Water Filtration Spring 2013

rapidcrimsonMécanique

22 févr. 2014 (il y a 3 années et 4 mois)

60 vue(s)

Water Filtration


Spring 2013

Lesson from:
http://water.epa.gov/learn/kids/drinkingwater/upload/2005_03_10_kids_activity_grades_4
-
8_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 purificati
on 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 natur
al 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
s
ponge
is like
wetlands, forests, natural gr
asslands, 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

bacter
ia)
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 w
ater 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

k
ind of pollution might
typically
come from far
m

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

pr
ovided by
our

ecosystem
s
!


Materials (per group) :




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 bott
le 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 smal
l 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 escap
e and adds oxygen to
the water.


Place the cap on the bottle and vigorously shake
the bottle for 30 sec
onds.


Continue the aeration process by pouring the water into
the bottle with its top cut off
, then
pouring the water back and for
th between them about 10 times.


Once aerated, gases have escaped (bubbles should be gone). Pour your ae
rated 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 wa
ter 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 ca
n easily be removed from water.


Add two tablespoons of

alum to the aerated water. S
lowly 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. W
hen 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 bo
ttom
of the cylin
der.


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 t
he 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 environ
ments. 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 run
off, than streams without dams.


5.

While you are waiting for sedimentation to proceed, c
onstr
uct 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
-
of
f 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 th
e filtered water in the beaker.


Q:

Does the water
move quickly or slowly through the filter?

A:

S
lowly.


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 proc
ess and remove pollutants
. Also, fast
-
moving water can cause eros
ion (washi
ng 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
-
l
ooking. 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 wa
ter 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?