Foam and scum in activated sludge plants is a common problem, especially in low loaded nitrifying plants. Gram-positive filamentous bacteria are to be found in such problematic situations in large numbers in the activated sludge. In particular, the types Microthrix parvicella and Nocardia, which have hydrophobic (water-repellent) properties, are very common. The foam consists of gas bubbles that are enveloped by a sludge membrane. Often, the foam is so tough that it cannot be destroyed by mechanical means or spraying.

If one finds these filamentous organisms in the activation plant, then problems in the digester are likely to be expected. The foam-causing bacteria enter the digestion with the surplus sludge. The foaming in the digester can lead to significant operational problems, in particular affecting the entire processes of gas collection and gas treatment.

Spraying of the surface area, foam traps, withdrawal of the floating sludge blanket or addition of chemicals (defoamer, flocculant, and precipitant) have only a limited effect.
If it is possible to change the biology in the activated sludge plant in such a way that the aforementioned filamentous organisms can no longer develop and maintain themselves, then the foaming problems in the digester are immediately solved.

With the S::Select® process, the biological system is dramatically changed to no longer have a (high) sludge retention time. Due to the aerobic granular activated sludge structure, two systems develop - a dominant aerobic granular sludge system that has a typical sludge retention time in the range of >>10d and a subordinate suspended system with a sludge retention time of about 2-4 days. In both systems, Microthrix parvicella and Nocardia have no chance to establish themselves.

In systems where for decades Microthrix parvicella and Nocardia had established themselves in the biological system and the corresponding problems in the activated sludge plant and in the digestion had to be combated, foaming problems are now mitigated since the transition to the S :: Select® system.


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