MadSci Network: Earth Sciences

Re: How is drinking water still contaminated after purification?

Date: Sun Nov 27 13:51:42 2005
Posted By: Dean Cliver, Faculty, Food Safety Unit, Uiversity of California, Davis
Area of science: Earth Sciences
ID: 1132724499.Es

	First, let’s recall that a water molecule is just an oxygen atom with a
couple of hydrogen atoms stuck onto it.  This simple little atom provides
the medium for all life on earth, yet most of the water on earth is in the
oceans, with so much salt in it that people and other land animals can’t
use it.  We need “fresh” water, which starts out as rain from clouds in the
sky.  The clouds get their water by evaporation of water from the oceans,
but also lakes, rivers, and anywhere there is water on the surface of the
earth.  Most water is simply recycled in this way, and has been for
millions of years.
	It is possible to make “new” water.  Early in the US space program, NASA
generated electricity on spacecraft with hydrogen-oxygen fuel cells.  The
by-product was ultrapure water, which the astronauts drank.  The trouble
with this is that water that pure is unnatural—it dissolves a little of
almost anything it touches, and then it’s not ultrapure anymore.
	When we want very pure water in the laboratory, we distill it.  This
starts with boiling water to evaporate it, then chilling the steam in a
“condenser” to turn it back into liquid.  Distillation gets rid of a lot of
impurities, but further steps are often needed.  This is expensive, and
storing the purified water without contaminating it is tricky.
	Drinking water comes either from a well (“groundwater”) or from a lake or river
(“surface water”).  Most homes out in the countryside, and some
cities, use groundwater.  Often, the groundwater is so purified by
filtration down through the soil that it doesn’t need much treatment to
make it safe to drink, but water that has passed through a lot of soil
tends to be “hard.”  It has dissolved minerals that make it turn soap to
scum, but the minerals are usually good for our bodies when we drink hard
water.  Some places, though, the dissolved minerals are harmful, so testing
is needed.
	Communities that use surface water have a harder time with purification
because water in lakes and rivers is water that has run off of surfaces
after rain falls, and it may carry many contaminants.  Also, the water may
already have been used by an upstream community and discharged into the
waterway as treated, disinfected (we hope!) wastewater (sewage). 
Purification is needed, but distillation (or making new water out of
hydrogen and oxygen) is too expensive in most cases; the energy cost to
boil the water would be huge.  Instead, water at the “intake” is
coarse-filtered to take out big objects, including fish.  The water may
also receive some chlorine at this stage.  Then, a “coagulant” such as alum
is added, to make smaller particles stick together, and these coagulated
particles are settled out in a step called “sedimentation.”  The water is
usually passed through of filter of sand or other fine material before
being disinfected with chlorine, or sometimes ozone or ultraviolet; note
that “disinfected” means viruses and bacteria that might cause infections
are killed—some bacteria are still alive.  At this point, it is called
“finished water,” but it’s still at the treatment plant rather than at your
house, where you can use it.  
	Sending the finished water out through “mains” to homes, schools, and
other users is not easy.  There are always things dissolved in the water
and bacteria that can use them, so water testing is done mainly from
samples in the distribution system, rather than in the treatment plant, to
see how things are going.  The main problems are build-up of slime from
bacterial growth and of slow accumulation of material that settles out of
the water when it’s standing still in the pipes.  Sometimes, though, water
has to be used in large quantities to fight a fire, and the pressure in the
pipes drops enough that water outside the pipes gets slurped in.  Of
course, the water that may be in the soil around the outside of the water
main is not pure, so when it gets slurped in, the “drinking water” gets
contaminated.  When things go really wrong, people are told to boil their
water before drinking it; this seldom lasts long.
	To prevent the development of coliform bacteria (these are called
“indicators”—most are harmless, but they show us that the water hasn’t been
properly taken care of), water suppliers try keep a low level of active
chlorine all the way to the user.  The nitrate you ask about is usually
present because the source water had been used before.  It can be removed
by a difficult process called “denitrification”; but the levels of nitrate
are usually harmless, so they are just left in the water.  Now, it’s being
found that used water may also contain people’s medications (aspirin, for
example), vitamins, and other things that got into sewage.  
	So, why not completely purify drinking water?  The main reason is that
most drinking water serves purposes other than drinking.  An average
grown-up’s daily water intake is about a half gallon, much of it as drinks
other than water (milk, soft drinks, coffee, etc.).  Kids need less.  The
average amount of “drinking water” an American uses, though, is from 50 to
100 gallons.  Much of the water that is delivered to your home is used for
washing dishes, clothes, bodies, and even cars.  Most homes in the U.S.
have flush toilets; these use 3 to 5 gallons of water each time they are
flushed (some are made to use less).  Of course, this water, as well as the
water that was used for washing things, now goes through the sewers to be
treated, disinfected, and discharged for someone else to use again.  Rivers
that join to make up the Mississippi drain half the U.S., including a lot
of cities, by the time they get to what used to be New Orleans.  This is
very “experienced” water, and drinking water treatment is a very demanding
and expensive task in downstream cities.  They may use special chemical
processes and even activated carbon, even if most of the drinking water is
used for other things than drinking.
	Those who don’t trust what comes out of their taps may choose to drink
bottled water, but there is no guarantee that the water in the bottle is
really purer.  In the U.S., the Environmental Protection Agency writes and
enforces the rules for tap water, and the Food and Drug Administration
regulates bottled water; they aren’t always “on the same page.”  Some
people also buy purification devices.  These can work if they are designed
to take care of the specific impurities that come with your tapwater (no
two tapwaters are quite the same), but they have to be maintained.  They
can easily make the water worse, rather than better, if they aren’t taken
care of.  
	I get to travel to other countries (Mexico, Ukraine, and Egypt this year, with
plane changes in Germany and Austria).  I have my own ideas of where I can
safely drink the tapwater, but I don’t worry about drinking water when
I visit places in the U.S.  We are very lucky to have enough water, and
pure enough water to know that what we drink from our taps won’t make us
sick.  Much of the rest of the world’s population is not so lucky.

Dean O. Cliver

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