If activated sludge contains a lot of gas (O2, CO₂, N2, etc.) when entering the sedimentation zone of the secondary clarification, gas bubbles will accumulate on the activated sludge. This leads to the flotation of sludge in the secondary clarifier.
Often, the cause of gas formation is an unintended denitrification in the secondary clarification. Long storage times of the sludge in the basin lead to these denitrification phenomena with formation of nitrogen gas.
To clarify whether the cause of the flotation is too strong of an aeration in the nitrification or indeed an unintended denitrification in the secondary clarifier, one can temporarily turn down the aeration while observing the sedimentation behavior of the activated sludge.
A low pH, e.g. caused by dosage of acidic precipitants, may lead to the dissolution of carbonates and thus to CO2 formation.


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