Glossary term

Readily Biodegradable COD

Wastewater COD fraction that microorganisms can use quickly, important for denitrification, EBPR, carbon dosing and biological process control.

Branch: Environmental Engineering
Glossary type: metric
Content: Glossary term
Updated: Jun 25, 2026
Revision: v1.0.0 · reviewed

Definition

metric

Readily biodegradable COD is the fraction of chemical oxygen demand that wastewater microorganisms can consume quickly under biological treatment conditions.

Readily biodegradable COD, commonly abbreviated rbCOD, is used to evaluate denitrification carbon supply, enhanced biological phosphorus removal, oxygen uptake, fermentation value, carbon dosing and biological treatment stability. It is not the same as total COD, soluble COD, BOD or TOC. Interpretation depends on fractionation method, filtration, respirometry, sample holding time, industrial contribution, primary treatment, fermentation, temperature, biomass acclimation and the process decision being made.

Readily biodegradable COD is the fraction of wastewater chemical oxygen demand that microorganisms can use quickly. It is commonly abbreviated rbCOD and reported in $\text{mg/L as COD}$ .

rbCOD matters because biological nutrient removal is often carbon limited. Denitrification needs an electron donor, and enhanced biological phosphorus removal needs readily biodegradable carbon in the anaerobic zone. Total COD can be high while the useful fast carbon fraction is too low.

Engineering Meaning

At a practical level:

COD=rbCOD+slow\ biodegradable\ COD+inert\ COD

The split is process dependent. Soluble COD is not automatically rbCOD, and BOD does not give the same time-scale information. Industrial wastewater, fermentation, primary clarification, sewer residence time and sample holding can all change the fraction available to biology.

rbCOD Fraction

A simple fraction is:

\displaystyle f_{rb}=\frac{rbCOD}{COD}

For:

rbCOD=85\ \text{mg/L},\quad COD=320\ \text{mg/L}

the readily biodegradable fraction is:

\displaystyle f_{rb}=\frac{85}{320}=0.266

or about $26.6\%$ . This fraction is useful only when the COD method and fractionation method are stated.

Carbon Load

For a wastewater flow:

L_{rbCOD}=Q(rbCOD)(0.001)

where $Q$ is in $\text{m}^3/\text{day}$ , rbCOD is in $\text{mg/L as COD}$ and $L_{rbCOD}$ is in $\text{kg COD/day}$ .

For:

Q=16000\ \text{m}^3/\text{day},\quad rbCOD=85\ \text{mg/L}

the load is:

L_{rbCOD}=16000(85)(0.001)=1360\ \text{kg COD/day}

Load is the right basis for carbon limitation, equalization, side-stream fermentation value and supplemental carbon comparison.

Denitrification Screen

A common screening demand for nitrate reduction is:

COD_N\approx2.86\Delta NO_x\text{-}N

If:

\Delta NO_x\text{-}N=6.5\ \text{mg/L as N}

then:

COD_N\approx2.86(6.5)=18.6\ \text{mg/L as COD}

If the anoxic zone receives:

rbCOD_{anoxic}=45\ \text{mg/L as COD}

the concentration margin is:

COD_{margin}=45-18.6=26.4\ \text{mg/L as COD}

This does not prove denitrification success because nitrate recycle, dissolved oxygen leakage, mixing, SRT, temperature and competing carbon use also matter.

EBPR Carbon Ratio

For EBPR screening, a carbon-to-phosphorus ratio can be written:

\displaystyle R_{rb/P}=\frac{rbCOD_{ana}}{P_{removed}}

For:

rbCOD_{ana}=180\ \text{mg/L},\quad P_{removed}=5.2\ \text{mg/L as P}

the ratio is:

\displaystyle R_{rb/P}=\frac{180}{5.2}=34.6\ \text{kg COD/kg P}

The useful fraction may be volatile fatty acids rather than all measured rbCOD, so this ratio is a screen, not a guarantee of PAO selection.

Measurement Boundary

rbCOD is often estimated by respirometry, wastewater characterization, fractionation models or plant-calibrated correlations. Filtered COD, soluble COD and rapid COD tests can help, but they do not automatically isolate the fraction microorganisms can use quickly.

Sampling should state grab or composite basis, holding time, filtration method, temperature, preservation, upstream fermentation, industrial batch discharges, primary clarifier performance and whether the result represents influent, settled wastewater, anoxic feed, anaerobic feed or side-stream carbon.

Validation Evidence

Useful evidence includes total COD, soluble COD, BOD, TOC, VFA where available, nitrate removal, phosphorus release/uptake, oxygen uptake rate, flow, temperature, SRT, MLSS, DO, ORP, recycle flows, carbon dose, fermentation operation, wet-weather dilution and historical treatment response.

Validation should connect rbCOD to the decision: denitrification capacity, EBPR stability, supplemental carbon dosing, primary treatment adjustment, fermentation upgrade, industrial source control or post-upset recovery.

Common Mistakes

Common mistakes include treating all COD as biodegradable, treating soluble COD as rbCOD, ignoring sample holding time, using one grab sample for a diurnal process, comparing rbCOD ratios without nutrient loads, and dosing external carbon without checking recycle flows or oxygen leakage. A strong rbCOD review states the method, fraction boundary, flow/load basis, nutrient target, process zone and validation evidence.

REF

Disciplines

Readily Biodegradable COD

Definition

Engineering Meaning

rbCOD Fraction

Carbon Load

Denitrification Screen

EBPR Carbon Ratio

Measurement Boundary

Validation Evidence

Common Mistakes

See also