COD and BOD in wastewater both quantify the oxygen needed to oxidise organic matter, but by different means: BOD measures biologically oxidisable material over 5 days, while COD chemically oxidises almost everything in hours. Their ratio indexes biodegradability, and both drive the oxygen-demand and reactor-sizing calculations at the heart of process design.

What exactly do BOD and COD measure?

Biochemical oxygen demand (BOD) is the mass of dissolved oxygen consumed by micro-organisms while they aerobically metabolise the organic (and, if unsuppressed, the nitrogenous) matter in a sample. The standard test incubates a diluted, seeded sample in the dark at 20 °C and measures dissolved-oxygen depletion over five days — hence BOD5, reported in mg O2/L. Adding a nitrification inhibitor (e.g. allylthiourea) gives the carbonaceous demand, cBOD5.

Chemical oxygen demand (COD) is the oxygen equivalent of the organic matter oxidisable by a strong chemical oxidant — acidified potassium dichromate at 148 °C for two hours, with a silver catalyst and mercuric sulphate to mask chlorides. Because dichromate attacks nearly all organic compounds (and some inorganics), COD is always ≥ BODu and returns a result the same day. COD is the practical master variable for load balances; BOD tells you how much of that load is biologically available.

What is theoretical oxygen demand (ThOD) and why does COD approximate it?

Theoretical oxygen demand (ThOD) is the stoichiometric oxygen required to oxidise a compound completely to CO2, H2O and (for organic N) a defined nitrogen end-state. It is computed directly from the balanced oxidation half-reaction, so it is an exact number for a known molecule. For glucose:

C6H12O6 + 6 O2 → 6 CO2 + 6 H2O
ThOD = (6 mol O2 × 32 g/mol) ÷ (180 g/mol glucose) = 1.07 g O2 per g glucose. General form: for CnHaOb, O2 demand = n + a/4 − b/2 moles per mole of substrate.

A well-run dichromate COD recovers 95–100 % of ThOD for most compounds, so COD ≈ ThOD in practice. The gap comes from species the oxidant cannot fully attack (pyridine, some aromatics, straight-chain hydrocarbons) or from reduced inorganics (sulphide, ferrous iron, nitrite) that inflate COD without being “organic.” This equivalence is what lets engineers translate a measured COD straight into an aeration oxygen requirement.

Why is the BOD:COD ratio the key biodegradability index?

Since COD captures total oxidisable matter and BOD only the biologically available fraction, their ratio measures how amenable an effluent is to biological treatment. A high ratio means a biological plant — activated sludge, MBBR, anaerobic digestion — will do the work cheaply; a low ratio signals refractory or inhibitory matter that may demand physicochemical polishing.

  • BOD5:COD > 0.5 — readily biodegradable; conventional biology is well suited.
  • 0.3–0.5 — biodegradable but slower; expect longer sludge age or acclimatised biomass.
  • < 0.3 — poorly biodegradable; consider pre-treatment (ozonation, Fenton, coagulation) or accept a persistent residual COD.

Tracking the ratio through a works is diagnostic: it should fall across a biological stage as the easy substrate is consumed, leaving a harder residual. For sector-specific benchmarks see our guide to industrial wastewater treatment, where the ratio drives the whole treatment train selection.

How is COD fractionated for process modelling?

Modern design does not treat COD as a single number. The IWA Activated Sludge Models (ASM1–3) partition total COD into fractions with distinct fates, because a simulator needs to know what is degradable, how fast, and what simply passes through.

FractionSymbolBehaviourTypical % of total COD
Readily biodegradable (soluble)SSTaken up almost immediately; drives peak oxygen uptake10–25 %
Slowly biodegradable (particulate)XSHydrolysed before uptake; rate-limiting for O2 demand40–60 %
Inert solubleSIPasses through unchanged; sets the effluent COD floor5–10 %
Inert particulateXIEnmeshed in sludge and wasted; adds to solids production10–15 %

The soluble inert SI is why even a perfect biological plant cannot reach zero COD — it fixes the achievable effluent floor. The particulate inert XI matters for sludge yield, not oxygen demand. Fractions are measured by respirometry and filtration/flocculation protocols and fed straight into the reactor mass balances that our activated sludge process design guide develops. A process design review of the influent fractionation is usually the fastest way to de-risk a biological upgrade.

Worked example: BODu, and converting concentration to a daily load

An industrial effluent measures BOD5 = 600 mg/L with a deoxygenation constant k = 0.23 d−1 at 20 °C, flowing at Q = 480 m3/d. Estimate the ultimate BOD and the daily oxygen-demand load.

  • Fraction of BODu exerted in 5 d: 1 − e−(0.23×5) = 1 − e−1.15 = 1 − 0.317 = 0.683.
  • Ultimate BOD: BODu = BOD5 / 0.683 = 600 / 0.683 = 879 mg/L (≈ 880 mg O2/L).
  • Convert to mass load: load (kg/d) = concentration (g/m3) × flow (m3/d) ÷ 1000. Using BOD5: 600 × 480 / 1000 = 288 kg BOD5/d.
  • Ultimate oxygen demand load: 879 × 480 / 1000 = 422 kg BODu/d — the figure that actually governs aeration supply.

If the same stream carries COD = 1,500 mg/L, the BOD5:COD ratio is 600/1500 = 0.40 — biodegradable but not effortlessly so, consistent with a longer sludge age. Sizing aeration on the 288 kg/d BOD5 figure alone would under-provide oxygen by roughly 30 %; the 422 kg/d ultimate load is the defensible design basis (before adding nitrogenous demand and the endogenous respiration allowance).

How do TOC and the other surrogates fit in?

Total organic carbon (TOC) measures the carbon itself — the sample is oxidised (combustion or UV/persulphate) and the evolved CO2 quantified. It is fast (minutes), needs no toxic dichromate or mercury, and is increasingly used for online monitoring. Because it counts carbon rather than oxygen demand, TOC correlates with COD through a site-specific factor, typically COD/TOC ≈ 2.5–4 g O2 per g C depending on the mean oxidation state of the organics (the theoretical ratio for carbohydrate is 2.67).

Establish the correlation once against parallel COD data, then use TOC for rapid trending and control while retaining COD/BOD for regulatory reporting and design. Choosing surrogates well is part of a sound wastewater characterisation and monitoring strategy, and it links directly to wider wastewater management decisions on instrumentation and consent compliance.

Frequently asked questions

Is COD always higher than BOD?

Yes. Chemical oxidation with dichromate attacks nearly all organic matter, whereas the BOD test only registers what micro-organisms can degrade in five days. COD therefore always exceeds even the ultimate BOD, and typically runs 1.5–3 times BOD5 for municipal sewage. A very high COD:BOD ratio flags refractory or toxic material.

Why use a 5-day BOD test rather than measuring ultimate BOD directly?

Five days is a historical compromise: it captures most of the carbonaceous demand while giving a workable turnaround, and it originated from the maximum travel time of British rivers to the sea. Ultimate BOD needs 20–30 days, which is impractical for routine control, so BOD5 is converted to BODu using the first-order rate constant.

What deoxygenation rate constant should I assume?

For raw municipal sewage, k (base-e, 20 °C) is usually 0.20–0.35 d−1; well-treated effluent is slower at 0.12–0.23 d−1 as the easy substrate is already gone. Correct for temperature with kT = k20·1.047(T−20), and confirm whether a quoted value is base-e or base-10.

What does the BOD:COD ratio tell a designer?

It indexes biodegradability. Above 0.5 the effluent is readily treated biologically; between 0.3 and 0.5 it degrades more slowly and needs a longer sludge age; below 0.3 it is poorly biodegradable and usually needs physicochemical pre-treatment. The ratio should decline across a biological stage as the accessible substrate is consumed.

Why fractionate COD into SS, XS, SI and XI?

Biological models such as ASM1 need to know not just how much organic matter is present but how it behaves. Readily biodegradable SS drives peak oxygen uptake, slowly biodegradable XS must be hydrolysed first, soluble inert SI sets the effluent COD floor, and particulate inert XI adds to sludge production. Lumping them together loses this predictive power.

Can TOC replace COD and BOD?

Not for compliance, but it is excellent for fast online monitoring. TOC measures carbon directly and correlates with COD through a site-specific factor of roughly 2.5–4 g O2 per g C. Calibrate it against parallel COD data, then use TOC for real-time trending while keeping COD and BOD for regulatory reporting and design.

Sources & further reading