One of the main problems encountered in biological waste water treatment plant of papermills is poor settling of activated sludge in secondary sedimentation tanks. In many cases, activated sludge volume indices stay above 150 ml/g. Moreover, bulking of sludges in activated sludge plants of papermills is more frequent than usual. Bulking sludge is characterized by excessive growth of filamentous microorganisms.

In a lab-scale activated sludge plant, tests were made to improve the settling of activated sludge by adding the following chemicals: Lime, alkaline activated bentonite, powdered lignite coke, polyelectrolytes used as flocculants.

The findings reveal that dosage of the above additives actually gives improved sedimentation. Moreover, biochemical degradation becomes more effective if bentonite and powdered lignite coke are added. Test results are evaluated to determine the extent to which settling of the activated sludge and plant efficiency are improved, taking into account operational and waste water parameters. Using lime, bentonite, lignite coke or polyelectrolytes is an economic method of fighting or preventing acute or permanent problems caused by bulking sludge and high sludge volume indices.

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