Introduction: "The biological foam in the factory is accumulating more and more, and it is overflowing into the biological pond aisle, how to quickly eliminate the foam?" Yesterday the operator almost slipped to......" I understand that you are very anxious, after all, biological foam is very unfavorable to the operation of the sewage plant: "In the aeration tank or secondary sedimentation tank, a large number of filamentous microorganisms appear, floating on the water surface and accumulating a large amount of foam; The concentration of organic matter and suspended solids in effluent increased. Produce odor or undesirable and harmful gases; The oxygen transfer efficiency of mechanical aeration is reduced. May result in a large amount of surface foam during sludge digestion......" However, in order to control biochemical sludge, avoid indiscriminate use of chemical defoamer, and the right medicine is the key.


01 Why do biobubbles occur?
A foam is created when a gas is introduced into a liquid, which expands and surrounds the gas. Its formation and stability need to have three necessary conditions: bubbles, surface active substances, hydrophobic particles. The bubbles are mainly produced by aerobic aeration, the surfactants come from the influent and the synthesis of microorganisms in the activated sludge, and the hydrophobic particles in the biological pool come from microorganisms with hydrophobic cell surface in the activated sludge.
02 How to judge foaming capacity?


1. The simplest way to judge the formation potential and stability of biological foam is to simulate the aeration of activated sludge in a certain volume cylinder at a standard rate, and classify it according to the volume and stability time of the foam.

2, repeated flotation, by determining the proportion of suspended solid mass after aeration and before aeration for classification. These two methods can get results in a short time, but often can not represent the actual state of the aeration tank.

3, cell surface hydrophobicity experiment, the microorganisms in the mixed solution are divided in the aqueous phase and the hydrophobic phase, and the foaming capacity of activated sludge is judged by the measurement of the absorbance of the aqueous phase before and after treatment. This method is effective in the culture experiment of pure bacteria in the laboratory, but the actual activated sludge samples are easily affected by complex substances in sewage.

4, the surface tension rule is the use of bacteria that lead to biological foam to reduce the surface tension of the solution characteristics, can be applied in the actual sludge mixture and pure culture solution. Unfortunately, both methods are susceptible to complex substances in sewage.

5. By using different staining methods such as Gram staining and Neisser staining on foam samples, recording the number of mycelia in the counting grid under phase contrast microscope or identifying the type and number of filament-like bacteria in foam through specific fluorescent probe labeling targeting rRNA is the most popular method at present, which can better restore the microbial state in actual sludge and simulated bacterial solution. Such methods require skilled technology and sophisticated laboratory support, and it takes a certain amount of time to get accurate results.

6. The foam scum index was used to evaluate the characteristics of activated sludge biofoam, including 7 dimensions such as foam color, bubble size, stability, coverage area, filamentally formed bacteria, foaming potential and total suspended solids content. The path analysis and structural equation model were used to give different weights to different dimensional characteristics to obtain the final foam scum index. There is a good linear correlation between the scum index of foam and the severity of biofoam.


03 What are the types of biofoam?
Compared with normal activated sludge, the biofoam 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 types of filamentous bacteria can cause biofoams, namely Nocardia type filamentous bacteria and microfilamentous bacteria.

1. Nocardia filamentous bacteria For a long time, Nocardia filamentous bacteria was considered to be the only filamentous bacteria responsible for the biofoam phenomenon. However, a series of studies have shown that another type of Gram-positive filamentous bacteria that has been shown to cause severe sludge bulking also selectively accumulates in the foam. Noca type is the customary name for filamentous bacteria that can be seen through a microscope with a linear appearance. It is a class of heterotrophic aerobic Gram-positive bacteria with distinct true branches. Its filamentous body exists in activated sludge floc and external liquid, usually 5.0~30um, 1.0um wide, single cell shape is irregular, lack of cell sheath and adhesion growth, no motility. Research has shown that Nocardia type filamentous bacteria have a very high capacity to store nutrients, which is the main reason they can survive in the nutrient-limited environment of foam or accumulate in the foam. Filamentous bacteria of Nocardia type can utilize a variety of carbon, nitrogen and phosphorus sources. Especially when the sewage contains rich hydrophobic nutrients, this competitive advantage is more obvious.

For a long time, Nocardia filamentous bacteria were thought to be the only filamentous bacteria responsible for the biofoam phenomenon. However, a series of studies have shown that another type of Gram-positive filamentous bacteria, which has been shown to cause severe sludge bulking, also selectively accumulate in the foam. M.parvicella belongs to a long branched, evolutionarily independent group of actinomycetes. The activated sludge floc is irregularly curled and surrounded inside and on the surface. Its width is 0.6~0.8um and its length is 50~200um. No single cell can be seen inside the filamentous body, and the outer sheath of the cell is missing. The filamentous body has no branching and no motility. Correlation analysis also showed that it was sensitive to high oxygen concentration stress and was more suitable for growth in low oxygen environment.
04 What factors affect biobubbles


1. The problem of temperature foam cycles or changes in severity with seasonal changes, and temperature is undoubtedly the most important factor. It has been shown that Gordonia amarae is the main cause of sludge foaming at high water temperature, and M.parvicella is the main cause of sludge foaming at low water temperature. Among them, the optimum growth temperature of M.parvicella is 12~15℃, which is very close to the annual water temperature stage of foam production in sewage plants. It should be noted that foaming may not be dominated by temperature, but it is triggered by changes in temperature, because temperature will also affect other factors such as oxygen and lipid solubility in water.

2. Filamentous bacteria caused by sludge age are a kind of microorganisms with slow growth rate and long cycle, and prolonged sludge age is conducive to the growth of microorganisms. In some low-load sewage treatment plants, the hydraulic retention time is long, and the foam phenomenon is easy to occur when the delayed aeration method occurs. At the same time, once the foam is formed, the biological retention time of the foam layer is completely independent of the sludge retention time in the aeration tank, thus forming a durable and stable foam. Studies have shown that when the sludge age is less than 9d, the foam problem will not appear. Of course, the age of sludge is related to temperature, and the growth rate of filamentous bacteria changes with temperature, so the age threshold of sludge to form biofoams in different seasons is different.

3, pH In general, the pH of municipal sewage is more stable in the range of 6.0-8.0. When the pH is reduced from 7.0 to 5.0 to 5.6, foam formation can be effectively reduced. Studies have shown that Nocardia type filamentous bacteria grow best at pH 6.5. For microfilaria, the pH range of its growth is 7.1~8.0. This explains why the wastewater plant with pure oxygen aeration is more likely to produce biofoam than the wastewater plant with air aeration, because the average pH of the sludge mixture with air aeration is 7.0, while the average pH of the sludge mixture with pure oxygen aeration is 6.5.

4, dissolved oxygen Nocardia type filamentous bacteria is strictly aerobic bacteria. It cannot grow or ingest acetate under anaerobic conditions (no nitrate, no oxygen) or anoxic conditions (nitrate, no oxygen). M.parvicella, on the other hand, can grow in a wide range of oxygen concentrations, but the optimal growth condition is the state of low oxygen. These results indicate that microfilaria is prone to have metabolic advantages in activated sludge microorganisms in the state of low dissolved oxygen, which leads to excessive proliferation, while high dissolved oxygen concentration (>6 mg/L) is harmful to microfilaria and will inhibit its growth.


5. Nutrients For Nocardia type filamentous bacteria and microfilaria, the biggest feature of carbon source utilization is the absorption and utilization of hydrophobic substrates such as oil, ester and other slow degradation substrates. Studies have shown that Rhodococcus and Gordons are easy to absorb long-chain alkanes and fatty acids, and when these substrates are at a high load level in sewage, they will cause the rapid proliferation of Nocardia type filamentous bacteria, and at a low concentration load, they will cause the proliferation of microfilamentous bacteria.



05 How to control Biofoam?



1. Physical and chemical methods Physical methods mainly include spraying water and manual or mechanical cleaning. Spraying water is a simple physical method, but the foam sprayed by water still exists in the mixture, so the foam phenomenon cannot be fundamentally eliminated. Removing the foam by manual or mechanical salvage will increase the production cost, and how to dispose of these biological foams is still a big problem. Chemical methods mainly include the addition of oxidants and disinfectants (chlorine, hypochlorous acid, hydrogen peroxide, ozone, quaternary ammonium salt, etc.) or coagulants (polyacrylamide, polyaluminum chloride, ferrous chloride, ferric chloride, etc.). Although these chemical methods can eliminate the phenomenon of biobubbles to a certain extent, this effect is only temporary because other factors leading to the occurrence of biobubbles have not been eliminated. At the same time, the use of disinfectants is non-selective, and the damage to this activated sludge system is far greater than the effective removal of biofoam. It is worth mentioning that when there is a biological foam, the random addition of defoamer will not only have no obvious effect, but will cause an increase in operating costs. This is because the antifoam agent is mainly aimed at chemical foam, and biological foam is more stable than chemical foam, and it is difficult to eliminate.

2, the most effective measure to control the age of sludge for biological foam is to reduce the age of sludge. A large number of studies have shown that reducing the residence time of the sludge in the aeration tank can effectively control the biofoam in the activated sludge process. Reducing sludge residence time is essentially a biological screening strategy, that is, using the characteristics of longer average generation time of foamed microorganisms to inhibit excessive proliferation of foamed microorganisms in the aeration tank or eliminate them to achieve the purpose of controlling biological foam. Experimental studies have reported that at low temperatures, the sludge age is controlled for less than 8 to 10 days, which can gradually inhibit the growth of Nocardia, but after the temperature rises, the sludge age must be controlled to 3 days to achieve the corresponding effect. However, the reduction of sludge age also has many aspects that are not applicable: when nitrification is required in the aeration tank, a relatively long sludge residence time is required, which is a contradiction with the adoption of this method. In addition, the growth of some other filamentous bacteria is relatively little affected by the change of mud age, and if the biological foam is mainly caused by these microorganisms, the effect of this method is not great.

3, ecological regulation of activated sludge prokaryotes, eukaryotes, phages and other interdependent and competitive, is a complex ecological network. In the actual production process, the physical and chemical control often can not achieve the expected effect of the operation management personnel, so the understanding and use of activated sludge ecological network is extremely important. Phages are able to invade and lyse their hosts, and thus become a powerful biological tool. Isolation of new filamentous bacteria in lytic foam from activated sludge has become a very hot research direction. However, it is important to note that any agent that induces mild phages in filamentous bacteria to enter the lysis cycle may also induce mild phages in other beneficial bacteria.

4, selector technology In addition to the above control means, biological foam control technology can also be combined with the control of sludge swelling and other abnormal operation of activated sludge, comprehensive regulation of activated sludge operation process. Setting anaerobic and aerobic selectors at the front end of the biological aeration tank can inhibit and eliminate the growth of some filamentous bacteria, thus reducing the probability of sludge swelling and biofoam. A selector is a mixing tank or channel through which the returned sludge flows before entering the aeration tank and has a high rate of rapid biodegradable organic matter removal, where almost all easily degradable organic matter in the selector is removed. There are usually 3 types of biological selectors according to their oxygen supply conditions: aerobic, anoxic, and anaerobic. The selectors can selectively promote the growth of microorganisms that form sludge floc, inhibit the growth of filamentous bacteria that cause foam, or selectively enrich filamentous bacteria to a certain position and then remove them to control their population in the sludge mixture.