NITRIFICATION AND ACTIVATED SLUDGE FOAMING ...

DISCUSSION
Nocardia sp. is a type of branched filamentous bacteria that is part of the population of microorganisms in
a healthy activated sludge system. However, excessive/disproportionate nocardial levels will lead to
increasing levels of foaming and scum production. To help minimize foaming in systems that are prone to
Nocardia, aeration systems should be carefully controlled so over-aeration does not occur. Excessive
aeration creates excessive foaming in elevated Nocardia conditions. The WRF has installed automated
DO control systems that help to minimize excessive aeration conditions. Key food sources for Nocardia
are fats/oils/grease and fatty acids. Limiting these foods will help to control nocardial populations.
Maintaining a near neutral pH in the aeration system is also a benefit to help control Nocardia and to
support nitrification performance. MCRT perhaps is the most important process parameter for Nocardia
control and for nitrification performance. Generally, nitrifiers require an older sludge or higher MCRT.
However, higher MCRT levels will promote Nocardia growth. Herein rests the critical balance -
maintaining high enough MCRT to support nitrification performance and low enough MCRT to minimize
Nocardia growth.
For the WRF, the MCRT operating range to control this balance is very narrow and presents a challenge
to maintain adequate MCRT to achieve complete nitrification while controlling foaming. Historically,
winter operations have presented the greatest challenge. During winter operations when nitrification rates
decrease due to lower aeration basin temperatures, to maintain adequate nitrification performance, the
MCRT generally needs to be raised in the effort to increase the population of nitrifiers in the MLSS. In
this effort, WAS is systematically reduced to raise the MLSS (increases MCRT). However, in systems
prone to nocardial foaming this increase in MCRT can lead to negative results. The increasing MCRT
promotes Nocardia, increases foaming, decreases effective MLSS, and decreases active nitrifiers
resulting in diminished nitrification performance. When nitrification performance starts to decrease, the
normal impulse is to preserve/build nitrifiers by reducing wasting. However, this creates more selective
forces for increasing Nocardia which creates further detrimental conditions for nitrification. Figure 6
presents a chronological plot showing the impact of decreasing wasting, increasing foaming, drop in
effective MLSS, and decrease in nitrification performance.
Figure 6 – Chronological Plot of Effective and Total MLSS, and Nitrification with Decreasing WAS
Additionally, even as temperatures increased during this period, nitrification performance did not improve.
Given the understanding of these relationships, the WRF instituted careful controls for MCRT, aeration
intensity, and alkalinity/pH to minimize Nocardia foam and scum in order to maintain more consistent