Environmental
NAME:________________________________
Environmental Decision Worksheet: Drinking Water Treatment
Many people living in modern societies today give little thought to the origin of things that are necessary
for life—such as from where does food come? How does gasoline get to the filling station, or where do
we get our clean water to drink? With this exercise, we hope to help you understand where and how
you get your drinking water.
There are some natural sources of water that are clean enough for humans to drink without treatment.
Most well waters, some natural streams in wilderness areas, and rain water in some parts of the world
fit this category. However, most surface waters (water in rivers, lakes, and streams) have various
contaminants that make the water unfit for human consumption.
One way of describing how “dirty” water is involves the measurement of something called “turbidity.”
Turbidity is defined technically as the resistance of water to the passage of light. It is caused by a variety
of materials such as clay, silt, finely divided organic material, microscopic organisms such as bacteria and
algae, and fine and coarse suspensions.
The removal of turbidity from drinking water is important for three reasons. First, turbid water looks
dirty, and people refrain from drinking it for aesthetic reasons. It “looks” bad. Second, turbid waters
tend to plug up the filters that are used to help clean the water. However, the most important reason
that turbidity needs to be removed is the fact that the particles that cause turbidity can shield harmful
bacteria and viruses from the disinfecting action of chlorine, ozone, or other disinfectants.
Turbidity has been measured by many methods and has been reported in many units over the last 100
years. Initially, the turbidity of natural water was compared visually to the turbidity caused by solutions
of silicon dioxide (SiO2). The units were given in terms of milligrams per liter—milligrams of SiO2 per liter
of water. Currently, turbidity is reported in terms of NTU (nephelometric turbidity unit); but for the
purposes of this discussion, these two units will be used interchangeably.
The particles causing turbidity are so small that they are influenced more by electrical charges on their
surfaces than by gravity. Thus, after several days of quiet standing, these waters still will exhibit some
turbidity. Because of this phenomenon, it has been the practice for many years to add chemicals to
turbid solutions that can neutralize the electrical charges and help gravity remove the particles from
suspension. One such chemical, which has been used extensively for this purpose, is aluminum sulfate,
commonly called alum. Its chemical formula is Al2(SO4)3· n H2O where n is the number of attached
water molecules (14, 18, or 14.3).
You are being asked to develop a treatment scheme to take river water and make it suitable for
drinking. One aspect of treatment is to determine a suitable chemical dosage to help remove the
turbidity. The driving question that needs to be answered is as follows:
How many pounds of aluminum sulfate would be needed to remove 20 units of turbidity from two
million gallons of river water as part of a coagulation/flocculation/sedimentation/
filtration process to produce clean drinking water?
ANSWER:_______________________________________pounds of alum
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