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COD vs BOD vs TOC: What Each Measures & When to Use It
COD, BOD and TOC all describe organic matter in water, but they answer three different questions. Choosing the wrong one for the job — or assuming a universal conversion between them — is one of the most common mistakes in effluent monitoring. Here is what each parameter actually measures and when each one earns its place on your panel.
What each parameter measures
| Parameter | What it measures | Method basis | Time to result | Question it answers |
|---|---|---|---|---|
| BOD₅ | Oxygen consumed by microorganisms degrading biodegradable organics (5 days, 20 °C) | Bioassay | 5 days | "Will this load overwhelm the biology of the receiving water or my plant?" |
| COD | Organics oxidizable by a strong chemical oxidant — includes hard-to-degrade fractions | Dichromate digestion (laboratory); faster online methods exist | 2–3 h lab; minutes online | "How chemically strong is this stream right now?" |
| TOC | Total carbon bound in organic compounds | High-temperature catalytic combustion or UV-persulfate oxidation, followed by CO₂ detection | Minutes | "How much organic carbon is there in total, including the non-biodegradable part?" |
BOD vs COD: the biodegradability clue
BOD captures only what microbes can degrade; COD captures everything a chemical oxidant can. The ratio of the two is a quick fingerprint of your wastewater: a BOD₅/COD ratio above roughly 0.3 is commonly used as a screening indicator of good biodegradability, while a low ratio points to a toxic or inert load that will pass through biological treatment. When the ratio suddenly drops in an industrial discharge, the biology in your aeration tank is about to have a bad week — this is one of the most useful early warnings a monitoring program can give you.
COD vs TOC: different units, no universal factor
COD is an oxygen demand (expressed as O₂ equivalents); TOC is a carbon mass. They are related through the oxidation state of the organic matter itself — methanol, acetic acid and phenol carry very different oxygen demand per unit of carbon. That is why a single universal COD/TOC factor does not exist. If you need to infer one from the other, build a site-specific correlation with paired samples on your own stream, and re-validate it whenever the production recipe changes.
Which one to monitor online
| Use case | Recommended focus |
|---|---|
| Aeration and process control | A fast surrogate (UV254 or online TOC/COD) correlated to your load — BOD₅ is far too slow to control anything |
| Discharge compliance | Whatever your permit names — commonly COD or BOD; match the permitted method exactly |
| Intake protection, organics spikes | TOC — it responds in minutes and catches non-biodegradable contamination that BOD ignores |
| Industrial discharge characterisation | COD plus BOD₅/COD ratio to understand treatability before design decisions |
How this connects to sensor selection
Online TOC and COD trends come from optical analyzers — UV254 absorbance for COD-like streams, combustion or UV-persulfate for true TOC. BOD remains a laboratory procedure in most permits. If your aim is early warning and process control, instrument the fast parameters and pull laboratory BOD₅ on a schedule for validation. Our deep dives cover each route in detail: COD sensor monitoring, BOD sensor monitoring and the TOC analyzer selection guide.
Frequently asked questions
Can I convert COD to BOD with a formula?
Only with a site-specific correlation built from paired samples on your own stream. Universal factors ignore the composition of your organics and will mislead you exactly when the stream changes.
Why is TOC lower than COD for the same sample?
Different units: TOC counts carbon mass, COD counts oxygen demand. The ratio between them depends on how oxidized the organic molecules are, so it varies by stream and even by production batch.
Is UV254 the same as COD?
No. UV254 is an absorbance surrogate that correlates with organic load in many waters. Treat it as an online trend that needs a laboratory correlation before its value can be reported as COD.
Which parameter should control my aeration blowers?
A fast surrogate with a validated correlation to load — typically UV254 or online TOC/COD. A five-day BOD result arrives long after the event it should have controlled.
Building an online organic load monitoring panel?
We help plants pair UV254 and TOC instrumentation with laboratory validation so the numbers actually control the process.
