Theoretical COD & BOD
Compute the theoretical oxygen demand (ThOD) of an organic compound from its formula by full-oxidation stoichiometry, the benchmark a measured COD should approach, and estimate BOD₅ from a BOD/COD ratio for your effluent class.
How to use this tool
Estimate how much oxygen it takes to break down an organic compound in water, the figure behind COD and BOD discharge limits. Use it to predict the oxygen demand of a known effluent before you measure it.
What to enter
- Molecular formula: the organic compound, e.g. C6H12O6. Handles C, H, O, N, S and halogens.
- Concentration: optional, in mg/L. Add it to turn the per-gram demand into a theoretical COD for your stream.
- BOD/COD ratio: how biodegradable the effluent is; pick the class (readily 0.6 → poorly 0.2). This scales COD down to an estimated BOD₅.
- Include nitrogenous demand: tick to also count oxygen used oxidising nitrogen to nitrate.
Reading the result
The headline is ThOD, the theoretical oxygen demand in grams O₂ per gram of compound, the ceiling a real COD measurement approaches. With a concentration you also get theoretical COD (mg/L) and an estimated BOD₅. Real measured values usually land a little lower.
Worked example
Glucose C6H12O6 needs 6 mol O₂ per mole (ThOD ≈ 1.07 g O₂/g); at 300 mg/L that is a theoretical COD of ≈ 320 mg/L and, at the typical municipal ratio, BOD₅ ≈ 160 mg/L.
Compound & load
Result
Methodology
For CcHhOoNnSsXx (X = halogen), the carbonaceous demand assumes complete oxidation with nitrogen leaving as ammonia: O₂ (mol) = c + (h − 3n − x)/4 − o/2 + s. ThOD per gram = O₂ × 31.998 / MW. With a concentration, theoretical COD (mg/L) = concentration × ThOD(g/g). BOD₅ = COD × the chosen BOD/COD ratio. Including nitrification (nitrogen oxidised to nitrate) adds 2 O₂ per nitrogen.
Known limits
- ThOD is the theoretical maximum. Measured COD is usually a little lower (incomplete dichromate oxidation of some structures) and BOD depends on the seed, acclimation, and toxicity.
- The BOD/COD ratio is an effluent-class rule of thumb; measure it for your stream where compliance matters.
- Inorganic oxygen demand (e.g. sulfide, ferrous iron) is not included.