Nitrogen Conversions for Aquaponics Biofilters

1. Basic Ratios (Based on Nitrogen Content N)

The conversions are based on the molar masses of the different nitrogen species:

Molar Masses:

• $\text{N (Nitrogen atom):}14.01\text{g/mol}$
• ${\text{NH}}_3\text{(Ammonia):}17.03\text{g/mol}$
• ${\text{NH}}_4^{+}\text{(Ammonium):}18.04\text{g/mol}$
• ${\text{NO}}_3^{-}\text{(Nitrate):}62.00\text{g/mol}$

2. Conversion Formulas

A) NH₄-N ↔ NH₃-N (Ammonia Nitrogen)

In this conversion, the nitrogen portion remains the same:

Important: 1 g NH₄-N corresponds to 1 g NH₃-N. The total nitrogen amount is identical.

B) NH₄-N ↔ Total NH₄⁺ (Ammonium Ion)

C) NH₄-N ↔ Total NH₃ (Ammonia)

D) NH₄-N ↔ NO₃-N (Nitrate Nitrogen)

With complete nitrification, 1 mol NH₄-N becomes 1 mol NO₃-N:

Important: The nitrogen amount remains the same, only the compound form changes.

E) NH₄-N ↔ Total NO₃⁻ (Nitrate Ion)

Maximum Concentration:

You can directly calculate the toxic NH₃ concentration with pH and temperature!

Formula for NH₃ fraction in total ammonium:

pKₐ value as a function of temperature (in °C):

Calculation Steps:

1. Calculate pKₐ from water temperature
2. Calculate NH₃ fraction (%) from pH and pKₐ
3. Convert total NH₄-N concentration to toxic NH₃: ${\text{NH}}_3\text{(g/m³)}={\text{NH₄-N (g/m³)}}_{}\times 1.216\times \frac{\text{NH₃-\%}}{100}$
4. Compare with the limits:
   • Salmonids (trout, salmon): < 0.0125 g/m³ (12.5 µg/L)
   • Cyprinids (carp, tilapia): < 0.031 g/m³ (31 µg/L)
   • Catfish: < 0.125 g/m³ (125 µg/L)