COD and Oxygen Demand Calculator

Calculate theoretical oxygen demand and treatment efficiency for wastewater.

COD measured (mg/L) (mg/L)

BOD5 measured (mg/L) (mg/L)

Flow rate Q (m³/day) (m³/day)

Target effluent COD (mg/L) (mg/L)

Typical discharge limit: 30-125mg/L

Please check your inputs and try again.

COD, BOD and What Their Ratio Tells You

Both measure oxygen demand, but by different routes. COD oxidises everything chemically using dichromate under strong acid at 150°C, taking about two hours. BOD measures what microorganisms consume over five days at 20°C. COD is therefore always the larger figure, because it includes material bacteria cannot digest.

BOD₅/COD ratio	Biodegradability	Treatment implication
> 0.6	Readily biodegradable	Biological treatment works well
0.3–0.6	Moderately biodegradable	Biological treatment viable
< 0.3	Poorly biodegradable	Needs chemical or physical pretreatment

That ratio is the key diagnostic when sizing a treatment plant. Municipal sewage typically runs 0.4 to 0.6 and responds well to activated sludge. Industrial effluent containing solvents, dyes or pharmaceuticals often falls below 0.3, where bacteria simply cannot process the load and advanced oxidation or adsorption is required first.

Load (kg/day) = COD (mg/L) × Q (m³/day) / 1000

Load matters more than concentration for plant sizing. A dilute high-volume stream and a concentrated low-volume one can impose identical demand on the treatment process.

Worked Examples

Example 1: Municipal wastewater: COD=450, BOD=200, Q=10000m³/day

BOD/COD=0.44, load=4500 kg/day

Result: Target 50mg/L: 88.9% removal needed

Standard municipal WWTP target

Example 2: Industrial: COD=2000, BOD=400, Q=500m³/day

BOD/COD=0.2 — poorly biodegradable

Result: Need physico-chemical pretreatment

Refinery or textile wastewater characteristic

Example 3: Poorly biodegradable industrial stream

COD 2,000, BOD 400 mg/L

Result: Ratio 0.20 — pretreatment required

Below 0.3 the organic load resists biological breakdown. Advanced oxidation or activated carbon adsorption is needed before conventional treatment.

Example 4: Removal efficiency required

Influent 450 mg/L, consent limit 50 mg/L

Result: 88.9% removal

Removal efficiency is (450 − 50)/450. This figure determines the process configuration and the required residence time.

Common Mistakes

⚠️

Comparing COD across different effluents without the ratio

COD alone says how much oxidisable material is present, not whether biology can remove it. The BOD/COD ratio is what determines treatment strategy.

⚠️

Assuming BOD₅ is total biological demand

The five-day test captures most carbonaceous demand but not all. Ultimate BOD is higher, and nitrogenous demand appears later unless inhibited.

⚠️

Sizing a plant on concentration rather than load

Load is concentration multiplied by flow. A dilute stream at high flow can impose the same oxygen demand as a concentrated trickle.

⚠️

Ignoring chloride interference in the COD test

Chloride is oxidised by dichromate and inflates the result. Mercuric sulfate is added to complex it, which is why the test generates hazardous waste.

Frequently Asked Questions

COD test procedure?

Dichromate oxidation at 150°C, 2 hours. K2Cr2O7 oxidizes most organic matter. Result in mg O2/L equivalent. ThOD (theoretical) = calculated from molecular formula. COD always ≥ BOD because it oxidizes nonbiodegradable compounds too.

Wastewater treatment sizing?

WWTP capacity often expressed in 'population equivalent' (PE = 60g BOD/day per person). Large municipal plant: 100,000 PE. Industrial plants can be 10-100× more concentrated. HRT (hydraulic retention time) determines tank volume.

What is the difference between COD and BOD?

COD chemically oxidises all organic matter in about two hours. BOD measures what microorganisms consume over five days, so it captures only the biodegradable fraction.

What does the BOD/COD ratio indicate?

Biodegradability. Above 0.6 is readily treatable biologically; below 0.3 indicates material bacteria cannot process, requiring chemical or physical pretreatment.

Why is COD always higher than BOD?

Because chemical oxidation attacks compounds that microorganisms cannot degrade, including many synthetic organics.

Why does the COD test use mercury salts?

Mercuric sulfate complexes chloride, which would otherwise be oxidised by dichromate and inflate the result. This is why COD testing produces hazardous waste.

Formula Explorer connections

Interpretation: This analytical relationship converts an instrument signal, separation measure or optical response into concentration, identity or performance. Assumption: Calibration, blank correction, linear range, path length, matrix effects and instrument settings must match the sample and method.

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