pH and alkalinity management in recirculating aquaculture systems (RAS) is crucial for maintaining optimal conditions for nitrification and fish. The overall chemical equation for nitrification is likely the most well-known equation in RAS water management.
Equation 1: Overall stoichiometry nitrification.
NH4++1.86O2+1.98HCO3-à0.020C5H7NO2+0.98NO3-+1.88H2CO3+1.04H2O
In theory, total nitrification consumes 1.98 mol of HCO3- for every mol of NH4+ oxidized, which equals 1.98 alkalinity equivalents used per mole of NH4 + (1.98 mole HCO3- /1 mole NH4+; equation 1). This 1.98 alkalinity equivalents consumed used for every mol of NH4+ oxidized will result in a pH drop unless the equivalents of a base (i.e. sodium carbonate) are provided. In practice however, only approximately half of the alkalinity equivalents are consumed per mol of NH4+ oxidized. It is suggested that fish excrete NH3 rather than NH4+, removing one H+ from the water to form NH4+ and consuming net only one alkalinity equivalent when oxidized.
Although nitrate, the ultimate product of nitrification, is not as harmful to fish as NH3/NH4+ , it still needs to be controlled in RAS since high levels might affect fish growth and health, which usually occurs through water exchange . Denitrification reactors can be used in aquaculture to remove nitrate, reducing the need to exchange water for nitrate control. Another benefit of using a denitrification reactor is the production of alkalinity.
Equation 2: Overall stoichiometry denitrification.
0.61C18H19O9N+4.89NO3-+0.39NH4++4.15H+àC5H7NO2+2.27N2+5.98CO2+5.15H2O
Denitrification removes approximately 0.91 equivalents of acid (producing alkalinity) per mole of NO3-N denitrified (4.15 mole H+/4.54 mole NO3-; equation 2). This implies that complete denitrification of produced NO3-N can nearly entirely compensate for alkalinity loss if fish excrete NH3, but quantitative data on this have never been published. An 8-week feeding experiment with tilapia in individual RAS (n=4) was performed, whereby the faecal waste produced was fed into an up flow sludge blanket reactor (no water exchange). An alkalinity balance was made using a nitrogen mass balance and alkalinity measurements. Because nitrate removal exceeded nitrate production during the experiment, more nitrate was added to the systems. Similar to the theory, higher nitrate removal relative to production increased alkalinity and pH while no base had to be added. To complete the balance, the alkalinity equivalent for nitrification was computed indirectly, using an alkalinity equivalent of 0.91 for denitrification, resulting in an average alkalinity consumption of 0.99. These results support the idea that fish excrete NH3 and that the net alkalinity loss owing to nitrification is close to one.