CHAPTER: 5 Aeration

37 Pages·2012·8.3 MB·English

ENVE 301
Environmental Engineering Unit Operations
CHAPTER: 5
Aeration
Assist. Prof. Bilge Alpaslan Kocamemi
Marmara University
Department of Environmental Engineering
1
Istanbul, Turkey
Aeration
Primary applications in water treatment
→
Removal of dissolved gases (degasification)
•
CO
2
High solubility of CO reduces the pH of water which causes excessive consumption of lime or other neutralizing
2
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 CO by 70-90%
2
CO > 10mg/L (cid:1)aeration is recommended
2
OOtthheerrwwiissee lliimmee aaddddiittiioonn sshhoouulldd bbee uusseedd ttoo nneeuuttrraalliizzee tthhee CCOO
2
•
CH
4
•
Volatile organics
•
taste &odor causing volatile compounds
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.
•
H S (highly soluble in water)
2
2
H S poisoning is one of the lauding causes of accidents in the field.
2
5ppm: Moderate odor.
10ppm: Eye irritation begins
Hazardous levels 30ppm: Strong , unpleasant odor of rotten egg
100ppm: Loss of smell
>300ppm: Unconsciousness, death
3
Primary application of aeration in water treatment (continue) :
•
oxygenation of water (gasification, absorption)
•
Removal of iron (oxidation) (gasification, absorption)
Primary application of aeration in waste water treatment:
→
To supply oxygen to aeorobic biological treatment processes
→
Air stripping to remove toxic volatile organics
→
Air stripping to remove volatile compounds
4
Aerators
Gravity Spray Diffused Air Mechanical
Aerators Aerators Aeration Aerators
Systems
e.g.
Cascade
aerators
5
Cascade Aerator (Gravity Aerator)
→
Water falls down a series of steps
utilize the potential energy of water to create
interfaces for efficient gas transfer
→→
tthhee ssppllaasshhiinngg ooff tthhee wwaatteerr ccrreeaatteess
turbulence and water droplets
Ref: Tchobanoglous and Scroeder, 1985, Addison-Wesley Publishing Company
→
Not efficient compared to other aeration methods
→
Require little space [50 –200 m2/ (m3/sec) ]
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Cascade Aerator (Continue)
1st Mechanism (effective for degasification)
Nappe
1
2
=
h gt (freefall )
2
2h
average time of exposure
=
t=
of the surface A to air
g
7
2nd Mechanism
(effective for gasification, e.g., oxygenation)
Nappe When the nappe submerge
into the receiving body of
water
significant amounts
ooff aaiirr iiss eennttrraaiinneedd..
22
Tail ‡
h
water 3
The entrained air is then
n 2 dispersed in the form of
=
h
2g bubbles throughout the
n = receiving body of water,
2gh
which leads to an intense
transfer of gases. 8
Cascade Aerator (continue)
The amount of air entrained depends on;
Nappe
→
velocity of nappe when passing the surface
of tail water
→
depth of tail water
22 DDeepptthh sshhoouulldd bbee cchhoooosseenn ssuucchh tthhaatt ;;
‡‡
hh
Tail
3
water Final vel. of jets Rising vel.
within the tail water = of bubbles
n 2
=
h before reaching the produced
2g
tank bottom
n =
2gh
‡ ‡‡‡
tail water depth (2/3) h
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(cid:1)
Subdivision into several steps & decreasing h promotes DESORPTION OF GAS
each step leads to formation of new interfacial area
(cid:1)
Decreasing the number of steps & increasing h promotes SORRPTION OF GAS
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