Potentially, another source of this salt

could come from salty industrial wastewaters.

So as you see, human activities can convert our freshwater rivers

into water that's more like our oceans.

So we need to act and do something before it's too late.

And I have a proposal.

We can take a three-step river-defense mechanism,

and if industrial-water users practice this defense mechanism,

we can put our rivers in a much safer position.

This involves, number one,

extracting less water from our rivers

by implementing water recycle and reuse operations.

Number two,

we need to take the salt out of these salty industrial wastewaters

and recover it and reuse it for other purposes.

And number three, we need to convert salt consumers,

who currently source our salt from mines

into salt consumers that source our salt from recycled salt sources.

This three-part defense mechanism is already in play.

This is what northern China and India are implementing

to help to rehabilitate the rivers.

But the proposal here

is to use this defense mechanism to protect our rivers,

so we don't need to rehabilitate them.

And the good news is, we have technology that can do this.

It's with membranes.

Membranes that can separate salt and water.

Membranes have been around for a number of years,

and they're based on polymeric materials that separate based on size,

or they can separate based on charge.

The membranes that are used to separate salt and water

typically separate based on charge.

And these membranes are negatively charged,

and help to repel the negatively charged chloride ions

that are in that dissolved salt.

So, as I said, these membranes have been around for a number of years,

and currently, they are purifying 25 million gallons of water every minute.

Even more than that, actually.

But they can do more.

These membranes are based under the principle of reverse osmosis.

Now osmosis is this natural process that happens in our bodies --

you know, how our cells work.

And osmosis is where you have two chambers

that separate two levels of salt concentration.

One with low salt concentration

and one with high salt concentration.

And separating the two chambers is the semipermeable membrane.

And under the natural osmosis process,

what happens is the water naturally transports across that membrane

from the area of low salt concentration

to the area of high salt concentration,

until an equilibrium is met.

Now reverse osmosis, it's the reverse of this natural process.

And in order to achieve this reversal,

what we do is we apply a pressure to the high-concentration side

and in doing so, we drive the water the opposite direction.

And so the high-concentration side becomes more salty,

more concentrated,

and the low-concentration side becomes your purified water.

So using reverse osmosis, we can take an industrial wastewater

and convert up to 95 percent of it into pure water,

leaving only five percent as this concentrated salty mixture.

Now, this five percent concentrated salty mixture

is not waste.

So scientists have also developed membranes

that are modified to allow some salts to pass through

and not others.

Using these membranes,

which are commonly referred to as nanofiltration membranes,

now this five percent concentrated salty solution

can be converted into a purified salt solution.

So, in total, using reverse osmosis and nanofiltration membranes,

we can convert industrial wastewater

into a resource of both water and salt.

And in doing so,

achieve pillars one and two of this river-defense mechanism.

Now, I've introduced this to a number of industrial-water users,

and the common response is,

"Yeah, but who is going to use my salt?"

So that's why pillar number three is so important.

We need to transform folks that are using mine salt

into consumers of recycled salt.

So who are these salt consumers?

Well, in 2018 in the United States,

I learned that 43 percent of the salt consumed in the US

was used for road salt deicing purposes.

Thirty-nine percent was used by the chemical industry.

So let's take a look at these two applications.

So, I was shocked.

In the 2018-2019 winter season,

one million tons of salt

was applied to the roads in the state of Pennsylvania.

One million tons of salt is enough

to fill two-thirds of an Empire State Building.

That's one million tons of salt mined from the earth,

applied to our roads,

and then it washes off into the environment and into our rivers.

So the proposal here is that we could at least

source that salt from a salty industrial wastewater,

and prevent that from going into our rivers,

and rather use that to apply to our roads.

So at least when the melt happens in the springtime

and you have this high-salinity runoff,

the rivers are at least in a better position

to defend themselves against that.

Now, as a chemist,

the opportunity though that I'm more psyched about

is the concept of introducing circular salt into the chemical industry.

And the chlor-alkali industry is perfect.

Chlor-alkali industry is the source of epoxies,

it's the source of urethanes and solvents

and a lot of useful products that we use in our everyday lives.

And it uses sodium chloride salt as its key feed stack.

So the idea here is,

well, first of all, let's look at linear economy.

So in a linear economy, they're sourcing that salt from a mine,

it goes through this chlor-alkali process,

made into a basic chemical,

which then can get converted into another new product,

or a more functional product.

But during that conversion process,

oftentimes salt is regenerated as the by-product,

and it ends up in the industrial wastewater.

So, the idea is that we can introduce circularity,

and we can recycle the water and salt from those industrial wastewater streams,

from the factories,

and we can send it to the front end of the chlor-alkali process.

Circular salt.

So how impactful is this?

Well, let's just take one example.

Fifty percent of the world's production of propylene oxide

is made through the chlor-alkali process.

And that's a total of about five million tons of propylene oxide

on an annual basis, made globally.

So that's five million tons of salt mined from the earth

converted through the chlor-alkali process into propylene oxide,

and then during that process,

five million tons of salt that ends up in wastewater streams.

So five million tons

is enough salt to fill three Empire State Buildings.

And that's on an annual basis.

So you can see how circular salt can provide a barrier

to our rivers from this excessive salty discharge.

So you might wonder,

"Well, gosh, these membranes have been around for a number of years,

so why aren't people implementing wastewater reuse?

Well, the bottom line is,

it costs money to implement wastewater reuse.

And second,

water in these regions is undervalued.

Until it's too late.

You know, if we don't plan for freshwater sustainability,

there are some severe consequences.

You can just ask one of the world's largest chemical manufacturers

who last year took a 280-million dollar hit

due to low river levels of the Rhine River in Germany.

You can ask the residents of Cape Town, South Africa,

who experienced a year-over-year drought drying up their water reserves,

and then being asked not to flush their toilets.

So you can see,

we have solutions here, with membranes,

where we can provide pure water,

we can provide pure salt,

using these membranes, both of these,

to help to protect our rivers for future generations.

Thank you.

(Applause)