Why biofoam is formed, how to control biofoam？

Why biofoam is formed, how to control biofoam？

THE HARM OF THE BIOLOGICAL FOAM

There are many effects of biofouling on the operation of wastewater plants:
The bio-foam is generally viscous. It will enclose a large amount of solid material such as activated sludge into the floating foam layer of the aeration tank. The foam layer will tumbling on the surface of the aeration tank, hindering the entry of oxygen into the aeration tank mixture and reducing Oxygenation efficiency, especially for mechanical exposure.

When the aeration tank mixed with foam enters the secondary settling tank, solid materials such as foam-wrapped activated sludge will increase the suspended solids content of the water and cause the effluent water quality to deteriorate, and a large amount of scum will be formed on the surface of the secondary settling tank. When the temperature in winter is low, the normal operation of the suction (scraping) mud bridge (machine) will be affected by the icing.

The bio-bubble spreads to the aisle plate, affecting inspection and equipment maintenance. Summer bio-frozen drifts in the wind, creating a series of environmental health problems that affect the health of the surrounding pedestrians. After freezing in winter, it is difficult to clean up, and it is also possible to slip the inspection and maintenance personnel.

The foam contained in the return sludge will cause similar flotation, which will damage the normal performance of the sludge. The bio-foam will enter the mud area with the sludge, and interfere with the sludge concentration and sludge digestion.


THE FORMATION OF BIOLOGICAL FOAM AND ITS INFLUENCING FACTOYS.

The formation mechanism of bio-foam

1) Foam-related microorganisms mostly contain lipids, such as M. parvicella, which has a lipid content of 35% of dry weight. Therefore, such microorganisms are lighter than water and easily float to the surface of the water.

2) The microbes related to foam are mostly filamentous or branched, easy to form a net, can sweep particles and bubbles, and float to the surface. The air bubbles surrounded by the screen increase the tension on the surface, making the bubbles less fragile and the foam more stable.

3) The air flotation caused by aeration bubbles is often the main driving force for foam formation. The particles use air bubbles to float, and must be small, light, and hydrophobic. Therefore, when there are oils, lipids and fat-containing microorganisms in the water, surface foaming is likely to occur.

The main factors in the formation of bio-foam
Sludge residence time

Long-sludge residence time (SRT) is beneficial to the growth of these microorganisms due to the low general growth rate and long growth cycle of the foam-producing microorganisms. If the delayed aeration method is adopted, the foaming phenomenon is easy to occur, and once the foam is formed, the biological residence time of the foam layer is independent of the sludge residence time in the aeration tank, and it is easy to form a stable and durable foam.

pH value

It has been reported that when the pH is lowered from 7.0 to 5.0 to 5.6, the formation of foam can be effectively reduced. The growth of Nocardia amarae is extremely sensitive to pH. The optimum pH is 7.8. When the pH is 5.0, the growth can be effectively controlled. Microthrix parvicella has an optimum pH of 7.7 to 8.0.

Dissolved oxygen (DO)

Nocardia is a strict aerobic bacterium that does not grow easily under anoxic or anaerobic conditions, but does not die. Microthrix parvicella can tolerate hypoxia.

Temperature

The fungi associated with the formation of biofoam have their own suitable growth temperature and optimum temperature (see the figure below). When the environment or water temperature is favorable for their growth, foaming may occur.
Hydrophobic substances 

Although the principle is not very clear, experiments have shown that insoluble or hydrophobic substances (such as oils, lipids, etc.) are conducive to the growth of actinomycetes.
Aeration mode 
It has been observed that different aeration methods produce different bubbles, and microbubbles or small bubbles are more conducive to the production of biofoam than large bubbles, and the foam layer tends to concentrate in areas with low aeration intensity.

THE BIOLOGICAL FOAM CONTROL MEASURES 
Spraying water 

This is one of the most common physical methods. Reduce foam by spraying water or water droplets to break up bubbles floating on the surface of the water. Part of the broken sludge particles restored the sedimentation performance, but the filamentous bacteria were still present in the mixture, so the foaming phenomenon could not be eliminated at all.
Add defoamer 
A bactericidal agent having strong oxidizing properties such as chlorine, ozone, and peroxide can be used. There are also commercially available pharmaceuticals produced by using polyethylene glycol and silicone, and a mixture of ferric chloride and copper pickling solutions. The effect of the agent only reduces the growth of the foam, but does not eliminate the formation of the foam. The widely used fungicides generally have a negative effect, because the excessive or improperly placed position will greatly reduce the number of bacteria and the total amount of bacteria in the reaction tank.

Reduce sludge age

Generally, the residence time of the sludge in the aeration tank is lowered to suppress the growth of the actinomycetes having a longer growth period. It has been proved that when the sludge residence time is 5-6 days, the growth of Nocardia can be effectively controlled to avoid the foam problem caused by it. However, there are many unapplicable aspects of reducing sludge age: when nitrification is required, the sludge residence time needs at least 6 days in the cold season, which is in contradiction with the use of this method; in addition, Microthrix parvicella and some filamentous bacteria are not contaminated. The effect of changes in mud age.

Reflux anaerobic digestion tank supernatant 

It has been shown that the method of refluxing the supernatant of the anaerobic digestion tank to the aeration tank can control the formation of bubbles on the surface of the aeration tank. The main function of the supernatant of the anaerobic digester is to inhibit Rhodococcus, but this method did not achieve the same success as the laboratory when it was actually used in several sewage treatment plants. Since the supernatant of the anaerobic digester contains high concentrations of aerobic substrates and ammonia nitrogen, they all affect the final effluent quality and should be used with caution.

Add special microorganisms 

Some studies have suggested that some special strains can eliminate the vitality of Nocardia, including protozoan kidneyworms. In addition, the addition of predatory and antagonistic microorganisms has a controlling effect on some foaming bacteria.

Set the bio selector 

Bio-selectors are designed to create preferentially growing microorganisms and eliminate other microorganisms by creating various reaction environments (oxygen, organic load or sludge concentration, etc.). Some studies have reported that the aerobic selector can control M. parvicella to a certain extent, but has no significant effect on Nocardia. The anoxic selector has a control effect on Nocardia, but has no effect on M. parvicella.
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