Gas Transfer in Recirculating Aquaculture Systems

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Gas Transfer in Recirculating Aquaculture Systems

• Raul H. Piedrahita, Ph.D.
• Biological and Agricultural Engineering
• University of California, Davis

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Topics

• Basic principles
• Gas transfer
• General design procedures

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Basic principles

• Concentration of gases in solution may be the
  water quality-limiting factor in recirculation
  aquaculture systems (RAS)

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Basic principles

• Concentration of gases in solution may be the
  water quality-limiting factor in recirculation
  aquaculture systems (RAS)
• Common problems with make-up water
• Oxygen (O2)
• Carbon dioxide (CO2)
• Nitrogen (N2) and Argon (Ar) (total gas pressure,
  or TGP)
• ...

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Basic principles

• Concentration of gases in solution may be the
  water quality-limiting factor in recirculation
  aquaculture systems (RAS)
• Common problems with culture water
• Oxygen (O2)
• Carbon dioxide (CO2)

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Basic principles

• Oxygen
• Consumed by fish and microorganisms
• 0.3-0.5 g O2/g feed
• Must be replenished oxygenation or aeration

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Basic principles

• Carbon Dioxide
• Produced by fish and microorganisms
• 0.4-0.7 g CO2 / g feed (1 mole CO2/mole O2)
• Must be reduced pH control and/or degassing

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Basic principles

• Saturation concentration of gas i is a function
  of
• the gas, temperature (T) and salinity(S)
• pressure (P)
• gas content in the "atmosphere" (Xi)
• ...

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Basic principles

• Saturation concentration of gas i is

Cs,i saturation concentration, mg/L Ki gas
"density", g/L, 1.429 for O2 and 1.977 for CO2
bi Bunsen coefficient, L/L-atm Xi mole
fraction in gas phase PBP barometric pressure,
mmHg Pwv vapor pressure of water, mmHg
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Basic principles-oxygen solubility
Situation XO2 PBP Pwv Cs,O2
Sea level, air, FW, 15C 0.209 760 12.79 10.072
Sea level, air, FW, 25C 0.209 760 23.77 8.244
FWfresh water SW sea water. Units XO2,
fraction by volume pressures, mmHg Cs,O2, mg/L.
After Colt, J. 1984
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Basic principles-solubility equilibrium between
gas and liquid
Mole fraction pressure
gas phase
Temperature salinity pressure
water
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Basic principles-oxygen solubility
Situation XO2 PBP Pwv Cs,O2
Sea level, air, FW, 15C 0.209 760 12.79 10.072
Sea level, air, SW, 15C 0.209 760 12.55 8.129
FWfresh water SW sea water. Units XO2,
fraction by volume pressures, mmHg Cs,O2, mg/L.
After Colt, J. 1984
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Basic principles-oxygen solubility
Situation XO2 PBP Pwv Cs,O2
Sea level, air, FW, 15C 0.209 760 12.79 10.072
1600 m, air, FW, 15C 0.209 631 12.79 8.328
FWfresh water SW sea water. Units XO2,
fraction by volume pressures, mmHg Cs,O2, mg/L.
After Colt, J. 1984
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Basic principles-oxygen solubility
Situation XO2 PBP Pwv Cs,O2
Sea level, air, FW, 15C 0.209 760 12.79 10.072
Sea level, pure O2, FW, 15C 1.00 760 12.79 48.19
FWfresh water SW sea water. Units XO2,
fraction by volume pressures, mmHg Cs,O2, mg/L.
After Colt, J. 1984
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Basic principles-oxygen solubility
Situation XO2 PBP Pwv Cs,O2
Sea level, air, FW, 15C 0.209 760 12.79 10.072
1 atm, pure O2, FW, 15C 1.00 1520 12.79 96.38
gauge pressure
FWfresh water SW sea water. Units XO2,
fraction by volume pressures, mmHg Cs,O2, mg/L.
After Colt, J. 1984
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Basic principles-CO2 solubility
Situation XCO2 PBP Pwv Cs,CO2
Sea level, air, FW, 15C 0.00038 760 12.79 0.76
Sea level, air, FW, 25C 0.00038 760 12.79 0.57
2006 value and rising... NOAA, 2006.
FWfresh water SW sea water. Units XCO2,
fraction by volume pressures, mmHg Cs,CO2,
mg/L.
After Weiss, R.F. 1974
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Basic principles - supersaturation

• Potential supersaturation caused by
• a temperature increase (water heating)
• Potential problem
• a pressure increase (e.g. caused by a pump)
• gas enrichment (e.g. pure oxygen use)

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Basic principles - supersaturation

• Potential supersaturation caused by
• a temperature increase (water heating)
• a pressure increase (e.g. caused by a pump)
• Potential problem
• gas enrichment (e.g. pure oxygen use)

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Basic principles - supersaturation

• Potential supersaturation caused by
• a temperature increase (water heating)
• a pressure increase (e.g. caused by a pump)
• gas enrichment (e.g. pure oxygen use)
• Used for pure oxygen injection

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Basic principles - pure O2

• Enriched O2 increases DO solubility
• Typically can have larger stocking densities than
  if air is used
• Less water needs to be oxygenated to add a given
  amount of oxygen
• CO2 can build up when pure O2 is used

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Basic principles - gas sources

• Air blowers

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Basic principles - gas sources

• Oxygen Transfer Systems

Oxygen - On-site generation - Liquid O2
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Basic principles - oxygen sources

• Enriched O2 can be produced on site using
  pressure swing absorption (PSA) equipment
• 85 to 95 purity
• requires PSA unit and
• air dryer,
• compressor to produce 90 to 150 psi,
• stand-by electrical generator.
• consumes about 1.1 kWh of electricity per kg O2
  produced.

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Basic principles - oxygen sources

• Enriched O2 can be purchased as a bulk liquid
  (LOX)
• 98 to 99 purity
• capital investment and risk are lower than PSA
• liquid O2 cost is highly location-specific
• LOX continues to be available if there is a power
  failure

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Gas transfer - rate

• Depends on
• the difference between the concentration in water
  (Ci) and saturation concentration (Cs,i)
• If Ci gt Cs,i (supersaturation) gas i will move
  from the water to the "atmosphere" degassing
• If Ci lt Cs,i (undersaturation) gas i will move
  from the "atmosphere" to the water
• the area of contact between the water and the
  "atmosphere"
• Diffusivity turbulence

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Gas transfer - rate

• Depends on
• the difference between the concentration in water
  (Ci) and saturation concentration (Cs,i)
• the area of contact between the water and the
  "atmosphere"
• increase by splashing the water or creating small
  bubbles
• Diffusivity turbulence

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Gas transfer - rate

• Depends on
• the difference between the concentration in water
  (Ci) and saturation concentration (Cs,i)
• the area of contact between the water and the
  "atmosphere"
• Diffusivity turbulence
• increase turbulence

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Gas transfer - devices

• Continuous liquid phase (bubbles in water)
• Bubble diffusers
• U-tubes
• Oxygenation cones (downflow bubble contactors)
• Oxygen aspirators/injectors
• ...

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Gas transfer - devices

• Airstones
• very inefficient (lt10 transfer efficiency)
• useful for emergency oxygenation
• used with air in airlift pumps

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U-Tube
Gas transfer - devices
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Gas transfer - devices

• U-tube
• down flow water velocity of 2 to 3 m/s
• depth usually gt 10 m
• does not vent N2 or CO2 effectively
• can achieve concentrations gtgt 40 mg/L
• transfer efficiency 50-80
• low pumping costs (low hydraulic head)
• construction costs site dependent
• limit gas flow to lt 25 of water flow

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Gas transfer - devices
Downflow bubble contactorOxygenation cone
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Gas transfer - devices

• Downflow bubble contactor
• widely used in Europe
• resists solids plugging
• can achieve concentrations gtgt 40 mg/L
• transfer efficiency can approach 100
• does not vent N2 or CO2 well

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Gas transfer - devices

• Oxygen aspiration/injection

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Gas transfer - devices

• Continuous gas phase (water drops in air)
• Packed or spray columns
• Multi-staged low head oxygenators (LHO)
• ...

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Gas transfer - devices

• Packed or spray columns

Gas out
Water in
Gas in
Water out
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Gas transfer - devices

• Packed or spray columns
• predictable performance
• can resist solids plugging
• can be used with air or oxygen
• can remove N2 and CO2 if used with air
• can be pressurized
• transfer efficiency can approach 100

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Gas transfer - devices

• Low head oxygenators - LHO

O2 in
off-gas
sump tank
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Gas transfer - devices

• LHOs
• effective O2 absorption with a low water drop
• degas N2 (but not CO2) while adding O2
• ratio of oxygen gaswater flow 0.5-2
• transfer efficiency drops rapidly for GLgt2
• "compact" and suitable for combining with PCA for
  degassing CO2

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Gas transfer - devices
CO2 Stripping
LHO
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Background - CO2

• CO2 is part of the carbonate system and its
  concentration depends on
• alkalinity (Alk meq/L, mg/L as CaCO3)
• total carbonate carbon (dissolved inorganic
  carbon) (CTCO3 mmol/L)
• pH
• temperature
• salinity

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Background - CO2

• The carbonate system
• H2CO3 ? HCO3 H Ka,1
• HCO3 ? CO3 H Ka,2
• where H2CO3 H2CO3 CO2 "free CO2"

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Background - CO2

• H2CO3 aH2CO3 . CTCO3
• or
• where

Alkc HCO3 2CO3 OH H
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Background - CO2
What it means
Can change the free CO2 concentration by changing
the pH
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Background - CO2
For freshwater at 25 C
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Background - CO2

• Its concentration can be reduced by degassing or
  by raising the pH

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Background - CO2

• If it is reduced by degassing
• pH rises
• CTCO3 concentration drops
• alkalinity does not change

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Degassing
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Background - CO2

• If it is reduced by raising the pH
• the aH2CO3 drops as the pH rises
• the concentration of CTCO3 does not change
• alkalinity increases due to the base addition

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Addition of a strong base (e.g. NaOH)
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Design principles

• Oxygenation(gO2/d) and CO2 reduction (gCO2/d)
  needed, based on
• feed (gfeed/gfish/d)
• physiology (gO2/gfeed, mgO2/L, gCO2/gfeed,
  mgCO2/L)
• mass balances, water make up rate, other
  processes
• treatment method?
• configuration and place in the treatment sequence
• preliminary calculations
• details

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Design principles

• Physiology
• Oxygen consumption and CO2 production data are
  scarce, especially for fish under commercial
  culture conditions
• If no detailed information is available, use
  generic values, such as
• 0.2-0.3 kg O2/kg of feed
• 1 kg O2/kg of feed
• respiratory quotient of 1mol CO2/mol O2

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Design principles

• Physiology
• Oxygen consumption and CO2 production data are
  scarce, especially for fish under commercial
  culture conditions
• If no detailed information is available, use
  generic values, such as
• 0.3-0.5 kg O2/kg of feed if solids are removed
  and biofilter oxygen demand is supplied/accounted
  for separately
• 1 kg O2/kg of feed
• respiratory quotient of 1mol CO2/mol O2

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Design principles

• Physiology
• Oxygen consumption and CO2 production is scarce,
  especially for fish under commercial culture
  conditions
• If no detailed information is available, use
  generic values, such as
• 0.2-0.5 kg O2/kg of feed
• up to 1 kg O2/kg of feed if solids tend to
  accumulate in the system and biofilter oxygen
  demand is not supplied/accounted for separately
• respiratory quotient of 1mol CO2/mol O2

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Design principles

• Physiology
• Oxygen consumption and CO2 production is scarce,
  especially for fish under commercial culture
  conditions
• If no detailed information is available, use
  generic values, such as
• 0.2-0.5 kg O2/kg of feed
• 1 kg O2/kg of feed
• oxygen consumption values and a respiratory
  quotient of 1 mol of CO2 produced/mol of O2
  consumed, or 1.4 kg of CO2/kg of O2

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Design principles

• Oxygenation (gO2/d) and CO2 reduction (gCO2/d)
  required
• treatment method?
• for O2 aeration, oxygenation, ...
• for CO2 degassing, base addition
• configuration and place in the treatment sequence
• preliminary calculations
• details

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Design principles

• Oxygenation (gO2/d) and CO2 reduction (gCO2/d)
  required
• treatment method?
• configuration and place in the treatment sequence
• system configuration
• sequence
• preliminary calculations
• details

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Design principles

• Oxygenation (gO2/d) and CO2 reduction (gCO2/d)
  required
• treatment method?
• configuration and place in the treatment sequence
• preliminary calculations
• O2 flow rates, concentrations, liquid oxygen
  consumption, ...
• CO2 flow rates, concentrations, chemical product
  consumption, ventilation, ...
• details

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Design principles

• Oxygenation (gO2/d) and CO2 reduction (gCO2/d)
  required
• treatment method?
• configuration and place in the treatment sequence
• preliminary calculations
• details
• equipment, design, alarms, back-up systems

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Design principles - precautions

• Use high GL ratios for degassing and low values
  for oxygenation
• G gas flow rate (L/min)
• L water flow rate (L/min)
• Avoid introducing air under pressure
• Choose the bases carefully taking into account
  the chemistry of the water to be treated
• Take into account metabolism fluctuations

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Design principles - precautions

• Use high GL ratios for degassing and low values
  for oxygenation
• Avoid introducing air under pressure
• it could cause supersaturation
• Choose the bases carefully taking into account
  the chemistry of the water to be treated
• Take into account metabolism fluctuations

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Design principles - precautions

• Use high GL ratios for degassing and low values
  for oxygenation
• Avoid introducing air under pressure
• Choose the bases carefully taking into account
  the chemistry of the water to be treated
• pH changes
• alkalinity and total carbonate carbon changes
• Take into account metabolism fluctuations

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Design principles - precautions

• Use high GL ratios for degassing and low values
  for oxygenation
• Avoid introducing air under pressure
• Choose the bases carefully taking into account
  the chemistry of the water to be treated
• Take into account metabolism fluctuations
• design for mean rates with safety factor
• design to respond to rate changes
• design for peak rates

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Design principles - layouts
O2 added and N2 and CO2 removed from influent
water
Influent
Effluent
O2
N2 and CO2
Useful to increase O2 and reduce excessive N2 and
CO2 in water supply
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Design principles - layouts
O2 addition and CO2 reduction in recycled water
Influent
Effluent
O2
and/or CO2 transformation through chemical
addition
CO2 removal through degassing
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Design principles - layouts
or
Influent
Effluent
O2
and/or CO2 transformation through chemical
addition
CO2 removal through degassing
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Design principles - layouts
Other Treatment
or
Influent
Effluent
O2
and/or CO2 transformation through chemical
addition
CO2 removal through degassing
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Design principles - layouts
or
Influent
Effluent
Other Treatment
O2
and/or CO2 transformation through chemical
addition
CO2 removal through degassing
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Challenges

• Fish physiology
• metabolic rates
• "safe" concentrations, especially for CO2
• consequence of non-optimum conditions
• Technology
• reduce costs
• improve CO2 control technologies
• improve analytical methods for CO2

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