Water Filtration – Spring 2013
Lesson from:
http://water.epa.gov/learn/kids/drinkingwater/upload/2005_03_10_kids_activity_grades_48_waterfiltration.pdf and http://sciencenetlinks.com/lessons/ecosystem-services-water-purification/
(Procedure from EPA site; introduction and interpretation from ScienceNetLinks site.)
Introduction:
Q: What is our theme for the quarter?
A: Ecology
Q: What did we make last week?
A: Our own miniature ecosystems.
Q: What elements did the ecosystems contain?
A: Dirt, plants, rocks, worms…key inputs were light and water.
Q: In your miniature ecosystem, was there any water pollution? What about in a real ecosystem?
A: The mini-ecosystems didn’t have any pollution. But real ecosystems can have pollution, including:
fertilizer, industrial chemicals, silt and other particulates, and pathogenic bacteria.
Q: How can water be purified?
A: One answer is in a water purification plant. This is where we get drinking water. But an important
other answer is that ecosystems themselves actually help purify water. This is an example of an
ecosystem service – a benefit provided to people by a healthy ecosystem.
Q: What kinds of natural ecosystems might function like these two items (glass with sinking sediment,
sponge) to purify water?
A: Glass with sinking sediment is like a pond: solid particles settle out, leaving clean water on top. The
sponge is like wetlands, forests, natural grasslands, and forests; water sinks into the ground and moves
slowly, which helps it get purified.
Q: Why does slowing water down help it get purified?
A: Because much of the purification is done by microbes (little organisms too small to see, like bacteria)
that live on surfaces underwater. Microbes can break down pesticides and herbicides, and remove toxic
heavy metals like mercury. The slower the water goes, the more time the microbes have to purify it.
Q: Suppose that a marshy field gets paved over with concrete. Is this good or bad for water purification?
A: Bad. Water cannot sink in to asphalt and other impervious surfaces, so it moves a lot faster, and many
of the microbes and other organisms that clean the water would lose their habitats.
Today, we will follow steps used in a water purification plant to clean up “polluted” water containing
lots of sediment and particles. This kind of pollution might typically come from farm fields. We will
consider how each of the steps we are doing is comparable to what happens in a natural ecosystem.
Does it take a lot of work to purify water? Can you get water to be as pure as the water you see in a
clear outdoor stream? Clean, healthy water is a valuable service provided by our ecosystems!
Materials (per group) :
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5 Liters of “polluted water” (or add 2 1/2 cups of sediment to 5 liters of water)
1 Two liter plastic soft drink bottle with its cap (or cork that fits tightly into the neck)
2 Two liter plastic soft drink bottles, one with its bottom cut off and one with the top cut off
1 large beaker (2 cups) or measuring bowl that will hold the inverted two liter bottle or you can
use another two liter plastic soft drink bottle with its top cut off so the other bottle will fit inside
of it.
2 tablespoons of alum (potassium aluminum sulfate available in the spice isle at grocery stores)
1 1/2 cups fine sand (white play sand or beach sand)
1 1/2 cups coarse sand (multi-purpose sand; wash in advance if dirty)
1 cup small pebbles (washed, natural color aquarium rocks work best)
1 coffee filter
1 rubber band
1 tablespoon (for the alum)
1 large spoon (for stirring)
A clock with a second hand or a stopwatch
Procedure:
1. Pour your polluted water into the two liter bottle with a cap. Have students describe the
appearance of the water.
Q: What kinds of pollutants can you see in the water?
A: Mostly sediment (fine dirt that can clog up natural waterways).
Q: Are all pollutants things you can see?
A: No, many of the worst pollutants are invisible (fertilizer, toxic chemicals, heavy metals, etc.)
2. Aeration adds air to water. It allows gases trapped in the water to escape and adds oxygen to
the water.
Place the cap on the bottle and vigorously shake the bottle for 30 seconds.
Continue the aeration process by pouring the water into the bottle with its top cut off, then
pouring the water back and forth between them about 10 times.
Once aerated, gases have escaped (bubbles should be gone). Pour your aerated water into
your bottle with its top cut off.
Q: How might oxygen get into water in natural ecosystems? Why is it needed there?
A: Oxygen can come from by photosynthesis of underwater plants, wind on the water
surface, or incoming streams or waterfalls. Many organisms that live in water (including
microbes that break down excess nutrients) need oxygen to live.
Note: this is a little different than why oxygenation is used in industrial water filtration,
where there are no organisms doing the water cleanup.
3. Coagulation is the process by which dirt and other suspended solid particles to chemically “stick
together” into floc (clumps of alum and sediment) so they can easily be removed from water.
Add two tablespoons of alum to the aerated water. Slowly stir the mixture for 5 minutes. You
will see particles in the water clinging together to make larger clumps.
Q: Where in ecosystems does coagulation happen?
A: A similar process happens in wetlands and riparian (streamside) forests. Pollutants
like organics, metals, and radioactive elements stick to particles of silt found in these
environments. When the silt settles, it pulls the pollutants out of the water.
4. Sedimentation is the process that occurs when gravity pulls the particles of floc to the bottom
of the cylinder.
Allow the water to stand undisturbed in the cylinder. Observe the water at 5 minute intervals
for a total of 20 minutes. Write down what you see - what is the appearance of the water at
each timepoint?
Q: Where in an ecosystem could a similar sedimentation process happen?
A: In places where water is moving slowly enough for sediment to sink. This can include
wetlands and streamside forests – 80 to 90% of fine particles sink and are removed in
these environments. It can also happen in the “lakes” upstream of beaver dams.
Fun fact: streams with beaver dams can retain up to 1000x more nitrogen, e.g., from
fertilizer runoff, than streams without dams.
5. While you are waiting for sedimentation to proceed, construct a filter from the bottle with its
bottom cut off as follows (see illustration):
a. Attach the coffee filter to the outside neck of the bottle with a rubber band.
b. Turn the bottle upside down placing it in a beaker or cut-off bottom of a two liter bottle.
c. Pour a layer of pebbles into
the bottle - the filter will
prevent the pebbles from
falling out of the neck.
d. Pour the coarse sand on top of
the pebbles.
e. Pour the fine sand on top of
the coarse sand.
f. Clean the filter by slowly and
carefully pouring through 1 L
(or more) of clean tap water.
Try not to disturb the top layer
of sand as you pour the water.
6. Filtration through a sand and pebble filter removes most of the impurities remaining in water
after coagulation and sedimentation have taken place.
After a large amount of sediment has settled on the bottom of the bottle of polluted water,
carefully - without disturbing the sediment - pour the top two-thirds of the water through the
filter. Collect the filtered water in the beaker.
Q: Does the water move quickly or slowly through the filter?
A: Slowly.
Q: Why would it be good for water to move slowly through soil in a natural ecosystem?
A: This allows more time for soil microbes to process and remove pollutants. Also, fastmoving water can cause erosion (washing away of soil). You can see an example of this if
you pour water into the top of the filter too hard, disrupting the upper layer of sand.
Q: What are some other examples of natural filters?
A: Some animals that live in rivers construct natural filters. One example is beaver dams.
Another is caddisflies, which live in streams and build nets to filter particles out of the
water. Blackflies also filter water, but they do so with special antennae on their heads!
Pour the remaining (one-third bottle) of polluted water back into the collection container.
Compare the purified and unpurified water.
Q: Has the purification changed how the polluted water looks? Is it completely clean?
A: Hopefully, water should be clearer and cleaner-looking. It may still have some fine
dirt in it.
Q: What is our water purification system missing that would be present in an
ecosystem?
A: Lots of good answers: microorganisms in water and soil, plants, animals, and insects,
all of which participate in water purification.
Q: Was it easy to purify the water?
A: Not really – it involved a lot of steps. A healthy ecosystem provides water purification
services for “free” – but that doesn’t mean they aren’t valuable! The “price” of this
ecosystem service is that we have to keep the ecosystem healthy and intact.
Additional references:
http://en.wikipedia.org/wiki/Water_aeration
Water Filtration Worksheet
At the beginning, our polluted water looked like:
During sedimentation (letting dirt sink), our water looked like:
After 5 minutes:
After 10 minutes:
After 15 minutes:
After 20 minutes:
After filtration, our water looked like:
Does the filtered water look different than the unfiltered water?