Biology Lab: Water Testing and
Purification
In this lab activity, students learn how to collect water samples aseptically and how to use a
microscope to look for and identify fresh water invertebrates. They also do some simple
chemical testing of their water samples and make and test simple, filter-purification systems.
Note: culturing and identification of bacteria in water samples will be done in another lab
Biology Lab: Water Testing and Purification

Biology Lab: Water
Testing and
Purification
TIPS
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Introduction
Water pollution is a major worldwide health problem. According to
the World Health Organization (WHO) >9 million deaths annually
are attributed to waterborne diseases. 38% of those deaths are of
children afflicted with diarrhea. 80% of all worldwide infectious
diseases are attributed to unsafe water supplies. Typical waterborne
infections are Giardia, Cholera, Schistosomiasis, Hepatitis,
Shigellosis, Typhoid and coliform-related diarrhea. Coliforms are
bacteria that inhabit all human or animal colons; many bacteria in
this group are Gram-negative bacilli (e.g. E.coli or Salmonella) or
Gram-positive cocci (e.g. fecal Streptococci). Because they are found
in all colons, they are often used as an indicator of seweragecontaminated water.
Choose several sources
of water to sample.
Select one that you
expect to be
contaminated or dirty
(e.g. pond water) as well
as one that you expect
It is impossible to keep water and food completely sterile for long –
to be clean (drinking
as soon as it is unwrapped (in the case of food), or transported
water) so you can
through streams or pipes it is going to come into contact with
compare them.
organisms. All we can do is keep contamination at a minimum by
keeping streams, pipes and reservoirs clean and separated from
waste or sewerage outlets, by storing food properly, by boiling
water before use, by cooking food properly, and by good personal
hygiene.
In this lab, you will also learn how simple, homemade filtration
systems may be used to purify water, and you will do an
experiment to determine how efficient they are at producing clean
drinking water.
Introduction  1
Biology Lab: Water Testing and Purification

This lab has four objectives:



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To learn how to collect water samples aseptically for quality
testing
To analyze water samples for common chemical contaminants
and learn what their presence means
To use a microscope to look for and identify invertebrates in
water samples
To make a homemade filtration system and test how well it can
remove contaminants from your water samples.
TIPS

You will learn more about culturing and identifying bacteria from
water samples in another lab, later (see Biology Lab: Culturing Water
Samples and Selective Media).
Protocol
1. Discuss with your lab partners (your group) and your teacher,
where you would like to collect your water samples from. The class
When collecting water
should aim to collect a wide a range of samples, but limit the number
samples, DO NOT
to two water samples per group.
immerse your feet or
2. Take two sterile tubes and two sterile pipets with you to the watersampling site.
hands in the water.
Stand at the edge of the
water source and use a
3. Carefully open your sterile tube using your littlest finger. Using the
pipet to collect your
thumb and first finger of the hand that is holding the lid, dip the tip of
sample. Use a clean
your pipet into the water, fill up the pipet and transfer it to the tube.
pipet for each sample
Repeat this until the tube is almost full then close the lid.
and throw it away after
use.
Figure 1. This photograph shows you
how to open a tube with one hand
Glass tubes and pipets
while holding an object, such as a
can be sterilized just
pipet, with the other hand using
before use by
aseptic technique.
immersing them in a pot
of water and boiling it
for 10 minutes. Use a
pair of tongs to retrieve
4. Take your water samples back to the lab.
them!
Introduction  2
Biology Lab: Water Testing and Purification

Phase 1: Chemical testing of water samples
You have been provided with a chemical testing kit to test for some
chemicals commonly found in water supplies. You may test for all the
chemicals in the kit for fun, but we suggest that you pay particular
attention to the following four:
TIPS

1. Chlorine: kills a lot of harmful microorganisms and is often added to
water supplies to keep them clean, but can be harmful to humans at high
doses. Doses of <90ppm or 2.6mg/L are perfectly safe for humans. Doses
higher than 90ppm may cause throat irritation or sickness.
2. Iron: sometimes appears in water that has traveled through rusty
pipes or has been stored in metal tanks for long periods. It can give the
water a reddish-brown color. Small amounts of iron may also leach out
from rocks. Iron is actually good for us; it is used by the body to make
hemoglobin. It is only dangerous in very high doses (>500ppm).
Repeated exposure to high doses may eventually lead to liver damage.
Hemoglobin is a
pigment that makes
3. Copper: small doses of copper sometimes enter water from particular,
your blood red. It
copper containing rocks (ores) or from copper piping. However, high
contains iron, and it
doses of copper in water are rare unless you live near a copper mine.
helps to transport
Copper levels above 1.5ppm (1.5 mg/L) are rare, but may be harmful to
oxygen around your
health.
body. People who don’t
4. Nitrates: come from chemical fertilizers. When fertilizers are used by
farmers, they may drain into rivers and water supplies, particularly after
get enough iron may
become anemic.
heavy rainfall. Human and animal feces also contain nitrates, so water
that is contaminated with animal or human feces may be high in nitrates.
Nitrate levels in drinking water above 10ppm (10mg/L) may be harmful
Using rivers or lakes as
to infants because nitrate lowers the ability of red blood cells to transport
a toilet can increase the
oxygen, resulting in diarrhea, vomiting, and/or lethargy and difficulty in
levels of nitrate in the
breathing.
water, which may be
toxic to other people
Follow the directions provided with the kit to test your water samples,
and record your results. You will re-test your water samples after
filtration later, and compare those results to these.
Introduction  3
who use the rivers and
lakes for swimming or
drinking water.
Biology Lab: Water Testing and Purification
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Phase 2: Microscopy
In phase three you will look at a small drop of each water sample under a microscope, to see if it has any
microscopic invertebrates in it. Most water supplies have some invertebrates in them and most are
harmless. However, there some harmful invertebrates too (see table 1 and the Fresh Water Invertebrates
Field Guide).
1. Transfer a few drops of water to the cavity slide using a pipet.
2. Place the cavity slide on the microscope. Carefully focus on a piece of debris in the water using a low
magnification, then look for moving organisms on/around the debris. Ask you teacher to check what you
are looking at. Some organisms may have moving parts such as tails, flagella or mouth-parts.
3. See how many you can identify and compare your findings to the rest of the class.
4. Estimate how many organisms you can see in ~5 fields of vision (5 different locations on the slide),
then record the total number.
Table 1: Fresh water invertebrate examples
Schistosomiasis miracidia
The larval stage that
infects humans and causes
schistosomiasis or
bilharzia
Giardia
A protist that can cause
diarrhea
Found in water
contaminated with
human urine and fresh
water snail hosts
Ameba
Some amebas can cause
severe diarrhea, but many
are harmless
Often found in water.
Those that cause diarrhea
or dysentery come from
human sewerage
Found in water
contaminated with
human feces
Rotifers
Are very common in
ponds and rivers, and are
harmless
Nematodes
Are very common in
ponds and rivers. Most
come from the soil and are
harmless
NOTE: A lot of
harmless protists
look similar. If you
see something like
this – don’t panic
NOTE: these are
really fun to watch
and have
interesting, rotating
mouth parts
Some nematodes may
come from cat or dog
feces, and can be harmful
to humans
Introduction  4
Biology Lab: Water Testing and Purification
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You may also see different types of algae, which are more closely related to
plants and are harmless (see figure 2 below). A lot of algae may be an
indication of high nitrate levels in your water sample. If you see a lot of
these, check the results of your nitrate test to see if it was high.
TIPS

Presence of large
amounts of algae in
water is often an
indication that nitrate
Figure 2. Some common, fresh water algae
levels are high. This
may suggest that the
water is contaminated
Phase 3: Making and testing a water filtration system.
with fertilizers or
Water treatment plants that provide drinking water to cities use a simple
filtration system to purify recycled water and make it safe for drinking. The
water is simply filtered through a series of beds that contain different
combinations of gravel sand or charcoal. Invertebrates don’t pass easily
through sand and gravel, and bacteria stick to charcoal. It is easy to make a
small scale water filter using sand or gravel or even crushed coal that is
effective and removes most debris and many organisms. To be completely
safe, the filtered water should be boiled before use.
sewerage.
1. Make a collection of sand, gravel, crushed coal and/or any other
particulate matter that you find lying around, by collecting them from the
environment. Dry each completely by baking in an oven for an hour, or
spreading them out on plastic or metal sheets in the sun for a few days.
Introduction  5
You can re-use the
cavity slides if you rinse
them with water and lay
them up-side down on a
piece of paper to dry.
Biology Lab: Water Testing and Purification

2. Store your gravel, sand etc. in plastic bags, buckets or plastic containers
somewhere dry.
3. Take a large syringe, pull out the plunger and push a tiny bit of cotton wool
(½ cm across) into the bottom with a pair of tweezers.
4. Make your filter by putting ~ ½ - 1 inch layers of the materials you
collected in step 1, into the syringe. Put the finest materials at the bottom (and
make these layers a little thinner, or the water will take a long time to filter
through it). You can make more than one type of filter by using different
combinations of materials, and test them all to see what works best.
TIPS

5. Hold your filter over a clean tube or clean glass container, and carefully
pour about ¾ of your dirtiest water sample through the filter. It may take a
long time, but your only need a few drops for testing.
6. Put a few drops of your filtered water sample into a clean cavity slide and
look at it under the microscope. Estimate the number or organisms in five
fields, as before, and record your results.
Are there fewer organisms than before? Use the equation below to calculate
the % decrease
You can re-use syringes
if you immerse them in
boiling water for ~10
minutes, allow them to
# organisms before filtration – # organisms after filtration
Total (# before + number after)
X 100%
dry then wrap them in
paper and store them in
a dry place.
Use ~ ¾ of your water
7. Retest your sample for iron, copper and nitrates to see if filtering reduces
the contamination level of these pollutants.
sample to test your
8. Take a culture dish (see Lab: Culturing Bacteria for protocol for making
culture dishes) with Tryptic Soy agar on it. Divide the dish into two sides by
drawing a line across the underside. Carefully spread a little unfiltered water
on one side. Label the outside of the dish so you know which side is the
unfiltered water. Then, carefully spread a little of the filtered water on other
side of the dish and label it.
but KEEP the rest to use
water filtration system,
as a control later.
Use a Q-tip to spread
your water on the
plates. If they are dry
before you use them,
9. Incubate your plates in a warm place for a few days then count the colonies
of bacteria on both sides. Use a similar equation as the one above to calculate
the % decrease in bacteria after filtering your water.
they will be clean
enough BUT DON’T
TOUCH THE TIP WITH
YOUR HANDS!
Introduction  6