Estimate residual alkalinity after nitrification and the chemical needed to reach a target.
Alkalinity Requirement for Nitrification
Formula
Consumed = 7.14 x NH3-N nitrified Recovered = 3.57 x NO3-N denitrified Residual = Influent alk - Consumed + Recovered Supplement, lb/d as CaCO₃ = Q x (Target - Residual) x 8.34 Product, lb/d = Supplement x chemical factor / strength
Inputs used
- Flow (MGD)
- Influent alkalinity (as CaCO₃) (mg/L)
- Ammonia-N nitrified (mg/L)
- Nitrate-N denitrified (mg/L)
- Target residual alkalinity (mg/L)
- Supplement chemical
- Product strength (%)
Worked example (White River WRRF, fictional)
- Consumed = 7.14 x 30 = 214.2 mg/L
- Recovered = 3.57 x 20 = 71.4 mg/L
- Residual = 180 - 214.2 + 71.4 = 37.2 mg/L
- Shortfall = 80 - 37.2 = 42.8 mg/L
- As CaCO₃ = 8.0 x 42.8 x 8.34 = 2,856 lb/d
- NaOH product = 2,856 x 0.7994 / 0.50 = 4,566 lb/d of 50% caustic
Tips
- Keep residual alkalinity above about 50 to 70 mg/L as CaCO3.
- Denitrification recovers about half of what nitrification consumes per mg/L of nitrate removed.
Results are estimates for operator use. Check units and inputs, and confirm decisions with plant data and procedures.