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Alkalinity Requirement Calculator

Calculate alkalinity consumption from nitrification, recovery from denitrification, and supplemental chemical dosing requirements for pH stability in an MBR system.

Process stability: Maintaining a residual alkalinity of 50–100 mg/L as CaCO₃ in the effluent is recommended to provide a pH buffer for the nitrification process. Insufficient alkalinity can lead to pH drops, inhibiting biological activity and potentially damaging membrane performance.

Input parameters
Define influent alkalinity, nitrogen loading, and chemical assumptions.
m³/day
Alkalinity balance
mg/L as CaCO₃
mg/L as CaCO₃ (Safety residual)
Nitrogen loading
mg/L as N
mg/L as N
mg/L as N
mg/L as N
Chemical dosing
fraction (e.g., 1.0 for dry, 0.5 for 50% solution)
kg/L (use 1.0 for dry chemicals)
Calculated results
Alkalinity mass balance and chemical dosing requirements.

Enter values and select Calculate to view the alkalinity requirement screen.

Equations used

The alkalinity balance accounts for the biological consumption of alkalinity during nitrification and the partial recovery during denitrification.

Alkalinity Consumed = Nitrified NH₄-N × 7.14 mg CaCO₃/mg N
Alkalinity Recovered = Denitrified NO₃-N × 3.57 mg CaCO₃/mg N
Net Alkalinity Change = Recovered − Consumed
Required Supply = max(Target Residual − (Influent + Net Change), 0)
Chemical Dose = Required Supply × Chemical Factor ÷ Strength

Stoichiometric factors

Standard engineering constants are used for the biological process:

  • Nitrification: 7.14 g alkalinity (as CaCO₃) consumed per g NH₄-N oxidized.
  • Denitrification: 3.57 g alkalinity (as CaCO₃) recovered per g NO₃-N reduced.

Chemical factors (kg chemical per kg CaCO₃ equivalent):

  • Sodium Bicarbonate: 1.68
  • Soda Ash: 1.06
  • Caustic Soda (100%): 0.80
  • Hydrated Lime: 0.74
  • Magnesium Hydroxide: 0.58