Hydrologic Cycle, Surface and Groundwater
Impurities
Abundant
supplies of fresh water are essential to the development of industry. Enormous
quantities are required for the cooling of products and equipment, for process
needs, for boiler feed, and for sanitary and potable water supply.
THE
PLANETARY WATER CYCLE
Industry
is a small participant in the global water cycle .The finite amount of water on
the planet participates in a very complicated recycling scheme that provides
for its reuse.
This recycling of water is termed the "Hydrologic
Cycle" (see Figure 1-1).
Evaporation
under the influence of sunlight takes water from a liquid to a gaseous phase.
The water may condense in clouds as the temperature drops in the upper atmosphere.
Wind transports the water over great distances before releasing it in some form
of precipitation. As the water condenses and falls to the ground, it absorbs
gases from the environment. This is the principal cause of acid rain and acid
snow.
WATER
AS A SOLVENT
Pure
water (H20) is colorless, tasteless, and odorless. It is composed of hydrogen
and oxygen. Because water becomes contaminated by the substances with which it
comes into contact, it is not available for use in its pure state.
To
some degree, water can dissolve every naturally occurring substance on the
earth. Because of this
property, water has been termed a "universal
solvent." Although beneficial to mankind, the solvency power of water can
pose a major threat to industrial equipment.
Corrosion
reactions cause the slow dissolution of metals by water. Deposition reactions,
which produce scale on heat transfer surfaces, represent a change in the
solvency power of water as its temperature is varied.
The control of corrosion
and scale is a major focus of water treatment technology.
WATER
IMPURITIES
Water
impurities include dissolved and suspended solids. Calcium bicarbonate is a
soluble salt. A solution of calcium bicarbonate is clear, because the calcium
and bicarbonate are present as atomic sized ions which are not large enough to
reflect light.
Suspended
solids are substances that are not completely soluble in water and are present
as particles. These particles usually impart a visible turbidity to the water.
Dissolved and suspended solids are present in most surface waters.
Seawater
is very high in soluble sodium chloride; suspended sand and silt make it
slightly cloudy.
An extensive list of soluble and suspended impurities found in
water is given in Table 1-1.
Table 1-1.
Common impurities found in fresh water.
Constituent
|
Chemical
Formula
|
Difficulties
Caused
|
Means
of Treatment
|
|||||
Turbidity
|
non-expressed in analysis as
units
|
imparts unsightly appearance to
water; deposits in water lines, process equipment, etc.; interferes with most
process uses
|
coagulation, settling, and
filtration
|
|||||
Hardness
|
calcium and magnesium salts,
expressed as CaCO3
|
chief source of scale in heat
exchange equipment, boilers, pipe lines, etc.; forms curds with soap,
interferes with dyeing, etc.
|
softening; demineralization;
internal boiler water treatment; surface active agents
|
|||||
Alkalinity
|
bicarbonate(HCO3-),
carbonate (CO32-), and hydroxide(OH-),
expressed as CaCO3
|
foam and carryover of solids
with steam; embrittlement of boiler steel; bicarbonate and carbonate produce
CO2 in steam, a source of corrosion in condensate lines
|
lime and lime-soda softening;
acid treatment; hydrogen zeolite softening; demineralization dealkalization
by anion exchange
|
|||||
Free Mineral Acid
|
H2SO4 ,
HCI. etc., expressed as CaCO3
|
corrosion
|
neutralization with alkalies
|
|||||
Carbon Dioxide
|
CO2
|
corrosion in water lines,
particularly steam and condensate lines
|
aeration, deaeration,
neutralization with alkalies
|
|||||
PH
|
hydrogen ion concentration
defined as:
|
pH varies according to acidic
or alkaline solids in water; most natural waters have a pH of 6.0-8.0
|
pH can be increased by alkalies
and decreased by acids
|
|||||
Sulfate
|
SO42-
|
adds to solids content of
water, but in itself is not usually significant, combines with calcium to
form calcium sulfate scale
|
demineralization, reverse
osmosis, electrodialysis, evaporation
|
|||||
Chloride
|
Cl -
|
adds to solids content and
increases corrosive character of water
|
demineralization, reverse
osmosis, electrodialysis, evaporation
|
|||||
Nitrate
|
NO3-
|
adds to solids content, but is
not usually significant industrially: high concentrations cause methemoglobinemia
in infants; useful for control of boiler metal embrittlement
|
demineralization, reverse
osmosis, electrodialysis, evaporation
|
|||||
Fluoride
|
F-
|
cause of mottled enamel in
teeth; also used for control of dental decay: not usually significant
industrially
|
adsorption with magnesium
hydroxide, calcium phosphate, or bone black; alum coagulation
|
|||||
Sodium
|
Na+
|
adds to solids content of
water: when combined with OH-, causes corrosion in boilers under
certain conditions
|
demineralization, reverse
osmosis, electrodialysis, evaporation
|
|||||
Silica
|
SiO2
|
scale in boilers and cooling
water systems; insoluble turbine blade deposits due to silica vaporization
|
hot and warm process removal by
magnesium salts; adsorption by highly basic anion exchange resins, in
conjunction with demineralization, reverse osmosis, evaporation
|
|||||
Iron
|
Fe2+ (ferrous)
Fe3+ (ferric) |
discolors water on
precipitation; source of deposits in water lines, boilers. etc.; interferes
with dyeing, tanning, papermaking, etc.
|
aeration; coagulation and
filtration; lime softening; cation exchange; contact filtration; surface
active agents for iron retention
|
|||||
Manganese
|
Mn2+
|
same as iron
|
same as iron
|
|||||
Aluminum
|
AI3+
|
usually present as a result of
floc carryover from clarifier; can cause deposits in cooling systems and
contribute to complex boiler scales
|
improved clarifier and filter
operation
|
|||||
Oxygen
|
O2
|
corrosion of water lines, heat
exchange equipment, boilers, return lines, etc.
|
deaeration; sodium sulfite;
corrosion inhibitors
|
|||||
Hydrogen Sulfide
|
H2S
|
cause of "rotten egg"
odor; corrosion
|
aeration; chlorination; highly
basic anion exchange
|
|||||
Ammonia
|
NH3
|
corrosion of copper and zinc
alloys by formation of complex soluble ion
|
cation exchange with hydrogen
zeolite; chlorination; deaeration
|
|||||
Dissolved Solids
|
none
|
refers to total amount of
dissolved matter, determined by evaporation; high concentrations are
objectionable because of process interference and as a cause of foaming in
boilers
|
lime softening and cation
exchange by hydrogen zeolite; demineralization, reverse osmosis,
electrodialysis, evaporation
|
|||||
Suspended Solids
|
none
|
refers to the measure of
undissolved matter, determined gravimetrically; deposits in heat exchange
equipment, boilers, water lines, etc.
|
subsidence; filtration, usually
preceded by coagulation and settling
|
|||||
Total Solids
|
none
|
refers to the sum of dissolved
and suspended solids, determined gravimetrically
|
see "Dissolved
Solids" and "Suspended Solids"
|
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source: gewater.com
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