NITRIFICATION AND ACTIVATED SLUDGE FOAMING ...

conditions. Excessive nocardial growth creates a persistent foam and scum conditions in treatment units
which can overcome the handling capacity for treatment systems. Foam on the aeration basins contains
the same organisms as in the MLSS except in more concentrated form. Activated sludge bacteria residing
in the foam are removed from the active mixture and not as available as those bacteria within the
liquid/mixed MLSS for stabilization of the wastewater constituents including ammonia nitrogen.
Figure 4 – Nocardial filaments
Evaluation of MLSS data initially from grab samples and later from online analyzers indicated that there
was significant variability in MLSS concentrations across the length aeration basins, the severity of which
was dependent upon the severity of foaming conditions. The MLSS profile data indicated a progressive
decline in the liquid concentration of solids in the MLSS in the aeration basins (samples collected from the
liquid beneath the foam layer). This liquid MLSS concentration was given the term “effective MLSS” as it
represents the MLSS that is most actively engaged in the aeration basin mixture. The term “total MLSS”
was given to the mixture of liquid and foam. Samples of this mixture was historically collected
downstream of the aeration basins and used for process control decisions. Figure 5 presents an example
of a trend of the change in Total and Effective MLSS as foaming conditions increase.
Figure 5 – Total Vs Effective MLSS Trend with Increasing Foaming
The ammonia nitrogen data from the aeration basin profiles indicated that incomplete nitrification is linked
to elevated foaming conditions causing lower effective MLSS concentrations.