Gas Transfer in Recirculating Aquaculture Systems
Raul H. Piedrahita, Ph.D.Biological and Agricultural Engineering
University of California, Davis
Topics
Basic principles Gas transfer General design procedures
Basic principles
Concentration of gases in solution may be the water quality-limiting factor in recirculation aquaculture systems (RAS)
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)
...
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)
Basic principles
OxygenConsumed by fish and microorganisms0.3-0.5 g O2/g feed
Must be replenished: oxygenation or aeration
Basic principles
Carbon DioxideProduced by fish and microorganisms0.4-0.7 g CO2 / g feed (1 mole CO2/mole O2)
Must be reduced: pH control and/or degassing
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) ...
i2i1i,s XPfS,TfC
Basic principles
Saturation concentration of gas i is:
760PP
XK1000C wvBPiiii,s
Cs,i = saturation concentration, mg/L; Ki = gas "density", g/L, 1.429 for O2 and 1.977 for CO2; = Bunsen coefficient,
L/L-atm; Xi = mole fraction in gas phase; PBP = barometric pressure, mmHg; Pwv = vapor pressure of water, mmHg
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
FW=fresh water; SW= sea water. Units: XO2, fraction by volume; pressures, mmHg; Cs,O2, mg/L.
After: Colt, J. 1984
Basic principles-solubility: equilibrium between gas and liquid
Temperaturesalinitypressure
Mole fractionpressure gas phase
water
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
FW=fresh water; SW= sea water. Units: XO2, fraction by volume; pressures, mmHg; Cs,O2, mg/L.
After: Colt, J. 1984
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
FW=fresh water; SW= sea water. Units: XO2, fraction by volume; pressures, mmHg; Cs,O2, mg/L.
After: Colt, J. 1984
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
FW=fresh water; SW= sea water. Units: XO2, fraction by volume; pressures, mmHg; Cs,O2, mg/L.
After: Colt, J. 1984
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
FW=fresh water; SW= sea water. Units: XO2, fraction by volume; pressures, mmHg; Cs,O2, mg/L.
After: Colt, J. 1984
* gauge pressure
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
FW=fresh water; SW= sea water. Units: XCO2, fraction by volume; pressures, mmHg; Cs,CO2, mg/L.
After: Weiss, R.F. 1974
* 2006 value and rising... NOAA, 2006.
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)
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)
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
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
Basic principles - gas sources
Air blowers
Basic principles - gas sources
Oxygen Transfer SystemsOxygen - On-site generation
- Liquid O2
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.
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
Depends on: the difference between the concentration
in water (Ci) and saturation concentration (Cs,i)• If Ci > Cs,i (supersaturation): gas i will move
from the water to the "atmosphere": degassing
• If Ci < 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
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
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: turbulenceincrease turbulence
Gas transfer - rate
Gas transfer - devices
Continuous liquid phase (bubbles in water) Bubble diffusers U-tubes Oxygenation cones (downflow bubble
contactors) Oxygen aspirators/injectors ...
Airstones very inefficient (<10% transfer efficiency) useful for emergency oxygenation used with air in airlift pumps
Gas transfer - devices
U-Tube
Gas transfer - devices
Gas transfer - devices
U-tube down flow water velocity of 2 to 3 m/s depth usually > 10 m does not vent N2 or CO2 effectively can achieve concentrations >> 40 mg/L transfer efficiency ~ 50-80 % low pumping costs (low hydraulic head) construction costs site dependent limit gas flow to < 25 % of water flow
Gas transfer - devices
flow returned toculture tanks
oxygen
pump
downflowbubble
contactor
off-gas vent
Downflow bubble contactorOxygenation cone
Gas transfer - devices
Downflow bubble contactor widely used in Europe resists solids plugging can achieve concentrations >> 40
mg/L transfer efficiency can approach 100
% does not vent N2 or CO2 well
Oxygen aspiration/injection
Gas transfer - devices
Continuous gas phase (water drops in air) Packed or spray columns Multi-staged low head oxygenators
(LHO) ...
Gas transfer - devices
Packed or spray columnsGas transfer - devices
Water in
Water out
Gas out
Gas in
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%
oxygenfeed gas
off-gasvent
flow
flowsump tank
Gas transfer - devices
O2 in off-gas
Low head oxygenators - LHO
LHOs effective O2 absorption with a low water
drop degas N2 (but not CO2) while adding O2
ratio of oxygen gas:water flow – 0.5-2% transfer efficiency drops rapidly for
G:L>2% "compact" and suitable for combining with
PCA for degassing CO2
Gas transfer - devices
LHO
CO2 Stripping
Gas transfer - devices
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
Background - CO2
The carbonate system
H2CO3* HCO3– + H+ Ka,1
HCO3– CO3
= + H+ Ka,2
where: [H2CO3*] [H2CO3] [CO2] = "free CO2"
[H2CO3*] = H2CO3* . CTCO3
or
where:
Alkc = [HCO3–] + 2[CO3
=] + [OH–] – [H+]
Background - CO2
[H2CO3*]
1Ka,1
[H]
2Ka,1Ka,2
[H]2
Alkc Kw
[H] [H]
Background - CO2
0.00.20.40.60.81.0
5 6 7 8 9pH
H
2C
O3
*
What it means:
Can change the free CO2 concentration by changing the pH
Background - CO2
876
0
25
50
75
100
0.250.5
1.0
2.0
3.04.0
0.250.51.02.03.04.0
CA
RB
ON
DIO
XID
E (
mg
/L)
ALKALINITY
CtCO3 mmol/L
meq/L
For freshwater at 25 °C
Its concentration can be reduced by degassing or by raising the pH
Background - CO2
If it is reduced by degassing•pH rises
•CTCO3 concentration drops
•alkalinity does not change
Background - CO2
Degassing
8760
25
50
75
100
0.250.5
1.02.03.04.0
0.250.51.02.03.04.0
pH
CA
RB
ON
DIO
XID
E (
mg
/L)
Alkalinity
CtCO3
meq/L
mmol/L
Alkalinity remains unchanged
If it is reduced by raising the pH:•the H2CO3*drops as the pH rises
•the concentration of CTCO3 does not change
•alkalinity increases due to the base addition
Background - CO2
Addition of a strong base (e.g. NaOH):
8760
25
50
75
100
0.250.5
1.02.03.04.0
0.250.51.02.03.04.0
pH
CA
RB
ON
DIO
XID
E (
mg
/L)
Alkalinity
CtCO3
meq/L
mmol/L
CTCO3 remains constant
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
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
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
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
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
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
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
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
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
Design principles - precautions
Use high G:L 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
Design principles - precautions
Use high G:L ratios for degassing and low values for oxygenation
Avoid introducing air under pressureit could cause supersaturation
Choose the bases carefully taking into account the chemistry of the water to be treated
Take into account metabolism fluctuations
Design principles - precautions
Use high G:L 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
Design principles - precautions
Use high G:L 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
Design principles - layouts
Influent
N2 and CO2
Effluent
O2 added and N2 and CO2 removed from influent water
Useful to increase O2 and reduce
excessive N2 and CO2 in water supply
O2
Design principles - layouts
Influent
CO2 removalthrough degassing
Effluent
O2 addition and CO2 reduction in recycled water
and/or CO2 transformationthrough chemical addition
O2
Design principles - layouts
InfluentEffluent
or
CO2 removalthrough degassing
and/or CO2 transformationthrough chemical addition
O2
Design principles - layouts
InfluentEffluent
orOther Treatment
CO2 removalthrough degassing
and/or CO2 transformationthrough chemical addition
O2
Design principles - layouts
Influent
Effluent
or
Other Treatment
CO2 removalthrough degassing
and/or CO2 transformationthrough chemical addition
O2
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