gas transfer in recirculating aquaculture systems raul h. piedrahita, ph.d. biological and...

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


Common problems with make-up water: Oxygen (O2)

Carbon dioxide (CO2)

Nitrogen (N2) and Argon (Ar) (total gas pressure, or TGP)

...

Basic principles


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


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


Situation

XO2 PBP Pwv Cs,O2


0.209 760 12.79 10.072

Sea level, air, SW, 15C

0.209 760 12.55 8.129




Situation

XO2 PBP Pwv Cs,O2


0.209 760 12.79 10.072

1600 m, air, FW, 15C

0.209 631 12.79 8.328




Situation

XO2 PBP Pwv Cs,O2


0.209 760 12.79 10.072

Sea level, pure O2, FW, 15C

1.00 760 12.79 48.19




Situation

XO2 PBP Pwv Cs,O2


0.209 760 12.79 10.072

1 atm*, pure O2, FW, 15C

1.00 1520 12.79 96.38



* gauge pressure

Basic principles-CO2 solubility

Situation

XCO2 PBP Pwv Cs,CO2


0.00038*

760 12.79 0.76


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)


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)


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


U-Tube



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


flow returned toculture tanks

oxygen

pump

downflowbubble

contactor

off-gas vent

Downflow bubble contactorOxygenation cone


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


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

(LHO) ...


Packed or spray columnsGas transfer - devices

Water in

Water out

Gas out

Gas in


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


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


LHO

CO2 Stripping


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



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


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


•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


treatment method?

configuration and place in the treatment sequence system configuration sequence

preliminary calculations details

Design principles


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


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


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



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



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


Influent

CO2 removalthrough degassing

Effluent

O2 addition and CO2 reduction in recycled water

and/or CO2 transformationthrough chemical addition

O2


InfluentEffluent

or



O2


InfluentEffluent

orOther Treatment



O2


Influent

Effluent

or

Other Treatment



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

gas transfer in recirculating aquaculture systems raul h. piedrahita, ph.d. biological and...

Documents

sea water

water quality

fresh water sw

culture water

makeup water

vapor pressure of water

mmhg cs

basic principlessolubility