Quite a few recirculated aquaculture projects have failed over time simply because the RAS technology developed for freshwater has been applied to seawater, with the consequence that at best the growth performance of the fish will far below expected, and therefore the facility will not fulfil the expected capacity. At worst massive mortalities will occur now and then.
Most people will agree that there is a slight difference between beer and milk, even if both liquids contain mostly water. 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 |
| Bromide (Br-) | - | - | 65 |
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 |
| Zinc (Zn) | 0.04-0.08 | 0.01 |
| Copper (Cu) | 0.01-0.02 | 0.003 |
| Boron (B) | 0.01-1.0 | 4.6 |
| Cobalt (Co) | 0.0005-0.0015 | 0.0005 |
| Molybdenum (Mo) | 0.0002-0.0008 | 0.01 |
Below we will address the key differences between seawater chemistry and freshwater chemistry.
The three principal differences relate to:
1) The bicarbonate equilibrium in seawater, - see further the section on Recirculation Chemistry,
2) The content of phosphor in seawater.
3) The surface tension in seawater
1) Every time the fish consumes one kg of oxygen, they will produce nearly 0,5 kg of Carbon dioxide, which then will be absorbed into the bicarbonate equilibrium in seawater.
The removal of CO2 is also impacted by surface tension, which essentially has the effect of making it more difficult to remove the CO2 produced from the process water.
Dealing with low levels of recirculation will be quite beneficial, as the produced CO2 in seawater will mostly be transferred into 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 will build up to critical levels and therefore will have a negative impact on the fish, and the farmer will experience that the fish will no longer take on weight.
At intensive recirculation, it is therefore of key importance, both in fresh water as well as in seawater, that an equivalent amount of CO2 compared to what is released from the fishes, is also removed from the water in the associated water treatment system.
In freshwater this is pretty simple, and simple means can be applied like simple aeration which may remove up to 80% of the free CO2, while if the same method is applied in Seawater, then the efficiency might well drop to 10-15 %.
It basically means that 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, there will be bacteria’s which can gain energy from oxidizing the organic matter using Nitrate (NO3), which then in several steps will be 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 is widely used in the aquaculture industry.
The problem arises in seawater when the nitrate levels drop in 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 will be the natural situation in sweater), then bacteria which can use the phosphate for oxidizing the organic material, will become competitive to the denitrifying bacteria.
The issue with the latter is that the products from the activities of the sulphate reducing bacteria’s, will include H2S, which is an incredible toxic compound that block the oxygen uptake of the blood cells of not just fish but also other animals as well, including human beings.
Even very low nonlethal levels beyond what can be measured with technology normally available for the fish farmers, will cause long term impact on the fish. The fish will generate a reduced ability to transfer oxygen through the gill, and thereby a loss in the oxygen capacity uptake to support normal growth. This impact may last for months.
In severe cases where the denitrification systems are integrated, and where there is a risk that the outlets flow from the denitrification filters can return back to the fish, there is a serious risk that the majority or at least a high proportion of the fish in the farm will be erased, should there be a technical or human error causing a wrong dosing from one of the dosing pumps which normally would be required for dosing an organic carbon source into the denitrification filters.
Du to this specific reason, it is recommended to not let the outlet flow from denitrification filters return back into the system. If denitrification filters are required, then discharge the outlet from the filters directly to the recipient, or to additional water treatment if required.