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External Carbon Source Dosing & Post-Anoxic Sizing Calculator

Translate a post-anoxic nitrate/nitrite-N polishing target into an external COD deficit, commercial liquid-carbon demand, feed-pump flow, storage, preliminary hydraulic volume, and alkalinity recovery.

Preliminary design tool only. Select the carbon source, COD equivalent, product density, dose margin, and control strategy using supplier data, measured plant response, chemical-safety requirements, and the applicable permit. This calculation does not establish a compliance dose or replace process modelling.
Used only to label the direct product-cost output; no currency conversion is applied.

Carbon source & post-anoxic inputs

Use measured nitrate, usable rbCOD, and commercial-product data wherever available.

Nitrate-polishing objective
m³/d; average flow for daily product demand
mg/L as nitrate/nitrite-N
mg/L as nitrate/nitrite-N
mg/L COD; enter only carbon available anoxically
Carbon-COD requirement
kg COD/kg NOx-N; editable site-specific design basis
% added to selected carbon-COD demand
Commercial carbon product
For example: methanol, acetate solution, ethanol, glycerol, or a proprietary product
kg COD per kg commercial product
kg/L; use supplier data
currency per kg commercial product
Post-anoxic hydraulics & storage
peak ÷ average flow
hours; preliminary hydraulic selection
days/year
days of average product use
% above coverage demand

Carbon dosing & post-anoxic results

All nitrogen results are as elemental N; product results use the entered commercial-product properties.

NOx-N removal load
13.00kg N/d
15.0 − 2.0 mg N/L at 1,000 m³/d
Available rbCOD
20.0kg COD/d
20.0 mg/L at average flow
External COD deficit
37.20kg COD/d
Demand less available rbCOD
Product volume
24.8L/d
At 1.50 kg COD/kg product
Peak product pump screen
1.55L/h
Continuous peak-flow equivalent
Post-anoxic volume
46.9m³
0.75 h at 1,500 m³/d peak flow
Denitrification loading
0.277kg N/m³·d
Removal load ÷ selected volume
Alkalinity recovery
46.41kg as CaCO₃/d
13.00 kg N/d × 3.57
Nominal product storage
856L
30 days + 15% margin
Annual product cost
USD 9,052
Commercial product only
External carbon is indicated by the default inputs. Confirm usable rbCOD, reaction kinetics, chemical feed control, and product handling requirements before selecting equipment.

Governing formulas

Nitrate-polishing and carbon balance

NOx-N removal load = Average flow × max(NOx-N in − NOx-N target, 0) ÷ 1,000

Carbon-COD demand = NOx-N removal load × selected COD/N factor × (1 + carbon margin)

External COD deficit = max(Carbon-COD demand − available rbCOD, 0)

Product mass = External COD deficit ÷ entered product COD equivalent

Product volume = Product mass ÷ entered product density

Post-anoxic hydraulics and alkalinity

Peak-flow post-anoxic volume = Average flow × peak-flow factor × selected peak-flow HRT ÷ 24

Denitrification loading = NOx-N removal load ÷ post-anoxic volume

Alkalinity recovery = NOx-N removal load × 3.57 kg as CaCO₃/kg NOx-N

EPA guidance addresses the role of carbon in nitrogen removal, including dosage, kinetics, and carbon-source selection. The theoretical electron-donor demand is lower than many practical design doses, so the COD/N factor and safety margin remain deliberately editable.[1][2]

Scope and hand-off

This calculator is intended to follow the Nitrogen Removal Calculator when an external carbon deficit remains. It stores the current product demand and annual cost in this browser session for a future operating-cost hand-off; it does not modify any other calculator automatically.