14 ChemMatters | OCTOBER/NOVEMBER 2017
U T T
What is going on? Is water safe to drink? We hear about “safe” drinking
water all the time. But what exactly does this mean? There are always contaminants in water, so how do we know whether the water we are drinking
is safe? If you fill a jar with water from a pond, you will see suspended
material such as algae, but what you won’t see are microbes and chemicals such as lead, copper, and carbon dioxide. While some aren’t cause for
concern, others are.
The contaminants we worry about include bacteria, viruses, parasites,
heavy metals, pesticides, and carcinogens, which are chemicals that
cause cancer. At what levels are these contaminants harmful? What health
problems can they cause? And how do we remove these contaminants
from water without worrying about adverse effects?
Bacteria and viruses
Common illnesses associated with bacteria in untreated water include dysentery,
typhoid, cholera, botulism, and legionella. Common viral infections include hepatitis A
and severe acute respiratory syndrome (SARS).
Residents of Columbia, S.C., are advised
to boil their water.
Residents of Flint, Mich., are told that they
should NOT drink or bathe in the town’s water.
Some schools in Baltimore, Md., have posted signs over
bathroom sinks telling students NOT TO DRINK the water.
The most common waterborne disease is
due to strains of Escherichia coli bacteria.
Most people are familiar with E. coli outbreaks
associated with improperly prepared food or
improper hygiene. E. coli are present in the
colon as part of a normal digestive system.
But there are strains of E. coli that cause serious illness. Symptoms of an infection can
range from a mild stomach ache to severe
bloody diarrhea and, in extreme cases, death.
Tracking viral infections to contaminated
drinking water is a bit like solving a murder
mystery. Even though contaminated water
may be the ultimate source of the infection,
people may not readily associate their illness
with drinking water. For example, the symptoms of SARS mimic the flu, and symptoms
from some viral infections may take several
days to appear.
To remove bacteria and viruses, water is
disinfected using techniques that include
large-scale chlorination and ozone treatment.
Depending on the treatment plant, chlorination
uses chlorine gas (Cl2) or sodium hypochlorite
(NaOCl), also known as bleach.
When chlorine is added to water, it forms
a weak acid called hypochlorous acid (HOCl).
Hypochlorous acid is lethal to bacteria and
viruses because it has no electric charge. The cell
walls around bacteria have a net negative charge,
so they repel other negatively charged particles in
water. Hypochlorous acid is neutral and passes
through the cell walls of bacteria and viruses.
Once inside the cell, HOCl wreaks havoc by
chemically attacking lipids that make up the cell
walls and by destroying enzymes and structures
inside the cell, eventually killing the cell.
Ozone (O3) can also remove bacteria and
viruses from drinking water. The chemical
breaks through the cell membranes of bacteria and interferes with the work of enzymes,
which leads to the destruction of bacteria.
Ozone also destroys viruses by diffusing
through their protein coats and damaging the
ribonucleic acid of the virus.
Ozone must be made onsite because it is
highly reactive and does not last long. This is
an advantage of ozone over chlorine, because
chlorine cannot be produced onsite and has
to be shipped or stored as a gas or as bleach.
Also, fewer unintended reactions occur in the
water being cleaned with O3.
By Frankie Wood-Black