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. Strong aeration may also cause problems associate with water temperature regulation, I therefore propose the following for pH and alkalinity regulation;

• 12 metric tonnes of MagPhlow in 1 of the pressure filters, with a contingency to start a second filter with MagPhlow media if found to be required
• 300 Kg per day of sodium bicarbonate to be added
• 25kg to 50kg per day of sodium hydroxide to be metered into the water