COD vs BOD vs TOC: Choosing the Right Organic Pollution Indicator

COD, BOD and TOC are the three headline indicators of organic pollution in water. They are related but not interchangeable, and choosing the wrong one leads to either compliance gaps or unnecessary cost. This guide explains what each parameter measures, what each is good for, and how they relate to one another.

The Common Thread: Organic Matter and Oxygen Demand

All three parameters describe the organic content of water, but from different angles. When organic matter enters a water body, microorganisms oxidise it, consuming dissolved oxygen in the process. Enough organic load and the oxygen is exhausted — which is why organic pollution is measured through the lens of oxygen demand.

COD: Chemical Oxygen Demand

COD measures the amount of oxygen required to chemically oxidise the organic matter in a sample, using a strong oxidising agent (typically dichromate) under acidic conditions with a catalyst and heat.

• What it captures: Nearly all organic matter, including compounds that resist biological degradation.
• Timescale: About two hours for the standard digestion method.
• Strength: Fast, repeatable, and independent of microbial activity.
• Limitation: Some inorganic species are also oxidised, and the result does not tell you whether the organic matter is biodegradable.

Because the result is available in hours rather than days, COD is the everyday process control parameter in industrial wastewater treatment.

BOD: Biochemical Oxygen Demand

BOD measures the oxygen consumed by microorganisms while they biologically degrade organic matter under controlled conditions — conventionally over five days at 20 °C in the dark (BOD₅).

• What it captures: Only the biodegradable fraction of the organic load.
• Timescale: Five days for BOD₅.
• Strength: Closer to what actually happens in a river or a biological treatment plant, which is why it remains the classic regulatory parameter.
• Limitation: Too slow for real-time control; results are sensitive to seeding, toxicity and sample handling.

A sample containing toxic industrial effluent can produce a misleadingly low BOD because the microorganisms are inhibited — a well-known pitfall when BOD is used alone.

TOC: Total Organic Carbon

TOC measures the amount of carbon bound in organic compounds. The sample is acidified and sparged to remove inorganic carbon, then the remaining organic carbon is oxidised to CO₂ and quantified.

• What it captures: Organic carbon directly, independent of oxidation state.
• Timescale: Minutes.
• Strength: Fast, reagent-light, and not confounded by inorganic species that interfere with COD.
• Limitation: Does not directly express oxygen demand, and gives no information about biodegradability.

TOC is the standard parameter in pharmaceutical and electronics ultrapure water, where organic contamination must be tracked at very low levels.

Side-by-Side Comparison

Aspect	COD	BOD₅	TOC
Measures	Oxygen to chemically oxidise organics	Oxygen consumed by microbes over 5 days	Carbon in organic compounds
Result available in	~2 hours	5 days	Minutes
Biodegradability information	No	Yes	No
Affected by toxic influent	No	Yes — suppressed result	No
Inorganic interference	Possible (e.g. chloride)	Limited	Removed by sparging
Typical primary use	Industrial process control	Regulatory discharge consent	Ultrapure water, rapid screening
Suited to online monitoring	Yes, with reagents	Not practically	Yes

The BOD/COD Ratio: A Useful Diagnostic

The ratio between BOD and COD indicates how treatable a wastewater is by biological means:

• Ratio above about 0.5: Readily biodegradable — biological treatment is well suited.
• Ratio between roughly 0.3 and 0.5: Partially biodegradable — biological treatment possible, often with acclimatisation or pre-treatment.
• Ratio below about 0.3: Poorly biodegradable — consider chemical or physical pre-treatment before biological stages.

Tracking this ratio over time is one of the cheapest ways to detect a change in the incoming waste stream before it upsets the treatment plant.

Choosing the Right Parameter

Your objective	Primary parameter	Why
Meeting a discharge consent	BOD₅ (and COD)	Most consents are written in these terms
Day-to-day plant control	COD	Fast enough to act on the same shift
Detecting toxic shock loads	COD or TOC	Not suppressed by microbial inhibition
Ultrapure water monitoring	TOC	Sensitive at very low concentrations
Continuous online river monitoring	TOC or optical surrogate	Rapid response without reagents
Assessing treatability	BOD/COD ratio	Indicates biodegradable fraction

Online Measurement: The Reagent-Free Alternative

Traditional COD analysis consumes hazardous reagents and produces chemical waste, which makes continuous online operation expensive and environmentally unattractive. UV-Vis full-spectrum sensors take a different approach: they measure the absorption spectrum of the water and derive multiple parameters from it.

A full-spectrum sensor such as the MT-GP-808 determines COD, BOD, TOC, turbidity, colour, total phosphorus, total nitrogen, ammonia nitrogen, nitrate and nitrite from a single optical measurement with no reagents at all. Because turbidity interferes with optical readings, automatic turbidity compensation is essential — on this class of sensor it is built in.

Approach	Reagents	Waste stream	Maintenance	Response time
Wet-chemistry COD analyser	Required	Chemical waste produced	Regular reagent and tube service	Minutes per cycle
UV-Vis full-spectrum sensor	None	None	Periodic cleaning and verification	Continuous / seconds

Conclusion

COD tells you the total oxidisable load quickly, BOD tells you what biology will actually consume, and TOC tells you the organic carbon directly. There is no single best parameter — the right choice depends on whether you are answering a regulator, running a plant, or protecting a receiving water body. In practice, most facilities measure COD for control and BOD for compliance, and use the ratio between them as an early warning of changing treatability.

Frequently Asked Questions

Can COD replace BOD entirely?

Not usually. Regulators generally specify BOD₅ in discharge consents because it reflects real oxygen depletion in the receiving water. COD is better suited to internal process control, where speed matters more.

Why is COD always higher than BOD?

COD oxidises virtually all organic matter chemically, including compounds resistant to biological breakdown. BOD only captures the fraction that microorganisms degrade during the test period, so it is necessarily a subset.

Are COD and BOD results interchangeable between sites?

No. The relationship between them is specific to the wastewater composition. Establish the correlation empirically for your own effluent before using one to estimate the other.

What makes TOC attractive for online monitoring?

Speed and cleanliness: results arrive in minutes, no hazardous reagents are consumed and no chemical waste is produced, which suits continuous unattended operation.

How does an optical sensor produce a BOD value?

It does not measure BOD directly. The UV-Vis spectrum correlates with BOD for a given water type, so the sensor is calibrated against laboratory BOD results for that application. Once correlated, it provides a continuous estimate suitable for trend monitoring and alarming, though periodic laboratory verification remains necessary.