(Figure 4-1) Turbulence increases the aeration of flowing streams. |
AERATION
Aeration is a unit process in
which air and water are brought into intimate contact.
Turbulence increases the aeration
of flowing streams (Figure 4-1).
In industrial processes, water
flow is usually directed countercurrent to atmospheric or forced-draft air
flow. The contact time and the ratio of air to water must be sufficient for
effective removal of the unwanted gas.
Aeration as a water treatment
practice is used for the following operations:
· carbon
dioxide reduction (decarbonation)
· oxidation
of iron and manganese found in many well waters (oxidation tower)
· ammonia
and hydrogen sulfide reduction (stripping)
Aeration is also an effective
method of bacteria control.
METHODS OF AERATION
Two general
methods may be used for the aeration of water:
- The most common in industrial
use is the water-fall aerator.
Through the use of spray nozzles, the water is broken up into small droplets or
a thin film to enhance counter-current air contact.
- In the air diffusion method of aeration, air is diffused into a receiving
vessel containing counter-current flowing water, creating very small air
bubbles. This ensures good air-water contact for "scrubbing" of
undesirable gases from the water.
Water-Fall Aerators
Many
variations of the water-fall principle are used for this type of
aeration.
The simplest configuration employs a vertical riser that discharges water by free fall into a basin (Figure 4-2).
The simplest configuration employs a vertical riser that discharges water by free fall into a basin (Figure 4-2).
Figure 4-2. Multicone aerator. |
The riser usually operates on the
available head of water. The efficiency of aeration is improved as the fall
distance is increased.
Also, steps or shelves may be
added to break up the fall and spread the water into thin sheets or films,
which increases contact time and aeration efficiency.
Coke tray and wood or plastic
slat water-fall aerators are relatively similar in design and have the
advantage of small space requirements.
Coke tray aerators are widely used in iron and
manganese oxidation because a catalytic effect is secured by contact of the
iron/manganese-bearing water with fresh precipitates.
These units consist of a series
of coke-filled trays through which the water percolates, with additional
aeration obtained during the free fall from one tray to the next.
Wood or plastic slat tray
aerators are similar to small atmospheric cooling towers. The tray slats are
staggered to break up the free fall of the water and create thin films before
the water finally drops into the basin.
Forced draft water-fall aerators are used for many industrial water conditioning purposes (see Figure 4-3).
Forced draft water-fall aerators are used for many industrial water conditioning purposes (see Figure 4-3).
Horizontal wood or plastic slat
trays, or towers filled with packing of various shapes and materials, are
designed to maximize disruption of the falling water into small streams for
greater air-water contact.
Air is forced through the unit by
a blower which produces uniform air distribution across the entire cross
section, cross current or counter-current to the fall of the water.
Because of these features, forced
draft aerators are more efficient for gas removal and require less space for a
given capacity.
Air Diffusion Aerators
Air
diffusion systems aerate by pumping air into water through perforated pipes,
strainers, porous plates, or tubes.
Aeration by diffusion is
theoretically superior to water-fall aeration because a fine bubble of air
rising through water is continually exposed to fresh liquid surfaces, providing
maximum water surface per unit volume of air.
Also, the velocity of bubbles
ascending through the water is much lower than the velocity of free-falling
drops of water, providing a longer contact time. Greatest efficiency is
achieved when water flow is counter-current to the rising air bubbles.
APPLICATIONS
In industrial
water conditioning, one of the major objectives of aeration is to remove carbon
dioxide.
Aeration is also used to oxidize
soluble iron and manganese (found in many well waters) to insoluble
precipitates.
Aeration is often used to reduce
the carbon dioxide liberated by a treatment process. For example, acid may be
fed to the effluent of sodium zeolite softeners for boiler alkalinity control.
Carbon dioxide is produced as a
result of the acid treatment, and aeration is employed to rid the water of this
corrosive gas.
Similarly, when the effluents of
hydrogen and sodium zeolite units are blended, the carbon dioxide formed is
removed by aeration.
In the case of cold lime
softening, carbon dioxide may be removed from the water before the water enters
the equipment.
When carbon dioxide removal is
the only objective, economics usually favor removal of high concentrations of
carbon dioxide by aeration rather than by chemical precipitation with lime.
Air stripping may be used to
reduce concentrations of volatile organics, such as chloroform, as well as
dissolved gases, such as hydrogen sulfide and ammonia.
Air pollution standards must be
considered when air stripping is used to reduce volatile organic compounds.
Iron and Manganese Removal
Iron and
manganese in well waters occur as soluble ferrous and manganous bicarbonates.
In the aeration process, the water is saturated with oxygen to promote the
following reactions:
4Fe(HCO3)2
|
+
|
O2
|
+
|
2H2O
|
=
|
4Fe(OH)3-
|
+
|
8CO2
|
||||||||
ferrous bicarbonate
|
oxygen
|
water
|
ferric hydroxide
|
carbon
dioxide
|
||||||||||||
2Mn(HCO3)2
|
+
|
O2
|
=
|
2MnO2
|
+
|
4CO2 -
|
+
|
2H2O
|
||||||||
manganese
bicarbonate
|
oxygen
|
manganese
dioxide
|
carbon
dioxide
|
water
|
||||||||||||
The oxidation products, ferric
hydroxide and manganese dioxide, are insoluble. After aeration, they are
removed by clarification or filtration.
Occasionally, strong chemical
oxidants such as chlorine (Cl2) or potassium permanganate (KMnO4)
may be used following aeration to ensure complete oxidation.
Dissolved Gas Reduction
Gases
dissolved in water follow the principle that the solubility of a gas in a
liquid (water) is directly proportional to the partial pressure of the gas
above the liquid at equilibrium.
This is known as Henry's Law and
may be expressed as follows:
Ctotal = kP
where
Ctotal = total concentration of
the gas in solution
P = partial pressure of
the gas above the solution
k = a
proportionality constant known as Henry's Law Constant
However, the gases frequently
encountered in water treatment (with the exception of oxygen) do not behave in
accordance with Henry's Law because they ionize when dissolved in water.
For example:
H2O
|
+
|
CO2
|
«
»
|
H+
|
+
|
HCO3-
|
||||||||||
water
|
carbon
dioxide
|
hydrogen
ion
|
bicarbonate
ion
|
|||||||||||||
H2S
|
«
|
H+
|
+
|
HS-
|
||||||||||||
hydrogen
sulfide
|
hydrogen
ion
|
hydrosulfide
ion
|
||||||||||||||
H2O
|
+
|
NH3
|
«
|
NH4+
|
+
|
OH-
|
||||||||||
water
|
ammonia
|
ammonium
ion
|
hydroxide
ion
|
|||||||||||||
Carbon dioxide, hydrogen sulfide,
and ammonia are soluble in water under certain conditions to the extent of
1,700, 3,900, and 531,000 ppm, respectively.
Rarely are these concentrations
encountered except in certain process condensates.
In a normal atmosphere, the
partial pressure of each of these gases is practically zero.
Consequently, the establishment
of a state of equilibrium between water and air by means of aeration results in
saturation of the water with nitrogen and oxygen and nearly complete removal of
other gases.
As the equations above show, ionization of
the gases in water is a reversible reaction. The common ion effect may be used
to obtain almost complete removal of these gases by aeration.
If the concentration of one of
the ions on the right side of the equation is increased, the reaction is driven
to the left, forming the gas.
In the case of carbon dioxide and
hydrogen sulfide, hydrogen ion concentration may be increased by the addition
of an acid.
Bicarbonate and carbonate ions in
the water will form carbon dioxide, which can be removed by aeration.
In a similar manner, an increase
in hydroxyl ion concentration through the addition of caustic soda aids in the
removal of ammonia.
Figures 4-4, 4-5, and 4-6 show the percentage of gas removal that may
be obtained at various pH levels.
Gas removal by aeration is
achieved as the level of gas in the water approaches equilibrium with the level
of the gas in the surrounding atmosphere.
The process is improved by an
increase in temperature, aeration time, the volume of air in contact with the
water, and the surface area of water exposed to the air.
The efficiency of aeration is greater where the
concentration of the gas to be removed is high in the water and low in the
atmosphere.
LIMITATIONS
Temperature
significantly affects the efficiency of air stripping processes. Therefore,
these processes may not be suitable for use in colder climates.
Theoretically, at 68°F the carbon
dioxide content of the water can be reduced to 0.5 ppm by aeration to
equilibrium conditions.
This is not always practical from
an economic standpoint, and reduction of carbon dioxide to 10 ppm is normally
considered satisfactory.
Although removal of free carbon
dioxide increases the pH of the water and renders it less corrosive from this
standpoint, aeration also results in the saturation of water with dissolved
oxygen.
This does not generally present a
problem when original oxygen content is already high.
However, in the case of a well
water supply that is high in carbon dioxide but devoid of oxygen, aeration
simply exchanges one corrosive gas for another.
The efficiency of aeration
increases as the initial concentration of the gas to be removed increases above
its equilibrium value.
Therefore, with waters containing
only a small amount of carbon dioxide, neutralization by alkali addition is
usually more cost-effective.
The complete removal of hydrogen
sulfide must be combined with pH reduction or chemical oxidation.
Nonvolatile organic compounds
cannot be removed by air stripping. For example, phenols and creosols are
unaffected by the aeration process alone.
source:
gewater.com
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