BOD tells you what your POTW can break down. COD tells you what it can’t — and that gap is the number your regulator is watching.
If BOD is on your permit, COD almost certainly is too. The two show up side by side on every lab report, they both measure oxygen demand, and operators routinely treat them as two names for the same thing. They are not the same thing — and the difference between them is often the difference between a clean inspection and a surcharge letter.
Here is what COD actually measures, what your COD:BOD ratio is telling you, and what to do when the number climbs.
COD, or chemical oxygen demand, is a measure of the total oxygen required to chemically oxidize every organic and inorganic compound in a water sample. Unlike BOD, it captures compounds that are chemically oxidizable but biologically resistant — the material a treatment plant’s microbes cannot break down. That word total is what makes COD matter.
Biological treatment at your POTW handles biodegradable organic material well. But not everything in industrial wastewater is biodegradable. Solvents, complexing agents, certain surfactants, and process byproducts can pass through biological treatment chemically unchanged. COD sees them. BOD does not.
The COD test works fast. A strong chemical oxidant — usually potassium dichromate — is added to the sample under heat and acidic conditions. Everything oxidizable is oxidized, and the oxygen equivalence is measured. Per Standard Methods 5220, a COD result is available in roughly 2–3 hours, against the five-day minimum for a BOD5 test.
Biochemical oxygen demand (BOD) is a measure of the amount of dissolved oxygen that microorganisms consume as they break down organic matter in wastewater over a five-day period at 20°C. The standard test — called BOD5 — is expressed in milligrams per liter (mg/L) and is used by regulators to assess the organic contamination level of effluent before it enters a waterway or publicly owned treatment works (POTW).
Think of it as a proxy for how much biological stress your discharge will place on the receiving environment. When bacteria in a waterway decompose high-BOD effluent, they consume oxygen. Below approximately 5 mg/L of dissolved oxygen, fish and other aquatic life begin to die. At zero dissolved oxygen — a condition called anoxia — ecosystems collapse. The BOD5 test predicts that risk before it happens.
High BOD = heavy organic load = high oxygen depletion risk. Your permit sets the ceiling. Exceeding it means you have crossed the line from treatment management into regulatory liability.
BOD measures only the oxygen demand microbes will exert on biodegradable material over five days. COD measures the oxygen demand of everything chemically oxidizable — biodegradable or not — in a few hours. BOD is a biological screening number; COD is a total-load number.
| COD | BOD | |
|---|---|---|
| What it measures | Total chemical oxygen demand, including non-biodegradable compounds | Oxygen demand from biodegradable organics only |
| Test method | Chemical oxidation (dichromate, heat, acid) | Biological activity over 5 days (BOD5) |
| Turnaround | 2–3 hours | 5 days minimum |
| What it misses | Effectively nothing measurable by oxidation | Compounds microbes cannot break down |
| Primary users | Industrial pretreatment, process control, POTW compliance | Municipal wastewater, permit baseline, organic load screening |
| Process-control value | High — same-day results enable real-time response | Low — five-day window limits real-time control |
For an industrial facility, that turnaround gap is the practical headline. A BOD5 result tells you what your wastewater looked like last week. A COD result tells you what it looks like today — in time to act before the next discharge.
A COD:BOD ratio around 2:1 generally indicates readily biodegradable wastewater that a POTW’s biological process can handle. As the ratio rises toward 3:1 and beyond, a growing share of the organic load is non-biodegradable or biologically inhibitory — material biological treatment cannot remove.
The COD:BOD ratio is one of the most useful diagnostic numbers in industrial pretreatment because it quantifies biodegradability directly. When the ratio climbs, it is telling you that part of your organic load will pass through your POTW untreated, land in the plant’s effluent, and put pressure on the permit that governs that effluent. Many POTWs surcharge or restrict on COD load alone — independent of BOD.
ChemREADY’s industrial wastewater analysis covers COD, BOD, and the parameters on your pretreatment permit.
This is why COD comes back in hours, not days. The chemistry doesn’t wait for microbes.
COD appears on industrial pretreatment permits because it gives the POTW a fast, total measure of the oxygen-demanding load a facility contributes. A permit’s local limits are set so the combined discharge from all industrial users stays within what the plant can treat without violating its own NPDES permit. COD is the parameter that lets the POTW track that load in hours rather than days.
When a facility exceeds its COD limit, the consequences are rarely a single dramatic event. They are cumulative and financial:
This is the core reason COD deserves equal attention to BOD: a lab report can show acceptable BOD next to elevated COD, and the second number is the one that drives the surcharge. Watching only BOD leaves the expensive parameter unmonitored.
COD problems concentrate in industries whose processes generate compounds that resist biological breakdown — food and beverage, metal finishing, chemical manufacturing, and pharmaceutical operations carry the highest risk. The common thread is process chemistry, not facility size.
| Industry | Common COD sources | Risk level |
|---|---|---|
| Food & Beverage | Fats, sugars, cleaning chemicals, sanitizers, high organic load from processing | HIGH |
| Metal Finishing | Complexing agents, chelators, surfactants, lubricants | HIGH |
| Chemical Manufacturing | Process byproducts, solvents, inhibitory compounds that suppress biological treatment | HIGH |
| Pharmaceutical | Active pharmaceutical ingredients, solvents, fermentation byproducts | HIGH |
| Plastics & Rubber | Plasticizers, additives, polymer residuals resistant to biodegradation | MEDIUM |
| Auto Manufacturing | Coolants, lubricants, paint booth wastewater, adhesive residuals | MEDIUM |
| Textile / Laundry | Dyes, surfactants, bleach residuals, high-temperature discharge | MEDIUM |
| General Industrial | Cleaning agents, process chemicals, janitorial discharge | LOWER |
If your facility sits in a high-risk row, elevated COD is not a question of whether — it is a question of how much, and whether you are measuring it.
There are four primary ways to reduce COD in wastewater: source reduction, coagulation and flocculation, biological augmentation, and advanced oxidation. The right choice depends on what is driving the COD. A high reading tells you something is non-biodegradable, but not what it is or where in the process it originates.
None of these decisions should be made without current data. Facilities that discover a COD problem during a regulatory inspection have no time to evaluate options carefully. The facilities that stay ahead of it test first, identify the driver, and match the treatment to the compound.
COD is not a secondary permit parameter to check after BOD. It measures a fundamentally different fraction of your wastewater load — the fraction biological treatment cannot reach — and it is the number that drives surcharges, triggers violations, and escalates monitoring when it climbs. Watching only BOD on a permit that lists both is leaving the expensive parameter unmonitored.
COD, or chemical oxygen demand, is a measure of the total oxygen required to chemically oxidize every organic and inorganic compound in a water sample. Unlike BOD, it captures compounds that are chemically oxidizable but biologically resistant — the material a treatment plant’s microbes cannot break down.
BOD measures only the oxygen demand microbes will exert on biodegradable material over five days. COD measures the oxygen demand of everything chemically oxidizable — biodegradable or not — in a few hours. BOD is a biological screening number; COD is a total-load number.
A COD:BOD ratio around 2:1 generally indicates readily biodegradable wastewater that a POTW’s biological process can handle. As the ratio rises toward 3:1 and beyond, a growing share of the organic load is non-biodegradable or biologically inhibitory — material biological treatment cannot remove.
COD appears on industrial pretreatment permits because it gives the POTW a fast, total measure of the oxygen-demanding load a facility contributes. COD is the parameter that lets the POTW track that load in hours rather than days.
The consequences are usually cumulative and financial: surcharges for organic load above a threshold, local limit violations with notices of violation and corrective-action plans, and more frequent sampling and reporting.
There are four primary ways to reduce COD in wastewater: source reduction, coagulation and flocculation, biological augmentation, and advanced oxidation. The right choice depends on what is driving the COD.
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Adeline Johnston writes WaterFACTS for ChemREADY on wastewater treatment and dewatering.
Technically reviewed by the ChemREADY wastewater team.