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Summary of causes of sudden increase of COD in effluent
DATE:2020-4-23 8:56:34

The reason for the sudden rise of COD and BOD in the effluent from the secondary sedimentation tank reflects the failure of sewage treatment. There are external reasons and internal reasons that lead to functional failure.

To identify its characteristics, COD of effluent should be measured first.

The increase of COD and BOD5 in effluent from sewage treatment is mainly caused by granular substances or dissolved compounds.


1. Reasons for the increase of COD effluent value in the activated sludge treatment process


The cause of the visible low is the presence of large flocculation behind the diving wall or in the outlet channel, or when the visibility in the secondary sedimentation tank is small or decreased, caused by suspended solids, and when the visibility of the effluent is less than 1.5 meters, caused by soluble sewage inclusions.

The surface load of the secondary sedimentation tank is too high, and the solid content in the activated sludge is too high. At this time, it must be noted that the reduction value of the sludge content in the activated sludge should not exceed 25% within 24 hours, and the increase of NH4 +N content in the effluent must be avoided due to the extraction of the remaining sludge.


2. Reasons for the increase of COD effluent value in activated sludge treatment process


If the sludge index is too high and the reflux ratio is sufficient, measures must be taken to improve the sludge index when the water depth in the clean water area is less than 1m and the sludge index exceeds 150ml/g.

The reflux ratio is too small. The reflux ratio is at least 50-70% of the inflow.

RV < 0.5 (= 50%) → increase the return flow of activated sludge → change the mud surface height of the secondary sedimentation tank.

If the sludge surface is not reduced, check the sludge volume load Q.


3. Reasons for the increase of COD effluent value in activated sludge treatment process


Flocculant fragmentation can lead to high concentration of filterable substances in the water (> 30mg/L or 12TE/F), insufficient degasses, gas formation, and long residence time of sludge in the secondary sedimentation tank.

The removal rate of diving walls and floaters is not effective.

The temperature difference of inlet and outlet water of the secondary sedimentation tank is too small.

The pipe is blocked.

The COD load of inlet water is too high.


4. Reasons for the increase of COD effluent value in activated sludge treatment process


The ammonium nitrogen value of effluent was too low.

Turbidity increase;

Excessive water intake;

Operation fault of the return sludge pump;

PH is too low or too high;

Sludge is mechanically crushed;

The nitrite content is elevated.