An abundant growth of zoogloeal colonies was observed periodically during some experiments carried out on a bench-scale plant fed with a complex synthetic wastewater. The plant had a configuration with an enhanced regeneration-denitrification-nitrification (R-D-N) activated sludge process for low alkalinity wastewaters.

Several operational changes were undertaken in order to suppress the excessive growth of zoogloeal colonies. They included: (1) changes in the wastewater composition, (2) volume changes in the compartmentalization of the anoxic selector, (3) increase of D.O. concentration in the regeneration and nitrification tanks.

Conclusions from the plant operation observations give rise to a hypothesis that none of the operational changes provoked a significant restriction in the abundant zoogloeal growth. When a temporary improvement was observed, after acclimatization on the new operational conditions the microorganisms culture returned to its previous state and continued to grow in a wave-mode. The reason for the appearance of this type of sludge might be closely related to the composition of the synthetic wastewater.

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