COD tells you the total oxygen demand of everything oxidizable in your water sample — in hours, not days. It's faster than BOD, broader in scope, and used daily for process control at facilities across the country.
If you're studying for your wastewater operator certification — or working in a plant lab — you need to understand COD. It shows up on the exam alongside BOD, it's used for daily process control at many facilities, and the relationship between COD and BOD tells you things about your wastewater that neither measurement alone can.
This article covers what COD is, how it's measured, how it compares to BOD, what the COD/BOD ratio tells you, and exactly what the operator exam tests on the topic.
Chemical Oxygen Demand (COD) is a measurement of the total amount of oxygen required to chemically oxidize all oxidizable material in a water sample. Unlike BOD — which uses bacteria to biologically oxidize organic material over 5 days — COD uses a strong chemical oxidant to chemically oxidize everything that can be oxidized, and it does it in 2–3 hours.
COD is measured in milligrams per liter (mg/L) and represents the oxygen equivalent of all oxidizable compounds in the sample — both biodegradable organic material and non-biodegradable compounds that bacteria can't break down.
BOD measures what bacteria can break down biologically. COD measures everything that can be oxidized chemically — including things bacteria can't touch. This is why COD is always equal to or greater than BOD. If your COD is significantly higher than your BOD, a meaningful fraction of your oxygen demand comes from non-biodegradable compounds.
Uses live bacteria to oxidize biodegradable organic material. Takes 5 days at 20°C in the dark. Measures only what microorganisms can decompose. Slower but reflects actual biological treatment demand. Required for most NPDES permit compliance reporting.
Uses a strong chemical oxidant (potassium dichromate) to oxidize all oxidizable material. Results in 2–3 hours. Measures both biodegradable and non-biodegradable compounds. Faster and more reproducible. Used for rapid process control and industrial monitoring.
The standard COD test uses potassium dichromate (K2Cr2O7) as the oxidizing agent in a strongly acidic solution. Here's how it runs:
A measured volume of the wastewater sample is combined with potassium dichromate, sulfuric acid, and a silver sulfate catalyst in a sealed digestion tube. The tube is heated to 150°C for 2 hours. During this time, the dichromate oxidizes virtually all oxidizable compounds in the sample — organic and inorganic alike.
After digestion, the amount of dichromate consumed is measured — either by colorimetry (the color change from orange to green as Cr6+ is reduced to Cr3+) using a spectrophotometer, or by titration with ferrous ammonium sulfate. The more oxidizable material in the sample, the more dichromate was consumed.
The oxygen equivalent of the dichromate consumed is calculated and expressed as mg/L of oxygen. Most modern labs use pre-made COD vials (Hach or equivalent) that include the reagents in sealed tubes — the technician adds the sample, digests, and reads the result directly on a colorimeter. The calculation is built into the instrument.
Chloride ions in the sample are oxidized by dichromate, giving falsely high COD readings. This is why silver sulfate is added as a catalyst — it precipitates chloride out of solution before it can interfere. For samples with very high chloride (saline water, some industrial waste), mercuric sulfate is also added. Chloride interference is a commonly tested exam detail.
The ratio of COD to BOD is one of the most useful diagnostic tools in wastewater characterization. It tells you how biodegradable your wastewater is:
| COD/BOD Ratio | What It Means | Implication |
|---|---|---|
| 1.0–2.0 | Highly biodegradable waste | Most of the COD is biodegradable — biological treatment will be very effective. Typical of fresh domestic wastewater (ratio ~1.5–2.0). |
| 2.0–3.5 | Moderately biodegradable | Significant non-biodegradable fraction present. Biological treatment will remove BOD but COD will remain elevated in effluent. Common with mixed municipal/industrial waste. |
| > 3.5 | Low biodegradability | Large non-biodegradable fraction. Biological treatment alone may not meet effluent limits. Chemical or advanced treatment may be needed. Common with certain industrial wastewaters. |
For typical domestic municipal wastewater, the COD/BOD ratio is usually 1.5 to 2.5. A ratio outside this range is a signal worth investigating — it may indicate an industrial discharge, a septage or high-strength waste event, or a change in service area.
The BOD5 test takes 5 days. You can't wait 5 days to know if your process is working. COD solves this problem.
Once a facility has established a reliable COD-to-BOD correlation for their specific wastewater, COD results from a 2-hour test can be used to estimate BOD in near-real time. This allows operators to:
To use COD as a BOD proxy, a facility runs parallel BOD and COD tests on the same samples over time and calculates a site-specific correlation factor. Once established, a COD result can be converted to an estimated BOD using that factor. Every facility's correlation is slightly different — you can't assume a generic ratio will work for your specific wastewater without validating it with your own data.
This is where BOD and COD get more nuanced — and where the exam gets more specific.
Total BOD in a sample actually has two components:
COD does not distinguish between these — it measures total chemical oxygen demand. But when comparing COD to BOD for process control purposes, it matters whether your BOD measurement includes nitrogenous demand or just carbonaceous demand. Many labs now run CBOD5 — a BOD test with a nitrification inhibitor added — to isolate the carbonaceous fraction and make a cleaner comparison to COD.
Some NPDES permits specify effluent limits as CBOD5 rather than BOD5 — particularly when the receiving water body has a separate concern about ammonia. If a permit specifies CBOD5, the lab runs the test with a nitrification inhibitor. This distinction shows up on Class II and above exams.
| Sample Point | Typical COD (mg/L) | Notes |
|---|---|---|
| Raw domestic influent | 300–600 mg/L | Varies with dilution, I/I, and industrial inputs |
| Primary effluent | 200–400 mg/L | 20–30% COD reduction through primary settling |
| Secondary effluent | 40–100 mg/L | Residual COD includes non-biodegradable compounds |
| Industrial wastewater | 500–50,000+ mg/L | Highly variable — food processing, pulp/paper, chemical plants |
| Digester supernatant | 1,000–5,000 mg/L | High-strength return stream — significant internal load |
| Topic | What to Know |
|---|---|
| Definition | Oxygen equivalent of all chemically oxidizable material — biodegradable and non-biodegradable |
| Test method | Potassium dichromate oxidant, 150°C digestion, 2–3 hours |
| COD vs. BOD | COD ≥ BOD always; COD is faster (hours vs. 5 days); COD includes non-biodegradable demand |
| Chloride interference | Chloride gives falsely high COD — silver sulfate is added to prevent this |
| COD/BOD ratio | ~1.5–2.5 for typical domestic wastewater; higher ratio = less biodegradable |
| Process control use | Faster than BOD — used to estimate BOD via established correlation for daily monitoring |
| CBOD vs. BOD5 | CBOD5 adds nitrification inhibitor to isolate carbonaceous demand from nitrogenous demand |
| Why COD stays elevated after treatment | Non-biodegradable fraction cannot be removed by biological secondary treatment |
| Parameter | COD | BOD5 |
|---|---|---|
| Full name | Chemical Oxygen Demand | Biochemical Oxygen Demand (5-day) |
| Test method | Chemical oxidation (K2Cr2O7) | Biological oxidation (bacteria) |
| Test duration | 2–3 hours | 5 days at 20°C |
| What it measures | All oxidizable material — biodegradable + non-biodegradable | Biodegradable organic material only |
| Units | mg/L | mg/L |
| Relationship | COD ≥ BOD always | BOD ≤ COD always |
| Typical domestic influent | 300–600 mg/L | 150–350 mg/L |
| Primary use | Process control, industrial monitoring, rapid screening | Permit compliance reporting, treatment performance |
| Limitation | Doesn't distinguish biodegradable from non-biodegradable | Slow — can't use for real-time process control |
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