CHAPTER: 5 Aeration - Marmara Üniversitesimimoza.marmara.edu.tr/~bilge.alpaslan/enve301/Lectures/Chp_5.pdf · Primary applications in water treatment High solubility of CO 2reduces

ENVE 301

Environmental Engineering Unit Operations

CHAPTER: 5

1

Assist. Prof. Bilge Alpaslan Kocamemi

Marmara UniversityDepartment of Environmental Engineering

Istanbul, Turkey

Aeration

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

H2S poisoning is one of the lauding causes of accidents in the field.

5ppm: Moderate odor.

10ppm: Eye irritation begins

3

Hazardous levels 30ppm: Strong , unpleasant odor of rotten egg

100ppm: Loss of smell

>300ppm: Unconsciousness, death

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:

4

To supply oxygen to aeorobic biological treatment processes

Air stripping to remove toxic volatile organicsAir stripping to remove volatile compounds

Aerators

Gravity

Aerators

Spray

Aerators

Diffused Air

Aeration

Systems

Mechanical

Aerators

5

e.g.

Cascade

aerators

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

6

Not efficient compared to other aeration methods

Require little space [50 200 m2/ (m3/sec) ]

Ref: Tchobanoglous and Scroeder, 1985, Addison-Wesley Publishing Company

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=

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

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

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

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 +=

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

13

Ref: Ppel, 1979, Delft University

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

=

=

=

=

14

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

30

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=====


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======

16

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======


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

29

Ref: Metcalf Eddy,1991 , McGraw Hill

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

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

31

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

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

DOME DIFFUSER DOME DIFFUSER DISC DIFFUSER

Typical Porous Diffusers


33

Dome, disc diffusers are mounted on or screwed into air manifolds

Ref: Metcalf & Eddy, 1991, McGraw Hill

Typical Non Porous Diffusers

Valved orifice diffuser Perforated tube diffuser


34

Produce larger bubbles than porous diffusers

Lower aeration efficiency

Lower cost, less maintanance


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.


Jet aerator

Aspirating aerators

Typical Other Diffusion Devices


35

Jet aerator

discharges a mixture of pumped liquid and

compressed air through a nozzle.

36Ref: Metcalf Eddy,1991 , McGraw Hill

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

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CHAPTER: 5 Aeration - Marmara Üniversitesimimoza.marmara.edu.tr/~bilge.alpaslan/enve301/Lectures/Chp_5.pdf · Primary applications in water treatment High solubility of CO 2reduces