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Chemistry 1002 Chapter 20Environment & Water |
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Introduction
Water is the single most essential substance needed
to support life on this planet and possibly in the entire physical
universe. It would undoubtedly be easier for life to evolve to
use an energy molecule other than ATP than it would be for life
to evolve to not need water. Because of this it is essential
that we manage our water supply in such a way that we always have
access to plenty of clean potable (drinkable) water.
Water Supply
About 4.35 trillion gallons fresh water (rain &
snow) fall to earth every day in continental US. A little over
70% of this water evaporates back into air, and additionally,
a bit under 20% of this water drains into Gulf of Mexico (via
Mississippi River) each day. This leaves about 10% of daily rainfall
(450 billion gallons) available to us as drinking water.
PROBLEM 5.
Where does most of the water go?
@ Evaporation.
When rainwater hits the ground and adds to rivers &
lakes, forms puddles, floods streets, etc. it is called surface
water. When surface water soaks into (under) ground it called
groundwater.
PROBLEM 6.
How's rainwater become groundwater?
@ Soaks into ground.
Water Consumption
In 1950 we were consuming about 45% of the available
water in continental US. By 1980 we were consuming almost 96%
of the available daily water supply in the continental US.
Obviously some (drier) parts of US (ie. California)
were consuming water much faster than it could be recharged while
other parts of US (ie. Louisiana) were still in good shape. This
situation led to depletion of many underground aquifers, resulting
in some bizarre consequences (sinkholes, brackish water, people
putting water table markers on utility poles, loss of rapids in
Colorado River, etc.).
Due to emerging imbalance in supply/demand for fresh
water in 1980's many state legislatures started passing tough
water-conservation laws, and by 1994 US was only using about 21%
of available water.
Water Pollution
Water pollution sometimes hard to define. Best definition
probably involves any human-caused or unusual natural change made
to body of water making it unfit for some particular purpose.
We don't consider ocean water polluted even though it's unfit
to drink because it's normally unfit to drink. If someone spills
oil on ocean making it unfit for birds and fish to live in/on
we call the result pollution.
Some forms of "pollution" weren't recognized
until recently. "Thermal" pollution occurs when industry
uses water to cool exothermic reactions and then returns the warmed
up water to its source. When this practice causes a change in
the ecosystem of some body of water it can often affect some other
user of that body of water (ie. fishermen) adversely.
Table 20.5: "clean" water impurities.
CAUSES OF WATER POLLUTION
Industrial waste. Industry usually takes
the brunt of the blame for environmental pollution (both air and
water) even though individual consumers contribute to this problem
as much as industry does. There are three ways industry can deal
with toxic wastes produced during manufacture of of products.
It can put them in landfills, burn them, or attempt to find some
way to use (recycle) them.
As cost of waste disposal rises industry becoming very
interested in developing processes which minimize waste or generate
waste which can be used for some other purpose. Nevertheless
recycling and reuse of waste is not always possible today and
industry still needs to dispose of large quantities of waste by
landfilling or burning it.
Landfilling is water-pollution hazard (leaching of
waste into water supply), and incineration is air pollution hazard.
PROBLEM 18.
Name 2 disposal methods and which is water-pollution
hazard.
@ Incineration & landfilling; landfilling.
Consumer waste. State and federal governments
police industry to try to assure that waste either gets burned
or put into secure landfills. Consumers not as heavily policed;
majority of our waste goes into ordinary landfills. Often no
easy way for us to get rid of hazardous consumer chemicals; usually
end up in the municipal wastewater stream or in garbage dump.
Rainwater can leach them into water table.
MEASURING WATER POLLUTION
Five common measurements of pollutants in water are:
Total Dissolved Solids, dissolved metals, Biochemical Oxygen Demand,
Volatile Organic Compounds, and Total Organic Compounds.
Total Dissolved Solids. Measured by
evaporating water and weighing solid left behind. Not as commonly
used for water purity as BOD, TOC, VOC, and dissolved metals measurements.
Biochemical Oxygen Demand (BOD). Measurement
of quantity of biodegradeable (able to be decomposed by
bacteria) organic contaminants dissolved in water. Bacteria added
to contaminated water consume oxygen in order to oxidize the biodegradeable
organic contaminants. The higher the contaminant concentration
the more oxygen is needed by the bacteria. BOD does not measure
organics directly; rather measures how fast oxygen is consumed
by bacteria in order to decompose them.
PROBLEM 25.
How does BOD measure water purity?
@ Measures biodegradeable organic content indirectly
by measuring rate of oxygen consumption by bacteria involved in
decomposing organics.
TOC and VOC measurements. Volatile organics
are organic compounds which evaporate easily. They can be measured
by sparging a water sample with inert gas (ie. helium) which carries
VOC's out of water and into a recording instrument (usually a
GC/MS setup). Total organics are measured by extracting all organics
out of water sample with a volatile organic solvent, evaporating
organic solvent (along with VOC's), recording quantity and identity
of nonvolatile organics, and adding result to previous VOC measurement.
TOC and VOC measurements are often made to determine quantity
and identity of nonbiodegradeable organics. TOC/VOC measurements
can be made after biodegradeables have been destroyed by bacterial
treatment.
Metal measurements. Metal ions dissolved
in water can't be decomposed by bacteria, and won't be detected
by mass spectroscopy. Generally AA (Atomic Absorbtion spectroscopy)
is used to measure them.
WATER PURIFICATION
Natural. Nature purifies water by two
major means. Bacteria decompose all natural volatile biodegradeable
organics into simple gases (ie. carbon dioxide, nitrogen, etc.).
Metals and other nonvolatile substances are left on the earth
when water evaporates. Rainwater is therefore reasonably pure
("distillation"), as long as we do not release too much
volatile nonbiodegradeable material into the environment (ie.
nitrogen and sulfur oxides, components of "acid rain").
When rain falls to earth it can become contaminated
by organics on the ground before settling into aquifers. Water
moving over rocks in streams picks up oxygen from air ("aeration")
enabling bacteria to decompose these organics. As water settles
into aquifers deep underground bacteria are removed by "filtration."
Wastewater treatment. Humans treat wastewater
in from one to three stages, depending on how polluted the wastewater
is and how clean it needs to be when it is returned to the environment.
These stages are known as primary, secondary, and
tertiary wastewater treatment.
Primary wastewater treatment involves merely screening
out solid material (paper, cloth, rock, etc.) and allowing sludge
to settle.
Secondary treatment uses bacteria and aeration to break
down biodegradeable organics, followed by chemical disinfection
to kill bacteria. Two most common disinfection agents are chlorine
and ozone. In US municipal wastewater and drinking water treated
with chlorine to kill pathogenic bacteria. Much of Europe uses
ozone.
PROBLEM 37.
What does chlorine do to treated water?
@ Kill bacteria.
Europe uses ozone for disinfection of drinking water
rather than chlorine, even though it is more expensive, because
chlorine is thought to give more toxic "disinfection byproducts."
Disinfection byproducts result from reactions of chemical
disinfectants with trace impurities in treated water. They are
thought to be mildly carcinogenic. Chlorine yields chlorinated
organics on reaction with trace quantities of organics in water
supply, and ozone yields bromate ion by reaction with trace bromide
ion in water supply.
Problem 45.
What are disinfection byproducts & how are they
toxic?
@ Reaction products between chemical disinfectants
and trace impurities in water; may be mildly carcinogenic.
PROBLEM 46.
Similarity & difference between ozone and chlorine?
@ Both kill bacteria; ozone generates safer disinfection
byproducts.
Tertiary wastewater treatment generally
involves at least one of four different processes performed on
secondary-treated water when secondary treatment does not produce
wastewater clean enough to return to a particular sector of environment
(ie. water table). These processes are use of denitrifying bacteria,
distillation, reverse osmosis, and filtration through carbon black
(also known as activated charcoal).
Distillation is rarely done, since cheapest form (solar
distillation) requires land and plentiful sunlight.
Reverse osmosis often used to purify sea water. Saudi
Arabia, Kuwait, Malta, and other places use reverse osmosis to
produce drinking water from seawater. Reverse osmosis can remove
most inorganic and hydrophilic organic chemicals from water, but
does not work well for bacteria or hydrophobics.
Carbon filtration mostly useful for removing nonbiodegradeable
organics.
PROBLEM 50.
Which tertiary treatment used in home best for dissolved
organics?
@ Carbon filtration.
Show Fig's. 20.12, 20.13, & 20.14.
Comments?
Last Revised : Wednesday, December 3, 1997
Copyright © 1997
Louisiana State University, Department of Chemistry.
All rights reserved.
http://www.chem.lsu.edu/lucid/courseinfo/chem1002/ch20.html
