Transcript OXYGENATION Brian Vinci, Ph.D. Steven Summerfelt, Ph.D. The Freshwater Institute, Shepherdstown, WV

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OXYGENATION
Brian Vinci, Ph.D.
Steven Summerfelt, Ph.D.
The Freshwater Institute, Shepherdstown, WV
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Carrying Capacity
Issues

Fish require O2 for respiration:
0.3-0.5 kg O2 consumed per 1.0 kg feed;
DO  4-6 mg/L can reduce growth.
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Carrying Capacity
Issues

Methods that increase carrying capacity
allow feed rates to be increased


because more DO is available or
TAN, CO2, spatial, or other limitations are
reduced
are used to increase production

if profitability is also increased
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Carrying Capacity Issues

EXAMPLE
Increasing inlet DO from 10 to 18 mg/L




assuming an outlet DO of 6 mg/L and no other
limitations
TRIPLES the available DO and carrying capacity
TRIPLES the potential production
TRIPLES the concentration of waste produced
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Carrying Capacity Issues

Intensification with oxygenation &
aeration is limited!
Every 10 mg/L DO consumed adds:
 1.0-1.4 mg/L TAN
 13-14 mg/L CO2
 10-20 mg/L TSS
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Carrying Capacity Issues

CO2 becomes limiting:
cumulative DO consumption > 10-22 mg/L

depending on pH, alkalinity, temp., species
without stripping or pH control
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Gas Transfer

AERATION -- Air is contacted with water.
dissolved gases approach equilibrium with the
partial pressures in the atmosphere.
air
O2
water
air
air
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Oxygenation

Purified O2 gas is contacted with water;
dissolved O2 super-saturation produced
some N2 gas is stripped.
O2
O2
O2
water
O2
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
Gas Transfer
When only aeration is used to provide O2:
fish loading levels are relatively low;
air-water contact strips CO2 and avoids toxic
accumulations (Speece, 1973).
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
Gas Transfer
The rate of gas transfer depends on:
gas-water interfacial area
rate of surface film renewal
concentration gradient
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
Gas Transfer
gas-water interfacial area is increased by:
using packing,
creating fine bubbles/droplets.

rate of surface film renewal is increased by:
creating more turbulence
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Gas Transfer

Driving force for gas transfer out of water
= concentration gradient
= {(bulk conc.) - (saturation conc.)}
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Gas Transfer

Increase concentration gradient with:
methods to increase saturation concentration
 pure O2 feed gas
 pressurized systems
increasing G:L
 keeps gas-phase partial pressures from large changes
across transfer unit
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Oxygenation

Enriched O2 increases DO solubility nearly
5-fold compared to air.
 48.1 mg/L vs. 10.1 mg/L (@ 15ºC)

Increasing pressure from 1 to 2 atm doubles
the DO solubility.
 97 mg/L vs 48 mg/L (@ 15ºC)
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Oxygen Source - PSA

Enriched O2 can be produced on site using
pressure swing adsorption (PSA) equipment:
85 to 95% purity
requires PSA unit and
 air dryer,
 compressor to produce 90 to 150 psi,
 stand-by electrical generator.
costs about 1.1 kWh of electricity per kg O2
produced.
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Oxygen Source - LOX

Enriched O2 can be purchased as a bulk liquid:
98 to 99% purity
Capital investment and risk are lower than PSA,
Annual liquid O2 cost can be 3-times > PSA O2
 Location specific
 Transportation costs are a MAJOR component of the total
LOX cost
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Oxygenation Devices

Oxygen transfer equipment used:
Continuous liquid phase (bubbles in water)
 U-tubes,
 Oxygenation cones (downflow bubble contactors),
 Oxygen aspirators,
 Bubble diffusers,
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
Oxygenation
Oxygen transfer equipment used:
Continuous gas phase (water drops in air)
 Multi-staged low head oxygenators (LHO),
 Packed or spray columns,
 Pressurized columns,
 Enclosed mechanical surface mixers.
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
Oxygenation
Three units used to oxygenate large flows within
recirculating systems:
 Multi-stage low head oxygenators (LHO).
 U-tubes,
 Oxygenation cone (down flow bubble contactor)

Advantages:
 Readily scaled-up,
 Easy to control,
 Modest hydraulic head w/ good O2 adsorption eff.
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Low Head Oxygenators
flow
oxygen
feed gas
sump tank
off-gas
vent
flow
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Low Head Oxygenators

LHO’s and solids:
use without packing
construct within cone
bottom cylinders
 avoid sludge build-up
 reduce foot print
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Multi-Stage LHO

Maximize O2 adsorption efficiency:
reuse O2 through a series of chambers;
reduces gas short-circuiting.
LHO units degas N2 while adding O2
 Oxygen Gas:Water Flow – 0.5-2%
 Hydraulic Loading – 50-100 gpm/ft2

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LHO Chambers
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Multi-Stage LHO
O2 Transfer experiments in cold water (12-17ºC).
Oxygen Gas Vol. to O2 Transfer
Added Liq. Vol. Efficiency
> 8 mg/L
< 0.01:1
60-90%
~15 mg/L
0.02:1
50%
Reference
Dwyer & Peterson, 1993
Wagner et al., 1995
Weber et al., 1995
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100.00%
Absorption Efficiency (%)
90.00%
80.00%
70.00%
60.00%
50.00%
G/L = 0.5%
G/L = 1%
G/L = 2%
40.00%
1
2
3
4
5
6
7
8
9
10
Number of Chambers
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100.00%
24
90.00%
22
80.00%
20
70.00%
18
60.00%
16
50.00%
14
DOout
Absorption Efficiency
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Absorption
Efficiency
40.00%
12
DOout
30.00%
10
0.5
1
1.5
2
2.5
3
3.5
G/L Ratio
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Stacked CO2 Stripping and LHO
CO2 Stripping
LHO
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feed gas
U-Tube
Oxygenator
off-gas
recycling
mechanism
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U-Tube Oxygenator

Diffuse O2 into down-pipe:
water velocity of 1.0 to 3.0 m/s,
entrain the bubbles down,
buoyant velocity of bubbles = 0.3 m/s
O2 transfer increases as flow passes 10-45 m
depths.
 Does not vent N2 effectively.

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U-Tube Oxygenator

20-40 mg/L outlet DO can be achieved, but
 O2 adsorption of only 30-50%,
 off-gas recycling improves adsorption to 55-80%.

Only1-6 m of water head required to operate.
 larger pipes with large flows have lower water head
requirements than smaller pipes.
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Oxygenation Cone
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Oxygen
Cones
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Oxygenation Cone

Also called a down flow bubble contactor:
widely used in European eel farms ;
some tilapia farms;
resists solids plugging;
 can be pressurized to obtain 20-40 mg/L
oxygen concentrations,

does not vent N2 well when pressurized
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Downflow
Bubble
Contactor
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Guidelines for O2 & CO2 Control

Strip CO2 after it reaches it highest
level and before O2 supersaturations
are produced:
after biofilter.
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Guidelines for O2 & CO2 Control

Air strip before the oxygenation unit:
air stripping elevates DO to ~90% saturation level
pure O2 should only go toward DO supersaturation
don’t waste pure O2 to add DO at levels <
saturation
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Guidelines for O2 & CO2 Control

Produce DO supersaturation just before the
water enters the culture tank:
keep the supersaturated DO from atmospheric
contact.
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