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
g O2/g feed
Must be replenished: oxygenation or aeration

7. Basic principles Carbon Dioxide Produced by fish and microorganisms
g CO2 / g feed (1 mole CO2/mole O2)
Must be reduced: pH control and/or degassing

9. Saturation concentration of gas i is a function of:
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)
...
Before one can discuss gas transfer and the management of gases in RAS, one must have an understanding of gas solubility.
Gas solubility indicates the concentration of a gas that may be present in the water for it to be at equilibrium with the atmosphere with which it is in contact.

10. Saturation concentration of gas i is:
Basic principles
Saturation concentration of gas i is:
Tables are available that list the saturation concentrations of the main gases under different conditions. The most complete for aquaculture use is by Colt, J Computation of dissolved gas concentrations in water as functions of temperature, salinity, and pressure. I
Cs,i = saturation concentration, mg/L; Ki = gas "density", g/L, for O2 and 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
23.77
8.244
This and the following slides illustrate how various factors can affect the saturation concentration of oxygen in water. The impact of temperature is illustrated here: as temperature rises the saturation concentration decreases.
FW=fresh 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
Saturation depends on the gas phase or atmosphere that is in direct contact with the water. If water sits in an open container, the gas phase is the same as the atmosphere at the site. If water comes in contact with a pure oxygen atmosphere in an oxygenation device, then the gas phase would be a pure oxygen atmosphere and the saturation concentration in the device would be determined by the pure oxygen atmosphere. Similarly if the system is pressurized, etc.
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
12.55
8.129
The impact of salinity is illustrated here: as salinity increases, saturation concentration decreases.
FW=fresh 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
631
8.328
The impact of barometric pressure or elevation is illustrated here. As the site's elevation above sea level increases, the saturation concentration decreases.
FW=fresh 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
48.19
Exposing the water to a pure oxygen atmosphere can have dramatic impact on saturation concentration, increasing it almost five fold.
FW=fresh 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
96.38
Pressurizing the system can also increase saturation concentration. This is illustrated here for pure oxygen, but would also happen with air. The risk when this happens with air is that all the gases would increase in saturation, creating conditions of total gas supersaturation when the water is in the culture tanks.
* gauge pressure
FW=fresh 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 *
760
12.79
0.76
Sea level, air, FW, 25C
0.57
Carbon dioxide concentrations at saturation are very low, typically much lower than the concentrations observed in RAS. This is due to the low concentration in the air. A small build up of CO2 in the gas phase can have a substantial impact on the saturation concentration in the water and can affect the performance of gas transfer equipment.
* 2006 value and rising... NOAA, 2006.
FW=fresh 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)
Increasing the temperature of water without allowing for gas equilibration can result in gas supersaturation.

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)
Introducing gases under pressure can also cause gas supersaturation.

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
Aeration Systems – Air Stones, Packed Towers
Sources of Air
The standard sources of air in aquaculture are blowers, air pumps, or compressors. The primary differences between them are the pressure requirements and the volume of the discharge. Blowers supply high volumes of air at low pressure, while compressors supply small volumes of high pressure air. In specifying the type of air source required, two design parameters need to be determined: the required pressure and the required air volume. The operating pressure is determined by the requirements to overcome the water pressure at the diffuser’s depth, the pipe friction losses, and the diffuser’s resistance to air flows. For a typical application of air stones in a shallow (1 m or 3 ft) tank, this is about 2 to 3 psi (125 mm Hg). In deeper tanks or with diffusers requiring higher pressures, i.e., those with smaller bubbles or clogged pores, this could be considerably higher. The air volume required is determined by the mass of oxygen required and the overall transfer efficiency of the system. For example, a 9 inch (23 cm) air stone operating in 3 feet (1 m) of water with 0.7 cfm (1.2 m3/h) air supply transfers only lbs/hr (0.25 kg/day) of oxygen.
Regenerative blowers are designed to provide large volumes of air at low pressure, typically less than 4 psi (190 mm Hg). They are most commonly used with either air stones or airlift systems. Advantages of regenerative blowers include their low noise levels, reliability, energy efficient motors, and lower comparative cost. Air pumps operate in the mid-range of performance, between blowers and compressors. Compressors are designed for high pressure operations, such as in very deep tanks or where long airlines are required.

23. Basic principles - gas sources
Oxygen Transfer Systems
Oxygen - On-site generation
- Liquid O2
Sources of Oxygen
In aquaculture, three sources of oxygen are commonly used: high-pressure oxygen gas, liquid oxygen (LOX), and on-site oxygen generations. To insure availability and as backup, usually at least two sources are available at most facilities. High pressure oxygen gas is easily available in cylinders containing from 100 to 250 ft3 (3 to 7 m3) of gas at 2550 psi (170 atmospheres of pressure). A number of cylinders can be connected together using commercially available manifolds to increase the total capacity. Due to their cost and limited capacity, oxygen cylinders are normally used only as emergency backup systems.

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.
Oxygen can be generated on-site using either a pressure swing adsorption (PSA) or a vacuum swing adsorption (VSA) unit. In both cases, a molecular sieve material is used to selectively adsorb or absorb nitrogen from the air, producing an oxygen-enriched gas. Commercially available units can produce anywhere from 1 to 30 lbs (0.5 to 14 kg) of oxygen per hour at from 10 to 50 psi (0.7 to 3.3 atmospheres). A source of dry, filtered air at 90 to 150 psi (6.0 to 10.0 atmosphere) is required to produce an oxygen stream that is from 85–95% pure. PSA and VSA units operate on a demand basis and produce oxygen only when needed. They have proven to be very reliable and require little maintenance. However, they are both expensive in terms of capital and operationally expensive, due to the compressed air requirements. Also, since they require electrical power, some other source of oxygen is needed in the event of power failures or else the facility must be equipped with large backup generators and transfer switches.

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
In many areas, liquid oxygen is commercially available in bulk and can readily be transported and stored in on-site Dewar’s type storage containers. At one atmosphere, liquid oxygen boils at F ( C), thus special insulated cryogenic containers are required for storage. These containers range in size from 30 gal (0.11 m3 liquid) to a much as 10,000 gal (38 m3 liquid), and are usually rented or leased from the suppliers, although the smaller units can be purchased. One gallon of liquid oxygen is equal to 115 ft3 (3.26 m3) of gaseous oxygen. The maximum gas pressure in these containers is in the range of 150 to 200 psi (11.7 atmospheres). Prior to its use, the LOX is vaporized by directing it through heat exchanger coils. A liquid oxygen supply system will consist of a storage tank, vaporizer, filters, and pressure regulators. The economics of LOX use are dependent upon the transport cost, and the reduced capital and maintenance cost as compared to pressure swing adsorption (PSA) systems. In general, a LOX system is very reliable, operating even during power failures. Failures on farms using LOX systems as backup to power outage are caused by under-sizing the LOX system in the first place or unanticipated severe weather conditions that extend longer than predicted. Carefully consider your risks for such cases and size your LOX system with these potential dangers in mind. As a minimum, a LOX system should be able to maintain a facility with oxygen for 30 days. Remember that upon the first sign of major weather problems, it is probably prudent to take your fish off of feed, which will lower their oxygen demand dramatically over the next 24 hours.

28. Gas transfer - rate 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
Gases move into solution or out of solution depending on their concentration with respect to saturation concentration. The speed at which the gases move into or out of solution (rate) decreases as the difference between saturation concentration and the concentration in solution decreases.

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: Diffusivity: turbulence
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
In designing and operating aeration equipment one tries to take advantage of the factors that result in higher transfer rates within the limitations imposed by the system and energy considerations.

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 (<10% transfer efficiency)
useful for emergency oxygenation
used with air in airlift pumps
Due to their low absorption efficiency, the use of diffusers or air stones have been limited mainly to emergency oxygenation and fish live-haul systems. Although some of the recent fine-bubble diffusers (bubbles 100 to 500 microns) perform well in deep tanks (50% oxygen transfer efficiency), they require a high pressure source of oxygen (25–50 psi) and are subject to both chemical and organic fouling.

33. Gas transfer - devices U-Tube
The U-tube aerator operates by increasing the gas pressure and solubility, thus increasing the overall gas transfer rate. It consists of either two concentric pipes or two pipes in a vertical shaft 30 to 150 ft (9 to 45 m) deep. Oxygen is added at the upper end of the down-leg of the U-tube and as the water/gas moves downward through the contact loop, an increase in hydrostatic pressure increase the oxygen transfer rate. The overall oxygen transfer efficiency is a function of the depth of the U-tube, inlet gas flow rate, water velocity, diffuser depth and inlet DO concentration. Concentrations of dissolved oxygen ranging from 20–40 mg/L can be achieved, but the overall oxygen transfer efficiency is only 30–50%. Off-gas recycling can improve the absorption efficiency to 55–80%. Two advantages of the U-tube are the low hydraulic head requirements that allow operation with no external power if sufficient head is available, and that it can be used with water containing high levels of particulates or organics. Its chief disadvantages are that it does not vent off gasses such as nitrogen or carbon dioxide very efficiently and construction costs can be high, particularly if bedrock is present.

34. 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 ~ %
low pumping costs (low hydraulic head)
construction costs site dependent
limit gas flow to < 25 % of water flow
U-tubes are designed for flows where the downflow velocity is between 1.8 to 3.0 m/s. A potential problem with U-tubes is that if too much oxygen is added a gas bubble blockage can occur that results in flow interruption. This will tend to happen if gas-liquid ratios exceed 25%.

35. Gas transfer - devices Downflow bubble contactor Oxygenation cone
The aeration cone, bicone, or downflow bubble-contact aerator consists of a cone-shaped cylinder or a series of pipes with reducing diameters. Water and oxygen enter at the top of the cone, flow downward, and out. As the cone’s diameter increases, the water velocity decreases, until the downward velocity of the water equals the upward buoyant velocity of the bubbles. Thus, the bubbles are held in suspension, until they dissolve into the water. The performance of aeration cones is determined by gas and water flow rates, influent DO concentration, cone geometry and operating pressure. Absorption efficiency range from 95–100% with effluent concentrations from 30 to 90 mg/L. Commercial units are available that transfer from 0.4 to 10.8 lbs of oxygen per hour (0.2 to 4.9 kg/hr) at 25 mg/L, at flow rates from 45 to 600 gpm (170 to 2,300 Lpm).

36. 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

37. Gas transfer - devices Oxygen aspiration/injection Oxygen Injection
The most widely used form of oxygen injection takes advantage of the increased pressure available when pumping water. Oxygen is injected though a venturi nozzle or orifice, creating a fine bubble suspension in the pressurized line. Pressures of 30–235 psi (2 to 22 atmospheres) are needed to achieve satisfactory absorption, with contact times of 6–12 seconds. Absorption efficiency ranges from 15 to 70% with effluent DO concentration from 30–50 mg/L

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
Low Head Oxygenators (LHO) are being used more frequently, particularly because of their adaptability to high flows using minimal hydraulic head, hence their name Low Head Oxygenator. The original LHO design was developed and patented by Watten (1989). LHO’s vary in configuration, but all are fundamentally similar in operation. These units consist of a distribution plate positioned over multiple (5 to 10) rectangular chambers. Water flows over the dam boards at the end of a raceway or is pumped upwards from a fish tank, through the distribution plate, and then falls through the rectangular chambers. These chambers provide the gas-liquid interface needed for mixing and gas transfer. The streams of falling water impact a collection pool at the bottom of each chamber where the effluent water flows away from each chamber. All of the pure oxygen is introduced into the outer or first rectangular chamber. The mixture of gases in the first chamber passes sequentially through the remaining chambers. The gaseous mixture will decrease in oxygen concentration from chamber to chamber as the oxygen is absorbed. Finally the gaseous mixture will exit from the last chamber. This gas is referred to as off-gas. Each of the rectangular chambers is gas tight and the orifices between the chambers are sized and located to reduce back-mixing between chambers.
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 gas:water flow – 0.5-2%
transfer efficiency drops rapidly for G:L>2%
"compact" and suitable for combining with PCA for degassing CO2

43. Gas transfer - devices CO2 Stripping LHO
A common design is to place a CO2 stripping tower directly above an LHO and allow the water to cascade down.
LHO

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
An additional complication when looking at CO2, relative to other gases, is that it is part of the carbonate chemical equilibrium system. As such, the concentration in solution is governed not only by gas solubility and transfer processes but also by water pH and other parameters.

46. Background - CO2 The carbonate system H2CO3*  HCO3– + H+ Ka,1
HCO3–  CO3= + H+ Ka,2
where: [H2CO3*] = [H2CO3] + [CO2] = "free CO2"
These are the species of the carbonate system: carbonic acid, bicarbonate ion, and carbonate ion. The Ks are the equilibrium constants for the reactions and depend on temperature and salinity of the water.
The concentration of carbon dioxide one normally uses is in fact the "free CO2" concentration and includes not only CO2 but also carbonic acid or H2CO3.

47. Background - CO2 or where: [H2CO3*] = aH2CO3* . CTCO3
These are some of the equations and definitions one can use to calculate carbon dioxide concentration.
The alkalinity definition presented here includes only the carbonate, bicarbonate, hydroxyl and hydrogen terms. I aquaculture waters, contributions by ammonia and phosphate may also be significant. Similarly, in sea water one should also take into account the contribution by boron.
Alkc = [HCO3–] + 2[CO3=] + [OH–] – [H+]

48. Background - CO2 What it means:
As pH increases the proportion of the dissolved inorganic carbon that is present as "free CO2" decreases. Therefore, one can change the concentration of "free CO2" by adjusting the pH and not change the concentration of dissolved inorganic carbon present.
Can change the free CO2 concentration by changing the pH

49. Background - CO2
mmol/L
meq/L
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 Alkalinity remains unchanged 100 CARBON DIOXIDE (mg/L) pH8 7 6 25 50 75
100
0.25
0.5
1.0
2.0
3.0
4.0 pH
CARBON DIOXIDE (mg/L)
Alkalinity
CtCO3
meq/L
mmol/L
Alkalinity remains unchanged

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):8 7 6 25 50 75
100
0.25
0.5
1.0
2.0
3.0
4.0 pH
CARBON DIOXIDE (mg/L)
Alkalinity
CtCO3
meq/L
mmol/L
CTCO3 remains constant

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:
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:
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:
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:
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 treatment method? for O2: aeration, oxygenation, ...
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 configuration and place in the treatment sequence
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 preliminary calculations
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 details equipment, design, alarms, back-up systems
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 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

66. Design principles - precautions
Use high G:L 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 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

68. 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

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 - layoutsor
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 - layoutsor
Influent
Effluent
Other Treatment O2
and/or CO2 transformation
through chemical addition
CO2 removal
through degassing

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