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Quite a few recirculated aquaculture projects have failed over time simply because RAS technology developed for freshwater was applied to seawater, resulting in at best facility capacity and fish growth performance far below expectations, and at worst occasional massive mortalities. The chemical differences between freshwater and seawater are of great magnitude, and therefore the same technologies cannot be used for both media.
Seawater contains 10 times more sulphate than freshwater, so when the concentration of Nitrate drops, the sulphate reduction may take over in an anaerobic environment potentially creating dangerous Hydrogen Sulphide (H2S).
1. Some of the Chemical elements:
|In Seawater||In fresh (sweet) water|
|Sulphate (SO42-)||2,7 mg/l||0,3 mg/l|
|Chloride (CL-)||19,0 mg/l||0,2 mg/l|
|Sodium (Na+)||10,6 mg/l||0,04 mg/l|
|Magnesium (Mg2+)||1,3 mg/l||0,3 mg/l|
|Calcium (CA2+)||400 mg/l||60,9 mg/l|
|Potassium (K+)||380 mg/l||3 mg/l|
|Bicarbonate (HCO3-)||140 mg/l||3,5 mg/l|
2. Denitrification, to be applied for the reduction of nitrogen discharge into recipient waters :
- Denitrification requires an anaerobic environment, where bacteria in a series of steps can transform Nitrate(N) into free nitrogen.
Net equation: 2 NO3- + 10 e- + 12 H+ -> N2 + 6 H2O
But within seawater if an anaerobic environment emerges dissimilatory sulphate reduction easily becomes dominant, where bacteria take energy from organic matter.
Net equation: 2 CH2O + SO42- -> 2 HCO3 + H2S 0,1 ppm may have impact on the fish, and 4 ppm may cause 100 % mortality.
If we allow sulphate reduction to happen in parallel to the denitrification, and the water is re-introduced to the production system, Hydrogen Sulphide may reach the fish resulting either fish loss or a long term growth reduction..
3. The key problems when dealing with RAS in seawater are all related to gasses.
- The Carbon Dioxide / Bicarbonate equilibrium may cause problems, as a build-up of Bicarbonate may cause growth reduction, if the CO2 stripping capacity in the facility is insufficient. This is likely to happen if the technology has been dimensioned for freshwater use.
CO2 + H2O <=> H2CO3 <=> H+ + HCO3- <=> H+ + H+ + CO32-
Supersaturation of Nitrogen can be a problem, but only if the installation has not been well designed for the prevention of Nitrogen supersaturation.
The biggest single risk factor, is without comparison Hydrogen Sulphide. Even a minor Hydrogen Sulphide exposure to the fish can have fatal consequences.
Gasses behave differently in seawater, and sulphate, which can be reduced to Sulphide, is much more abundant in seawater compared to freshwater.
Quite a few RAS projects have failed over time simply because RAS technology developed for freshwater was applied to seawater resulting in at best facility capacity and fish growth performance far below expectations and at worst occasional mass mortalities. The chemical differences between freshwater and seawater are of great magnitude, and therefore the same technologies cannot be used for both media.
Table 1: Major ion concentrations (mg/L) in different ponds vs seawater (Boyd, 1990).
|Major Ions||Range for typical fresh water (mg/L)||Range for typical fresh water (mg/L)||Seawater (mg/L)|
|Bicarbonate/carbonate (HCO3- /CO3=)||11.6 to 136||11.6 to 244||142|
|Chloride (Cl−)||2.6 to 29||2.6 to 29||19 000|
|Sulfate (SO4=)||1.4 to 28||1.4 to 64||2 700|
|Calcium (Ca++)||2.7 to 41||2.7 to 53||400|
|Magnesium (Mg++)||1.4 to 9.1||1.4 to 15||1 350|
|Potassium (K+)||1.2 to 2.6||1.2 to 10||380|
|Sodium (Na+)||1.4 to 2.2||1.4 to 34||10 500|
Table 1: Trace element concentrations (mg/L) in typical freshwater vs seawater. (Boyd, 1990)
|Trace elements||Range for typical freshwater (mg/L)||Seawater (mg/L)|
|Iron (Fe)||Trace to 1.0||0.01|
|Manganese (Mg)||Trace to 0.25||0.002|
Below we will address the key differences between seawater chemistry and freshwater chemistry.
The two principal differences relate to:
1) The bicarbonate equilibrium in seawater / surface tension in seawater, - see further the section on Recirculation Chemistry,
2) The content of phosphor in seawater.
1) When fish consume one kg of oxygen, they produce nearly 0,5 kg of Carbon Dioxide, which is then absorbed into the Bicarbonate equilibrium in seawater.
The removal of CO2 is also impacted by surface tension, which increases the difficulty of removing CO2 from the process water.
With low levels of recirculation the produced CO2 in seawater is mostly converted to bicarbonate, which has lower impact on the fish compared to free CO2. But at higher levels of recirculation there is a risk that the bicarbonate levels build up to critical levels which has a negative growth impact on fish.
With intensive recirculation, it is of key importance (both in fresh water and seawater) that an equivalent amount of CO2 is removed from the water through the water treatment system compared to what is released from the fish. In freshwater this is pretty simple using aeration which can remove up to 80% of the free CO2. However, the same method when applied to seawater, can remove only up to 10-15% of CO2. In summary, the methods traditionally applied in freshwater for stripping out CO2, do not work efficiently in seawater.
2) The Sulphate (SO4- ) content will basically only cause a problem if there is an accumulation of organic matter anywhere in the system (including in the outlet systems of the tanks) or if a denitrification process is included in the flow structure.
In presence of organic matter in an anaerobic environment, certain bacteria can gain energy from oxidizing organic matter using Nitrate (NO3), which then in several steps is reduced first into Nitrite (NO2-) and then into free nitrogen (N2), which is the dominating gas in the atmosphere. This is the process that we call denitrification and is widely used in the aquaculture industry.
The problem arises in seawater when the nitrate levels drop into the anaerobic zone, due to denitrification, typically at a level in the range of 20-25 mg of NO3(N)/l. If Sulphate (SO4) is then present as well (which is normal in seawater), then bacteria which can use Phosphate to oxidize organic material become competitive to the denitrifying bacteria. This is problematic as one of the results of this process by the sulphate reducing bacteria, includes H2S which is highly toxic and blocks oxygen uptake of the blood cells of fish, animals and human beings.
It should be noted that even very low nonlethal levels of H2S beyond what can be measured with technology normally available to fish farmers has a long term impact on fish. Specifically, in the presence of H2S fish have a months long reduced ability to transfer oxygen through the gills resulting in a loss of oxygen uptake capacity to support normal growth.
In severe cases where the denitrification systems are integrated into the process flow system there is a risk that the outlet flow from the denitrification filters can return back to fish tank. In this case, should there be a technical or human dosing error from one of the dosing pumps normally required for dosing an organic carbon source into the denitrification filters, the mortality of the majority or at least a high proportion of the fish in the farm can occur.