A circular economy for salt that keeps rivers clean

During the winter of 2018-2019, one million tons of salt were applied to icy roads in the state of Pennsylvania alone. The salt from industrial uses like this often ends up in freshwater rivers, making their water undrinkable and contributing to a growing global crisis. How can we better protect these precious natural resources? Physical organic chemist Tina Arrowood shares a three-step plan to keep salt out of rivers -- and create a circular salt economy that turns industrial byproducts into valuable resources.

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)