How to control biological foam? Many people mistakenly use defoamers

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Why is there a biological foam?
When the gas is introduced into the liquid, the liquid expands and surrounds the gas, foam will be produced.
Its formation and stability require three necessary conditions: bubbles, surfactants, and hydrophobic particles.
Bubbles are mainly generated by aerobic aeration, and surfactants come from the synthesis of microorganisms in the influent and activated sludge, while hydrophobic particles in the biological pool come from microorganisms with hydrophobic cell surfaces in the activated sludge.
1. The simplest way to judge the potential and stability of biological foam is to simulate the standard rate of aeration of activated sludge in a certain volume of measuring cylinder, and classify it according to the volume of foam and the stability time.
2. Repeated flotation is performed by determining the proportion of suspended solid mass after and before aeration for classification.
These two methods can obtain results in a short period of time, but often do not represent the actual state of the aeration tank.
3. Cell surface hydrophobicity experiment distributes microorganisms in the mixed solution in the aqueous and hydrophobic phases, and determines the foaming ability of activated sludge by measuring the absorbance of the aqueous phase before and after treatment. This method has shown good results in laboratory pure bacterial cultivation experiments, and actual activated sludge samples are easily affected by complex substances in wastewater.
4. The principle of surface tension is to use the characteristics of bacteria that cause biological foam to reduce the surface tension of solution, which can be applied to both actual sludge mixed solution and pure culture solution.
Unfortunately,
Both methods are susceptible to the influence of complex substances in wastewater.
5. It is the most popular method at present to record the number of hyphae in the counting grid under the phase contrast microscope or to identify the type and number of filamentous bacteria in the foam through specific fluorescent probe labeling targeting rRNA by Gram staining, Naiser staining and other staining methods on the foam sample, which can better reduce the microbial status in the actual sludge and simulated bacterial solution.
This type of method requires skilled technology and complete laboratory support, as well as a certain amount of time to obtain accurate results.
6. Foam scum index is used to evaluate the characteristics of activated sludge biological foam, including seven dimensions of foam color, bubble size, stability, coverage area, filamentous bacteria, foaming potential and total suspended solids content. Different dimensions are given different weights through path analysis and structural equation model to obtain the final foam scum index.
There is a good linear correlation between the foam scum index and the severity of biological foam.
Compared with normal activated sludge, biological foam has a very sticky appearance, in which a large number of filamentous microorganisms are enriched.
Although more than 30 different types of filamentous bacteria have been identified from municipal sewage plants
Only two filamentous bacterial types can cause biological foam
They are Nocardia filamentous bacteria and microfilaments.
1. Nocardia filamentous bacteria
For a long time in the past, Nocardia type filamentous bacteria were considered the only filamentous bacteria causing biological foam phenomenon. However, a series of studies have shown that another Gram positive filamentous bacteria, which has been proven to cause severe sludge bulking, also selectively accumulate in the foam.
The Nocardia type is a type of filamentous bacteria with an actinomorphic appearance that can be seen through a microscope