Surface Water
The ultimate course of rain or melting
snow depends on the nature of the terrain over which it flows.
In areas consisting of hard packed clay, very
little water penetrates the ground. In these cases, the water generates
"runoff."
The runoff collects in streams and rivers.
The rivers empty into bays and estuaries, and the water ultimately returns to
the sea, completing one major phase of the hydrologic cycle.
As water runs off along the surface, it stirs
up and suspends particles of sand and soil, creating silt in the surface water.
In addition, the streaming action erodes
rocky surfaces, producing more sand. As the surface water cascades over rocks,
it is aerated.
The combination of oxygen, inorganic
nutrients leached from the terrain, and sunlight supports a wide variety of
life forms in the water, including algae, fungi, bacteria, small crustaceans,
and fish.
Often, river beds are lined with trees, and
drainage areas feeding the rivers are forested. Leaves and pine needles
constitute a large percentage of the biological content of the water.
After it dissolves in the water, this
material becomes a major cause of fouling of ion exchange resin used in water
treatment.
The physical and chemical characteristics of
surface water contamination vary considerably over time. A sudden storm can
cause a dramatic short term change in the composition of a water supply.
Over a longer time period, surface water
chemistry varies with the seasons. During periods of high rainfall, high runoff
occurs.
This can have a favorable or unfavorable
impact on the characteristics of the water, depending on the geochemistry and
biology of the terrain.
Surface water chemistry also varies over
multi- year or multi-decade cycles of drought and rainfall.
Extended periods of drought severely affect
the availability of water for industrial use.
Where rivers discharge into the ocean, the
incursion of salt water up the river during periods of drought presents additional
problems.
Industrial users must take surface water
variability into account when designing water treatment plants and programs.
Groundwater
Water that falls on porous terrains,
such as sand or sandy loam, drains or percolates into the ground.
In these cases, the water encounters a wide
variety of mineral species arranged in complex layers, or strata. The minerals
may include granite, gneiss, basalt, and shale.
In some cases, there may be a layer of very
permeable sand beneath impermeable clay.
Water often follows a complex three
dimensional path in the ground. The science of groundwater hydrology involves
the tracking of these water movements.
Table
1-2. A comparison of surface water and groundwater characteristics.
Characteristic
|
Surface Water
|
Ground Water
|
Turbidity
|
high
|
low
|
Dissolved minerals
|
low-moderate
|
high
|
Biological content
|
high
|
low
|
Temporal variability
|
very high
|
low
|
In contrast to surface supplies,
groundwaters are relatively free from suspended contaminants, because they are
filtered as they move through the strata. The filtration also removes most of
the biological contamination.
Some groundwaters with a high
iron content contain sulfate reducing bacteria. These are a source of fouling
and corrosion in industrial water systems.
Groundwater chemistry tends to be
very stable over time. A groundwater may contain an undesirable level of scale
forming solids, but due to its fairly consistent chemistry it may be treated
effectively.
Mineral
Reactions: As
groundwater encounters different minerals, it dissolves them according to their
solubility characteristics. In some cases chemical reactions occur, enhancing
mineral solubility.
A good example is the reaction of
groundwater with limestone. Water percolating from the surface contains
atmospheric gases.
One of these gases is carbon
dioxide, which forms carbonic acid when dissolved in water.
The decomposition of organic
matter beneath the surface is another source of carbon dioxide.
Limestone is a mixture of calcium
and magnesium carbonate. The mineral, which is basic, is only slightly soluble
in neutral water.
The slightly acidic groundwater
reacts with basic limestone in a neutralization reaction that forms a salt and a
water of neutralization.
The salt formed by the reaction
is a mixture of calcium and magnesium bicarbonate. Both bicarbonates are quite
soluble.
This reaction is the source of
the most common deposition and corrosion problems faced by industrial users.
The calcium and magnesium
(hardness) form scale on heat transfer surfaces if the groundwater is not
treated before use in industrial cooling and boiler systems.
In boiler feedwater applications,
the thermal breakdown of the bicarbonate in the boiler leads to high levels of
carbon dioxide in condensate return systems. This can cause severe system
corrosion.
Structurally, limestone is
porous. That is, it contains small holes and channels called "interstices."
A large formation of limestone
can hold vast quantities of groundwater in its structure.
Limestone formations that contain
these large quantities of water are called aquifers,
a term derived from Latin roots meaning water bearing.
If a well is drilled into a
limestone aquifer, the water can he withdrawn continuously for decades and used
for domestic and industrial applications.
Unfortunately, the water is very
hard, due to the neutralization/dissolution reactions described above. This
necessitates extensive water treatment for most uses.
CHEMICAL
REACTIONS
Numerous
chemical tests must be conducted to ensure effective control of a water
treatment program.
Because of their significance in
many systems, three tests, pH, alkalinity, and silica, are discussed here as
well.
pH
Control
Good pH control
is essential for effective control of deposition and corrosion in many water
systems. Therefore, it is important to have a good understanding of the meaning
of pH and the factors that affect it.
Pure H2O exists as an
equilibrium between the acid species, H+ (more correctly
expressed as a protonated water molecule, the hydronium ion, H30+)
and the hydroxyl radical, OH -.
In neutral water the acid
concentration equals the hydroxyl concentration and at room temperature they
both are present at 10-7 gram equivalents (or moles) per liter.
The "p" function is
used in chemistry to handle very small numbers. It is the negative logarithm of
the number being expressed.
Water that has 10-7 gram
equivalents per liter of hydrogen ions is said to have a pH of 7. Thus, a
neutral solution exhibits a pH of 7.
The pH meter can be a source of
confusion, because the pH scale on the meter is linear, extending from 0 to 14
in even increments.
Because pH is a logarithmic
function, a change of I pH unit corresponds to a 10 fold change in acid
concentration. A decrease of 2 pH units represents a 100 fold change in acid
concentration.
Alkalinity
Alkalinity
tests are used to control lime-soda softening processes and boiler blowdown and
to predict the potential for calcium scaling in cooling water systems.
For most water systems, it is
important to recognize the sources of alkalinity and maintain proper alkalinity
control.
Carbon dioxide dissolves in water
as a gas. The dissolved carbon dioxide reacts with solvent water molecules and
forms carbonic acid according to the following reaction:
CO2 +
H2O = H2CO3
Only a trace amount of carbonic
acid is formed, but it is acidic enough to lower pH from the neutral point of
7.
Carbonic acid is a weak acid, so
it does not lower pH below 4.3. However, this level is low enough to cause
significant corrosion of system metals.
If the initial loading of CO2 is
held constant and the pH is raised, a gradual transformation into the
bicarbonate ion HCO3- occurs.
The transformation is complete at
pH 8.3. Further elevation of the pH forces a second transformation into
carbonate, CO32-.
The three species carbonic acid,
bicarbonate, and carbonate can be converted from one to another by means of
changing the pH of the water.
Variations in pH can be reduced
through "buffering" the addition of acid (or caustic).
When acid (or caustic) is added
to a water containing carbonate/bicarbonate species, the pH of the system does
not change as quickly as it does in pure water.
Much of the added acid (or
caustic) is consumed as the carbonate/bicarbonate (or bicarbonate/carbonic
acid) ratio is shifted.
Alkalinity is the ability of a
natural water to neutralize acid (i.e., to reduce the pH depression expected
from a strong acid by the buffering mechanism mentioned above).
Confusion arises in that alkaline
pH conditions exist at a pH above 7, whereas alkalinity in a natural water
exists at a pH above 4.4.
Alkalinity is measured by a
double titration; acid is added to a sample to the Phenolphthalein end point
(pH 8.3) and the Methyl Orange end point (pH 4.4).
Titration to the Phenolphthalein
end point (the P-alkalinity) measures OH - and 1/2 CO32-;
titration to the Methyl Orange end point (the M-alkalinity) measures OH -,
CO32- and HCO3 .
Silica
When not
properly controlled, silica forms highly insulating, difficult to remove
deposits in cooling systems, boilers, and turbines.
An understanding of some of the
possible variations in silica testing is valuable.
Most salts, although present as
complicated crystalline structures in the solid phase, assume fairly simple
ionic forms in solution. Silica exhibits complicated structures even in
solution.
Silica exists in a wide range of
structures, from a simple silicate to a complicated polymeric material. The
polymeric structure can persist when the material is dissolved in surface
waters.
The size of the silica polymer
can be substantial, ranging up to the colloidal state.
Colloidal silica is rarely
present in groundwaters. It is most commonly present in surface waters during
periods of high runoff.
The polymeric form of silica does
not produce color in the standard molybdate based colorimetric test for silica.
This form of silica is termed "nonreactive".
The polymeric form of silica is
not thermally stable and when heated in a boiler reverts to the basic silicate
monomer, which is reactive with molybdate.
As a result, molybdate testing of
a boiler feedwater may reveal little or no silica, while boiler blowdown
measurements show a level of silica that is above control limits.
High boiler water silica and low
feedwater values are often a first sign that colloidal silica is present in the
makeup.
One method of identifying
colloidal silica problems is the use of atomic emission or absorption to
measure feedwater silica.
This method, unlike the molybdate
chemistry, measures total silica irrespective of the degree of polymerization.
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Multi-Media Filter, Highly-Activated Carbon Filter,
Zeolite-Process Water Softener With Brine Tank,
Fiberglass Ballast-Type Pressure Tank
(fully automatic backwash & regeneration)
|
PURICARE
Water
Treatment
Systems
.
...
...
Aganan, Pavia, Iloilo, Philippines
...
CLICK HERE . . . to view company profile . . .
.
.
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