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Gas Transfer in Recirculating Aquaculture Systems

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Raul H. Piedrahita, Ph.D. Biological and Agricultural Engineering University of California, Davis Topics Basic principles Gas transfer General design procedures Basic ... – PowerPoint PPT presentation

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Title: Gas Transfer in Recirculating Aquaculture Systems


1
Gas Transfer in Recirculating Aquaculture Systems
  • Raul H. Piedrahita, Ph.D.
  • Biological and Agricultural Engineering
  • University of California, Davis

2
Topics
  • Basic principles
  • Gas transfer
  • General design procedures

3
Basic principles
  • Concentration of gases in solution may be the
    water quality-limiting factor in recirculation
    aquaculture systems (RAS)

4
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)
  • ...

5
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)

6
Basic principles
  • Oxygen
  • Consumed by fish and microorganisms
  • 0.3-0.5 g O2/g feed
  • Must be replenished oxygenation or aeration

7
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

8
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9
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)
  • ...

10
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
11
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
12
Basic principles-solubility equilibrium between
gas and liquid
Mole fraction pressure
gas phase
Temperature salinity pressure
water
13
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
14
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
15
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
16
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
17
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
18
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)

19
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)

20
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

21
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

22
Basic principles - gas sources
  • Air blowers

23
Basic principles - gas sources
  • Oxygen Transfer Systems

Oxygen - On-site generation - Liquid O2
24
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.

25
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26
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

27
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28
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

29
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

30
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

31
Gas transfer - devices
  • Continuous liquid phase (bubbles in water)
  • Bubble diffusers
  • U-tubes
  • Oxygenation cones (downflow bubble contactors)
  • Oxygen aspirators/injectors
  • ...

32
Gas transfer - devices
  • Airstones
  • very inefficient (lt10 transfer efficiency)
  • useful for emergency oxygenation
  • used with air in airlift pumps

33
U-Tube
Gas transfer - devices
34
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

35
Gas transfer - devices
Downflow bubble contactorOxygenation cone
36
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

37
Gas transfer - devices
  • Oxygen aspiration/injection

38
Gas transfer - devices
  • Continuous gas phase (water drops in air)
  • Packed or spray columns
  • Multi-staged low head oxygenators (LHO)
  • ...

39
Gas transfer - devices
  • Packed or spray columns

Gas out
Water in
Gas in
Water out
40
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

41
Gas transfer - devices
  • Low head oxygenators - LHO

O2 in
off-gas
sump tank
42
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

43
Gas transfer - devices
CO2 Stripping
LHO
44
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45
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

46
Background - CO2
  • The carbonate system
  • H2CO3 ? HCO3 H Ka,1
  • HCO3 ? CO3 H Ka,2
  • where H2CO3 H2CO3 CO2 "free CO2"

47
Background - CO2
  • H2CO3 aH2CO3 . CTCO3
  • or
  • where

Alkc HCO3 2CO3 OH H
48
Background - CO2
What it means
Can change the free CO2 concentration by changing
the pH
49
Background - CO2
For freshwater at 25 C
50
Background - CO2
  • Its concentration can be reduced by degassing or
    by raising the pH

51
Background - CO2
  • If it is reduced by degassing
  • pH rises
  • CTCO3 concentration drops
  • alkalinity does not change

52
Degassing
53
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

54
Addition of a strong base (e.g. NaOH)
55
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56
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

57
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

58
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

59
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

60
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

61
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

62
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

63
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

64
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

65
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

66
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

67
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

68
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

69
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70
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
71
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
72
Design principles - layouts
or
Influent
Effluent
O2
and/or CO2 transformation through chemical
addition
CO2 removal through degassing
73
Design principles - layouts
Other Treatment
or
Influent
Effluent
O2
and/or CO2 transformation through chemical
addition
CO2 removal through degassing
74
Design principles - layouts
or
Influent
Effluent
Other Treatment
O2
and/or CO2 transformation through chemical
addition
CO2 removal through degassing
75
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

76
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