Total COD

The total COD of wastewater is broken down into three fractions:

  1. Particle and colloidal COD
  2. Biodegradable soluble COD
  3. Refractory COD (or hard COD)

Particle and colloidal COD

Particulate and colloidal COD represent around 35% of total COD. It consists mainly of suspended solids (TSS) and is essential for understanding the impact of suspended solids on the organic pollution load. The particulate COD is obtained by the difference between the raw COD and the COD after filtration of the sample.

According to an IRSTEA study on wastewater in prolonged aeration activated sludge, 1 mg of SS at the outlet of a municipal plant provides an average of 1.2 mg of COD and 0.5 mg of DBO5.

Knowing that this TSS is about 20 mg/L, the corresponding particulate COD is 24mg COD/L (20 x 1.2).

The particle or colloidal COD is determined by the difference between the raw COD and the COD after filtrationof the sample. It is mainly constituted of TSS.

If you would like more precise information on the relationship between these parameters, please click on the form below.

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Biodegradable soluble COD

At the outlet of the treatment plant, the soluble biodegradable COD represents only a small part of the total COD. In other words, around 10% of total COD. It represents the quantity of oxygen required to chemically oxidise organic matter dissolved in water, which is easily degraded by biological processes.

It is an essential indicator for assessing the effectiveness of biological treatment systems, such as mobile biofilm reactors (MBBRs) and activated sludge systems. A high proportion means that organic pollutants are easily biodegradable, making them easier to eliminate.

This leakage of soluble COD normally degraded by bacteria can be related to either:

  • an overdose of reagent (use of methanol for denitrification for example)
  • insufficient residence time in the ponds to allow full biodegradation.

This leak may also indicate:

  • a bypass of a treatment step
  • hydraulic drilling at some biological filters
  • poor ventilation.

To determine the biodegradable soluble COD at the municipal plant outlet, we use:

  • Total BOD5 (about 12 mg/L O2)
  • TSS concentration (20 mg/L) multiplied by a factor of 2.4:

Total soluble COD = (Total BOD5 – SS BOD5) x 2.4 = (12 – 20 x 0.5) x 2.4 = 4.8 mg/L O2

Refractory COD known as "hard COD"

Hard COD is the residual COD of a sample after an ultimate BOD5 analysis. It represents around 55% of 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 total COD.

Hard COD = Total COD – Particle COD – Soluble COD

Still according to IRSTEA, with a total COD concentration at the outlet of 70 mg/L, 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 3 to 5% ratio is confirmed.

This content in the 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.).

Thus, the nature of the non-domestic water must be analyzed before approving inputs to the collection system. 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:

  • the proportion of refractory stubs for domestic wastewater
  • non-domestic wastewater.

A high COD standard for wastewater containing high hard COD means the installation of extensive treatment equipment and higher operating costs.

COD/BOD5 ratio

The COD/BOD5 ratio is an important indicator of the biodegradability of an effluent and the origin of organic pollution. It is used to characterise the origin and nature of the organic pollution.
As a general rule, the higher the COD/BOD5 ratio, the more difficult it is to biodegrade the pollution. For domestic wastewater, this ratio is generally between 2 and 2.5.
A COD/BOD5 ratio greater than 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 that level indicate that the water is polluted.

The measurements close to 1 of the ratio COD / BOD5 a very good biodegradability (milk, yogurt).

  • From 1 to 2: wastewater from food processing industries, containing elements that bacteria love, resulting in high BOD5 levels.
  • Between 2 and 3: urban wastewater.
  • From 3 to 4: Wastewater is not easily biodegradable.
  • >4: Hardly biodegradable effluent
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