chapter: 5 aeration - marmara...
TRANSCRIPT
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ENVE 301
Environmental Engineering Unit Operations
CHAPTER: 5
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Assist. Prof. Bilge Alpaslan Kocamemi
Marmara UniversityDepartment of Environmental Engineering
Istanbul, Turkey
Aeration
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Aeration
Removal of dissolved gases (degasification)
CO2
Primary applications in water treatment
High solubility of CO2 reduces the pH of water which causes excessive consumption of lime or other neutralizing
agents in coagulation and softening process. The corrosiveness of water is also higher at lower pH values.
Exposure of water droplets to air for 2sec will lower the CO2 by 70-90%
CO2 > 10mg/L aeration is recommended
Otherwise lime addition should be used to neutralize the CO
CH4 Volatile organics
taste &odor causing volatile compounds
H2S (highly soluble in water) 2
not an efficient method for removing the taste & odor compounds produced by algae
because the algal oils causing taste and odor not volatile to a significant extent.
Otherwise lime addition should be used to neutralize the CO2
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H2S poisoning is one of the lauding causes of accidents in the field.
5ppm: Moderate odor.
10ppm: Eye irritation begins
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Hazardous levels 30ppm: Strong , unpleasant odor of rotten egg
100ppm: Loss of smell
>300ppm: Unconsciousness, death
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oxygenation of water (gasification, absorption)
Removal of iron (oxidation) (gasification, absorption)
Primary application of aeration in water treatment (continue) :
Primary application of aeration in waste water treatment:
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To supply oxygen to aeorobic biological treatment processes
Air stripping to remove toxic volatile organicsAir stripping to remove volatile compounds
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Aerators
Gravity
Aerators
Spray
Aerators
Diffused Air
Aeration
Systems
Mechanical
Aerators
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e.g.
Cascade
aerators
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Cascade Aerator (Gravity Aerator)
Water falls down a series of steps
utilize the potential energy of water to create
interfaces for efficient gas transfer
the splashing of the water creates the splashing of the water createsturbulence and water droplets
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Not efficient compared to other aeration methods
Require little space [50 200 m2/ (m3/sec) ]
Ref: Tchobanoglous and Scroeder, 1985, Addison-Wesley Publishing Company
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Cascade Aerator (Continue)
1st Mechanism (effective for degasification)
Nappe
)freefall(gt2
1h
2=
average time of exposure
of the surface A to air g
h2=
7
t=
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2nd Mechanism
(effective for gasification, e.g., oxygenation)
When the nappe submerge
into the receiving body of
water
significant amounts
of air is entrained.
Nappe
2 of air is entrained.
The entrained air is then
dispersed in the form of
bubbles throughout the
receiving body of water,
which leads to an intense
transfer of gases.
Tail
water
gh2
g2h
2
=
=
8
h3
2
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Cascade Aerator (continue)
The amount of air entrained depends on;
velocity of nappe when passing the surface of tail water
depth of tail water
Depth should be choosen such that ;
Nappe
h2 Depth should be choosen such that ;
Final vel. of jets Rising vel.
within the tail water = of bubbles
before reaching the produced
tank bottom
tail water depth (2/3) h9
Tail
water
gh2
g2h
2
=
=
h3
2
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Subdivision into several steps & decreasing hpromotes DESORPTION OF GAS
each step leads to formation of new interfacial area
Decreasing the number of steps & increasing h promotes SORRPTION OF GAS
10
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coeff. 1C-C 0 efficiencyKe tLaK ===
For gasification (sorption of gas)
tLaK
os
se
CC
CC =
tLaK
os
s eCC
CC =
11 )(
11
coeff. 1C 0s
0 efficiencyKeC
La ===
( )
0s0
0s0
KCKCCC
CCKCC
=
=
s0KCKCoCC +=
( ) s0 KCK1CC +=
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If the height of weir being divided in n equal steps each having an efficiency
coeff.
( ) snn01 CKK1CC +=
( ) snn12 CKK1CC +=
( ) snn23 CKK1CC +=..
( )n
0ssn n
K1CCCC
=
.
.
K unpolluted water = 0.45(1+0.046T)h
polluted water = 0.36(1+0.046T)h
sewage = 0.29(1+0.046T)h 12
T=temperature, oC
h= m
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Ref: Ppel, 1979, Delft University
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EXAMPLE 1: Raw water with 2g O2/m3 and a temperature of 100C is passed
over a straight weir of a height of 0,65m. Estimate the downstream oxygen
content.
For unpolluted water:
K=0.45(1+0.046T)h ( )n
3s
0
K
m/g3.11C C10@
=
K=0.45(1+0.046T)h
K=0.45(1+0.046.10)0.65
K=0.427
( )
( ) 31
n
0ss
m/g97.51
427.0123.113.11C
1n
n
K1CCCC
=
=
=
=
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EXAMPLE 2: Determine the number of steps of a cascade to achieve
maximum oxygenation , assuming an available head of 1.5m , an efficiency
coefficient K depending on the weir height h as stated by Fig 3.1 given
below
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3s
m/g2C m/g10C ==
One step:
K7.0
7K 7KC 1.3.Fig from m5.1h =====
15
( )( )( )
31
11
n
0ss1
m/g6.7C
7.0121010C
n
K1CCCC
==
=
n
K7.0
10
7K 7KC 1.3.Fig from m5.1h
s=====
Ref: Ppel, 1979, Delft University
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Two steps:
( )( )( )
32
22
n
0ss2
m/g8C
5.0121010C
n
K1CCCC
==
=
n
K5.0
10
5K 5KC graph from m75.0
2
5.1h
s======
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Three steps:
( )( )( )
33
33
n
0ss3
m/g8.7C
35.0121010C
n
K1CCCC
==
=
35.010
5.3
n
KK 5.3KC graph from m5.0
3
5.1h
s======
Ref: Ppel, 1979, Delft University
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Spray Aerator
applied in the course of water treatment for absorption of oxygen and/or desorption of gases
water is distributed into air in the form of small droplets by means of
orifices
nozzles
mounted on a stationary pipe system
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Ref: Metcalf Eddy,1991 , McGraw Hill
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orifices and nozzles may be costructed to discharge water
vertically
at an angle
in upward or downward direction
Spray Aerator (Continue)
Require relatively large area to collect
the water
30
Ref: Metcalf Eddy,1991 , McGraw Hill
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Diffused Air Aeration Systems
air is introduced into liquid being aerated in the form of bubbles whichtypically rise through the liquid
common device for ;
transferring oxygen in aerobic biological treatment systems
air stripping of volatile organics
the size of bubbles varies from coarse to fine
fine-bubble diffusers
coarse bubble diffusers
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air stripping of volatile organics
COARSE BUBBLE
FINE BUBBLE
Ref: http://www.brightwaterfli.com/diffused_aeration_systems.htm
Ref:http://www.hellotrade.com/diffused-gas-technologies-incorporated/ss-series-plenum-coarse-bubble-diffusers.html
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Diffused Air Aeration Systems (continue)
gas transfer rate size of bubbles
smaller bubbles greater A/ more efficient than larger sizedbubbles for mass transfer
Porous diffusers (e.g., plate, dome, disc, tubular diffusers)
Nonporous diffusers (e.g., fixed orifice, valved orifice)
Other diffusers (e.g., jet aeration)32
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DOME DIFFUSER DOME DIFFUSER DISC DIFFUSER
Typical Porous Diffusers
Diffused Air Aeration Systems (continue)
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Dome, disc diffusers are mounted on or screwed into air manifolds
Ref: Metcalf & Eddy, 1991, McGraw Hill
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Typical Non Porous Diffusers
Valved orifice diffuser Perforated tube diffuser
Diffused Air Aeration Systems (continue)
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Produce larger bubbles than porous diffusers
Lower aeration efficiency
Lower cost, less maintanance
Ref: Metcalf & Eddy, 1991, McGraw Hill
VALVED ORIFICE DIFFUSER
(non porpous diffuser)
Device that contains a check value to
prevent backflow when air is shut off.
Mounts on air distribution piping.
Ref: Metcalf & Eddy, 1991, McGraw Hill
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Jet aerator
Aspirating aerators
Typical Other Diffusion Devices
Diffused Air Aeration Systems (continue)
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Jet aerator
discharges a mixture of pumped liquid and
compressed air through a nozzle.
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36Ref: Metcalf Eddy,1991 , McGraw Hill
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MECHANICAL AERATORS
By producing a large air-water interface the transfer of oxygen fromatmosphere is enhanced
Can be VERTICAL SHAFT or HORIZONTAL SHAFT
37Ref: http://www.waterandwastewater.com/www_services/newsletter/april_18_2011.htm
Ref: http://en.wikipedia.org/wiki/File:Surface_Aerator.jpg