Simply Understanding the Difference Between BOD and COD
The total COD of wastewater is broken down into three fractions:
- Particulate and colloidal COD
- Biodegradable soluble COD
- Refractory COD (or hard COD)
Why Use Both Parameters Together?
BOD and COD are not redundant. They are complementary in wastewater analysis:
- The BOD is useful for sizing biological wastewater treatment plants, as it reflects the biodegradable load that bacteria will need to consume.
- COD also allows for identifying the presence of non-biodegradable pollutants such as detergents, solvents, or hydrocarbons.
Ce test standardisé consiste à incuber un échantillon d’eau à 20°C pendant 5 jours, à l’abri de la lumière. On mesure la concentration d’oxygène dissous au début (J0) et à la fin (J5). La différence donne la DBO5, exprimée en mg O₂/L.
⚠️ Ce que la mesure ne dit pas : elle ne prend en compte que les composés biodégradables. La présence de toxiques, un temps d’adaptation microbien (lag) ou une température inadaptée peuvent fausser les résultats.
🔍 Pourquoi c’est utile : la DBO5 sert à dimensionner les traitements biologiques (boues activées, MBBR…), évaluer la pollution biodégradable, et respecter les normes de rejet.
Total COD
The total COD of wastewater is broken down into three fractions:
- Particulate and colloidal COD
- Biodegradable soluble COD
- Refractory COD (or hard COD)
Particulate and Colloidal COD
Particulate and colloidal COD represents about 35% of the total COD. It is mainly composed of suspended solids (SS) and is essential for understanding the impact of suspended solids on the organic pollution load. Particulate COD is obtained by the difference between raw COD and COD after sample filtration.
According to an IRSTEA study on extended aeration activated sludge wastewater, 1 mg of SS at the outlet of a municipal plant brings on average 1.2 mg of COD and 0.5 mg of BOD5
Knowing that these SS are about 20 mg/L, the corresponding particulate COD is 24 mg COD/L (20 x 1.2). These values are essential for interpreting results and better understanding the link between SS and pollutant load.
In a corrected BOD5 exercise, these values allow analyzing the efficiency of different treatment stages and adjusting parameters accordingly.
Particulate or colloidal COD is obtained by the difference between raw COD and COD after filtration of the sample. It is mainly composed of SS.
If you would like more precise information on the relationship between these parameters, please click on the form below.
Exemple
Exemple concret : Si un échantillon d’eau usée présente une DBO5 de 200 mg/L et une DCO de 400 mg/L, alors le rapport DBO5/DCO sera de 0,5. Cela signifie que 50 % de la charge organique est biodégradable. Un rapport supérieur à 0,4 indique une bonne biodégradabilité et une adaptation au traitement biologique. En revanche, un rapport plus faible peut nécessiter un prétraitement chimique.
| Élément | Valeur | Interprétation |
|---|---|---|
| DBO5 | 200 mg/L | Charge organique biodégradable mesurée sur 5 jours |
| DCO | 400 mg/L | Charge organique totale (biodégradable + réfractaire) |
| Rapport DBO5/DCO | 0,5 | Bonne biodégradabilité (50 % de la charge est biodégradable) |
| Traitement recommandé | Biologique | Adapté si le rapport est > 0,4 |
| Alternatives si rapport faible | Prétraitement chimique | Recommandé si le rapport est < 0,4 |
Biodegradable Soluble COD
At the outlet of a wastewater treatment plant, biodegradable soluble COD represents only a small fraction of the total COD, typically around 10%. It corresponds to the amount of oxygen required to chemically oxidize dissolved organic matter in water that is easily degraded by biological processes.
This parameter is essential for assessing the performance of biological treatment systems, such as moving bed biofilm reactors (MBBR) and activated sludge systems. A high proportion indicates that organic pollutants are readily biodegradable, making their removal easier.
A release of soluble COD that is normally degraded by bacteria may be caused by:
- overdosing of reagents (for example, methanol addition for denitrification),
- insufficient hydraulic retention time in the tanks, preventing complete biodegradation.
It may also indicate:
- a bypass of a treatment stage,
- hydraulic short-circuiting in certain biological filters,
- poor aeration conditions.
To determine biodegradable soluble COD at the outlet of a municipal wastewater treatment plant, the following data are used:
- total BOD₅ (approximately 12 mg/L O₂),
- suspended solids concentration (20 mg/L), multiplied by a factor of 2.4.
Total soluble COD = (Total BOD₅ – BOD₅ associated with suspended solids) × 2.4 = (12 – 20 × 0.5) × 2.4 = 4.8 mg/L O₂.
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MBBR 4N-M.N-S
Biological treatment
Refractory COD or "Hard COD"
Hard COD corresponds to the residual COD of a sample after an ultimate BOD5 analysis. It represents about 55% of the total COD. This is the most difficult fraction to biodegrade and is resistant to biological treatment. It is made up of complex organic substances and certain industrial effluents.
On average, in domestic wastewater, hard COD represents 3 to 5% of the total COD.
Hard COD = Total COD – Particulate COD – Soluble COD
Still according to IRSTEA, with a total COD concentration of 70 mg/L at the outlet, we can estimate that:
Hard COD = 70 – 24 – 4.8 = 41.2 mg/L O2.
Knowing that domestic wastewater contains approximately 900 mg/L O2 at the inlet, the ratio of 3 to 5% is confirmed.
This content in inlet water can increase in case of non-domestic wastewater additions. These contain organic matter that is resistant to biological treatment (certain industrial effluents, landfill leachate, etc.).
Therefore, the nature of non-domestic wastewater must be analyzed before approving inputs into the collection network. Furthermore, it is necessary to regularly ensure that these concentrations follow the thresholds defined by the local authorities.
When setting discharge standards for a plant, authorities should consider:
- of the proportion of refractory bases for domestic wastewater
- of non-domestic wastewater.
A strict COD standard for wastewater containing a lot of hard COD means the installation of advanced treatment equipment and higher operating costs.
What is COD used for in wastewater treatment?
The COD/BOD5 ratio is an important indicator of the biodegradability of an effluent and the origin of organic pollution. It allows characterizing the origin and nature of organic pollution.
Generally, the higher the COD/BOD5 ratio, the more difficult the pollution is to biodegrade. For domestic wastewater, this ratio is usually between 2 and 2.5.
A COD/BOD5 ratio higher than 3 indicates pollution of industrial origin or the presence of toxic substances inhibiting biological activity.
Rapport DCO/DBO5
The COD/BOD₅ ratio is an important indicator of the biodegradability of an effluent and the origin of organic pollution. It helps characterize both the source and the nature of organic contaminants.
In general, the higher the COD/BOD₅ ratio, the more difficult the pollution is to biodegrade. For domestic wastewater, this ratio typically ranges between 2 and 2.5.
A COD/BOD₅ ratio above 3 indicates pollution of industrial origin or the presence of toxic substances that inhibit biological activity.
The BOD5 of unpolluted surface water varies between 2 and 20 mg/l. Measurements beyond this indicate that the water is polluted.
Measurements close to 1 of the COD / BOD5 ratio indicate very good biodegradability (milk, yogurt).
- From 1 to 2: wastewater from food industries, containing elements that bacteria love, resulting in high BOD5.
- Between 2 to 3: urban wastewater.
- From 3 to 4: Less easily biodegradable wastewater.
- >4: Hardly biodegradable effluent
FAQ
What is the main difference between BOD and COD?
COD (Chemical Oxygen Demand) measures the total amount of oxygen required to chemically oxidize all organic matter present in water, whether biodegradable or not. BOD5 (5-day Biological Oxygen Demand), on the other hand, only measures the oxygen consumed by microorganisms to degrade biodegradable organic matter. In summary, COD provides an overall picture of organic pollution, while BOD5 reflects the biodegradable portion of this pollution.
What is the acceptable COD standard for discharge into the natural environment?
The COD standard depends on local regulations and the type of discharge (urban or industrial). In France, for a medium-sized urban wastewater treatment plant, the COD at the outlet should generally be less than 125 mg/L O₂. For industrial discharges, the limit may be stricter depending on the nature of the activity. It is essential to consult prefectural decrees or the Environmental Code to know the precise values.
How to effectively reduce COD in a wastewater treatment plant?
To reduce COD in a wastewater treatment plant, several processes are combined: physical pretreatment (screening, grit removal), biological treatment (activated sludge, MBBR), and sometimes advanced treatment (ozonation, activated carbon, coagulation-flocculation). For refractory COD, specific treatments such as advanced oxidation or membranes may be necessary. Optimizing retention time, aeration, and monitoring of suspended solids is also essential.
Why Do We Measure Both?
These two indicators complement each other to accurately characterize an effluent:
- BOD is used to size biological treatment, while
- COD reveals the presence of refractory substances (detergents, solvents, etc.) requiring chemical or advanced treatment.
How to Interpret a Very Low COD/BOD5 Ratio (< 1.5)?
A COD/BOD5 ratio below 1.5 indicates very good biodegradability of the effluent, often typical of wastewater from the food industry. This means that the majority of organic matter is easily degradable by microorganisms, making biological treatment very effective. This type of profile is favorable for optimizing plant performance and reducing operating costs.
