What Are BOD and COD in Wastewater?
BOD measures the oxygen microorganisms use to break down organic matter; COD measures the oxygen needed to oxidize it chemically. Both are in mg/L.
BOD (Biochemical Oxygen Demand) is the amount of oxygen, in mg/L, that microorganisms consume to break down the biodegradable organic matter in wastewater; COD (Chemical Oxygen Demand) is the oxygen, also in mg/L, needed to chemically oxidize all organic matter, whether biodegradable or not. In short: BOD measures what bacteria can eat; COD measures everything that is oxidizable. That is why COD is always greater than or equal to BOD in the same sample.
Both are the two most important parameters for characterizing an effluent and for sizing a treatment plant, because they indicate the organic load the system must remove.
How they are measured
They are two different laboratory tests:
- BOD5: the dissolved oxygen consumed by microorganisms is measured over 5 days at 20 °C in darkness. It is a biological test, which is why it takes time. It is reported as BOD5 in mg/L.
- COD: the sample is oxidized with a strong oxidant (potassium dichromate) under hot, acidic conditions. It is a chemical test that gives a result within hours. It is reported in mg/L.
COD is faster and more reproducible, so many plants use it for daily control, while BOD is used to verify regulatory compliance and to estimate the biodegradable fraction.
The BOD/COD ratio: why it matters
The BOD/COD ratio indicates how biodegradable an effluent is and, therefore, which treatment is best suited:
| BOD5/COD ratio | Interpretation | Recommended treatment |
|---|---|---|
| Above 0.5 | Highly biodegradable | Direct biological treatment |
| 0.3 to 0.5 | Biodegradable | Biological, possible adjustment |
| Below 0.3 | Poorly biodegradable | Physical-chemical pre or combined |
Slaughterhouse or dairy effluent usually has a high ratio (highly biodegradable), ideal for anaerobic treatment. Industrial effluent with toxics or recalcitrant compounds may have a low ratio and need pretreatment.
Typical load ranges
As a rough guide, to place magnitudes:
- Raw domestic wastewater: BOD5 on the order of 200–400 mg/L.
- Food-industry effluent: BOD5 typically 1,000 to 5,000 mg/L or higher.
- Discharge limit to a water body: usually requires BOD5 below low values set by the environmental authority.
A well-operated anaerobic reactor, such as a UASB reactor, removes on the order of 65–75% of the BOD in high-strength effluents, and often requires a downstream aerobic (polishing) stage to meet the discharge limit.
Where it applies in Tech Tank projects
Characterizing BOD and COD is the first step of any sanitation project. At Tech Tank these parameters define:
- The type of wastewater treatment plant (anaerobic, aerobic or combined).
- The volume of reactors and tanks (higher load means larger volume or retention time).
- Whether it is worth recovering biogas from the organic matter, which is viable when the load is high.
To understand the full flow of a turnkey plant, read our pillar on the turnkey wastewater treatment plant. In Paraguay, discharge limits are set within the MADES framework and Water Resources Law 3239/07; it is recommended to verify current regulations and the environmental license with MADES for each project. If you need to characterize your effluent and size the system, you can contact the Tech Tank technical team.
Frequently Asked Questions
Why is COD greater than BOD?
Because COD oxidizes all organic matter, including what bacteria cannot degrade. BOD only counts the biodegradable fraction, so it is always equal to or lower than COD in the same sample.
What does BOD5 mean?
The subscript 5 indicates the test runs for 5 days at 20 °C. It is the standard version of Biochemical Oxygen Demand used in most discharge regulations.
Can COD alone be used to control a plant?
For daily operational control, yes, because it is fast and reproducible. But regulatory compliance and biodegradability assessment usually also require BOD5.
What BOD/COD ratio is good for biogas?
A high ratio, above 0.5, indicates a highly biodegradable effluent, ideal for anaerobic treatment and biogas recovery. Slaughterhouse, dairy and beverage effluents usually meet this condition.
How are BOD and COD reduced?
Through biological treatment (anaerobic and/or aerobic) that degrades organic matter, complemented by physical-chemical stages when the effluent is poorly biodegradable. The goal is to meet the environmental authority's discharge limit.