Particle and colloidal COD

According to an IRSTEA study on wastewater in activated mud prolonged aeration, 1 mg of SS at the outlet of a municipal plant brings an average of 1.2 mg of COD and 0.5 mg of BOD5.

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.

Biodegradable soluble COD

At the outlet of the treatment plant, the soluble biodegradable COD represents only a small part of the total COD.

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 soluble biodegradable COD at the municipal plant outlet, the :

  • 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.

On average in domestic wastewater, hard COD represents 3 to 5% of the 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 a domestic wastewater contains about 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 BOD5 of unpolluted surface water varies between 2 and 20 mg/l. Measurements beyond that level indicate that the water is polluted.

The COD / BOD5 ratio gives an indication of the source and origin of the organic pollution.

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, which contain elements that bacteria love, resulting in a high COD5.
  • Between 2 and 3: urban wastewater.
  • From 3 to 4: Wastewater is not easily biodegradable.
  • >4: Hardly biodegradable effluent
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