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.
This calculator also appears on ActivatedSludge.com, our consulting site. Results are for training and operator use; always follow your plant’s procedures.