The bicarbonate component in seawater is referred to as a gas, the reason for this is that bicarbonate is in a state of equilibrium with carbonates and carbon dioxide in solution. The relative proportion of each component is a function of the pH of the water and the degree of air stripping of carbon dioxide.  In marine recycle systems it is absolutely essential that the pH is maintained between pH of 7.8 and 8.3.  If the pH is allowed to fall, carbonates are converted to carbonic acid and the pH becomes unstable.

Carbonates are also consumed by the nitrifying bacteria,  it is therefore important that the alkalinity is maintained, and this can be achieved by the addition of sodium bicarbonate.

The pH of seawater ranges from pH 8.0 to 8.3. In a recycle system the nitrifying bacteria are consuming inorganic carbon which has the effect of reducing the pH. The relationships are quite complex, but it is an important aspect of the system water quality maintenance.

Alkalinity buffering equation

1. H₂0 + CO₂         H₂CO₃       HCO₃ + H⁺           CO₃ + 2H+

Nitrification equations

2. NH₄⁺ + 1.5O2                 2H⁺ + 2H₂O + NO₂^-

3. NO₂^- + 0.5O₂                    NO₃^-

4. NH₄⁺ + 1.83 O₂ + 1.98 HCO3^-          0.021 C₅H₇0₂N + 0.98 NO₃^- + 1.041 H₂O + 1.88 H₂CO₃^-

5. NH₄+ + 1.9O₂ + 2HCO3^-                    1.9 CO₂ + 2.9 H₂O + 0.1 CH₂

From the above equations it can be calculated that for every gram of ammonium oxidised to nitrate, the following occurs;

In a recycle system carbon dioxide (carbonic acid) is being produced by the fish. The nitrifying bacteria also produce acid, and inorganic carbon consumed by the bacteria is converted into cell biomass. This has the net effect of depressing the pH of the water and reducing the alkalinity. Eventually all of the alkalinity in the system will be consumed, the pH will become unstable, and the water will become acidic. This can develop into a serious situation resulting in fish moralities.

Carbon dioxide is in state of equilibrium with carbonates (dependent on pH) and with the atmosphere (dependent on the degree of gas stripping). If there is no aeration in the system then sodium hydroxide may be used to lower the carbon dioxide level and maintain the pH. However this is a rather dangerous solution to the problem since it will be difficult to maintain stable conditions, and there is the risk of over-shoot. In the recycle system there will be aeration and gas stripping as the water splashes around the system and through the biofilters. There will therefore be a net loss of carbonates from the water, thus, additional carbonates must be added. The MagPhlow (MgOH) media in the pressure filters will dissolve into the water in direct response the pH. The MagPhlow is insoluble in the water above a pH of approximately 8.3, this is therefore a much safer approach than the addition of sodium hydroxide.

Strong aeration of the water or gas stripping of carbon dioxide will help to maintain the stability of the water, in turn it will necessitate the addition of sodium bicarbonate at a level close to the upper limit stated above.

In conclusion,  both marine and freshwater recycle systems require pH control, and this is best achieved using sodium bicarbonate, and MagpHlow media in the pressure filters.  In large system, the pH balance may be sumplimented by the direct injection of sodium hydroxide,  but caution must be excerised by this approach in case of pH over shoot.

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