Hello my name is David Schaad and I'm Professor of the Practice and Associate Chair in the Department
of Civil and Environmental Engineering here at Duke University. And I want to talk to you about
clean water, actually water quality, and one unit process today called reverse osmosis. So we
think about water quality and flushing toilets and drinking the water that we do we realize
that engineers save lives and make human civilization possible. It was just over
150 years ago that in the us people were dying of diarrheal illnesses.
Understanding germ theory and public health disease outbreaks was something that we hadn't
understood quite yet. So we have eradicated diarrheal illnesses and water-borne diseases
in the united states for the most part, but that isn't true globally. When we think about
the global disease burden associated with water-borne illnesses over a billion people
lack access to improved water quality supplies and 2 billion like access to improved sanitation.
So let's put that in context. How many is a billion, how many is a million? So let's talk
about a million seconds. How long is a million seconds? So if we do the math, million seconds,
it's a little under 12 days. So we could do anything for 12 days right? We could even
sleep outside for a basketball game. How long is a billion seconds? So let's do the math again...it's
over 30 years. We think about the disease burden between a million and a billion. A billion people,
that's three times the population of the United States that don't have access to clean drinking
water supplies, the numbers are staggering. And it's two billion that don't have access
to potable, to improved sanitation. So engineers bring the tools of scientific discovery to treat
these issues of clean drinking water. So when we think about drinking water we think about the way
contaminants distribute in the environment we talk about really two terms: advection and dispersion.
So advection is the principle where like when you stir a coffee cup we introduce turbulence,
we have velocity, and we have motion. Dispersion is when you drop a contaminant in and it just
disperses into the environment. So i'm going to show you a little example. So when we think about
dispersion we're going to let this blue dye disperse into this glass of water and then this
glass we're going to use advection. Same number of droplets but this time we're going to stir it up
and we can see that the makes it much easier to get the blue dye through the entire glass.
But at the end of this, once this one finally disperses, the same amount of glass will be blue.
So to understand dispersion. Let's not talk about water but let's talk about our lungs. Every
minute we take multiple breaths. We breathe in air which has 21% oxygen. That oxygen diffuses across
the membrane of our lungs into our bloodstream and then that blood is pumped to the various
parts of our body, the cellular membranes respire carbon dioxide that comes back to our lungs,
carbon dioxide goes into our lungs and then we exhale carbon dioxide.
This diffusion process is the one of the ways that we use to treat water. And so what ends up
happening is we use a process called reverse osmosis. So again, just like these glasses,
water concentration wants to move from areas of high concentration to low concentration, it wants
to equilibrate throughout the entire process. So if we have just like in our lungs we have
high concentration of oxygen, low concentration of oxygen the oxygen diffuses across the membrane of
our lungs, the same thing is true if we have salt water or other water that's got more solute on
one side than the other. So what's going to end up happening is if we have two containers equal
amounts of water but one with more salt in it, what will end up happening is, by osmosis, the
water will be drawn from the glass that has less salt over to the side that has more salt to dilute
the side that has more salt. And the water level will actually rise on this side of the glass.
That's the process of osmosis. Maybe your parents talk to you about understanding things by osmosis.
Well it actually does happen the osmotic pressure draws the water across that membrane.
And so what we do as a as a treatment process a unit process for treating water is we actually
reverse that process we'll induce a high pressure on one side of the glass which will drive the
fresh water across that membrane keeping back the reject water or the brine solution
on this side of the membrane. And then of course we can find a useful use for that but then this
side is clean water. And this is important in lots of different applications. For drinking
obviously but for high-tech manufacturing, for desalination, for lots of different future uses
that we're going to need as humanity. So water quality is really really important as we think
about again, engineers save lives and enable human civilization, water quality is going to
continue to be a bigger and bigger issue as we continue to grow in terms of population. As we
are already living in areas where water quality is scarce how are we going to address humanity's
future water needs. Thanks for watching I hope you learned something, appreciate your time.