Foam problem in biological wastewater treatment systems

I’m not sure if silicone defoamers are effective! If you need them, feel free to contact me!

It’s okay; the presence of bubbles does not affect production and no action is needed

Biological foam formation and control in the activated sludge process. Biological foam formation and control in the activated sludge process. The activated sludge process is the most widely used biological treatment method in wastewater treatment plants. For most wastewater treatment plants around the world that use the activated sludge process, surface foaming is a common problem [1, 2]. This poses difficulties in the operation, functioning, and control of wastewater treatment plants, and also severely affects the quality of the treated water. According to surveys of sewage treatment plants in Europe, 20% are affected by foam on a long-term basis, 50% are affected periodically, and 87% of those using delayed aeration methods are affected by foam [3]. Foam generally takes three forms [4]: ① start-up foam. At the beginning of operation in the activated sludge process, surface foam is likely to form due to the presence of certain surfactants in the wastewater. However, as the activated sludge matures, these surfactants are biodegraded, and the foaming phenomenon gradually disappears. ②Denitrification foam. If nitrification occurs in the wastewater treatment plant, denitrification takes place in the sedimentation tank or in areas with insufficient aeration, resulting in bubbles of nitrogen and other gases that cause some of the sludge to rise to the surface, leading to foaming. ③Biological foam. Due to the abnormal growth of filamentous microorganisms, the foam formed together with bubbles and floc particles is stable, persistent, and difficult to control. Biological foam is extremely detrimental to the operation of wastewater treatment plants: a large amount of filamentous microorganisms appear in the aeration tanks or secondary sedimentation tanks, and large quantities of foam float on the surface of the water ； This leads to an increase in the organic matter concentration and suspended solids in the effluent ； Produces foul odors or harmful gases ； Reduce the oxygen transfer efficiency of mechanical aeration methods ； It may cause a large amount of surface foam to form during subsequent sludge digestion [5, 6]. 1 Formation of biological foam and its influencing factors 1.1 Mechanism of biological foam formation ① Most microorganisms associated with foam contain lipid substances; for example, the lipid content in M. parvicella reaches 35% of its dry weight. Therefore, these microorganisms are lighter than water and tend to float to the surface. ②Most foam-related microorganisms are filamentous or branched in structure, capable of forming networks that can capture particles and bubbles, etc., and rise to the surface. Bubbles surrounded by a mesh increase the surface tension of their surfaces, making it harder for them to burst and thus enhancing the stability of the foam. ③The flotation effect generated by aerated bubbles is often the main driving force behind foam formation. Particles that are separated by bubble flotation must be small in size, light in weight, and hydrophobic. Therefore, when oil, lipids, and lipid-containing microorganisms are present in water, surface foaming tends to occur. 1.2 Genus of bacteria related to biofoam formation. The formation of biofoam is primarily associated with the growth and types of microorganisms in activated sludge, but many phenomena remain difficult to explain simply to this day. The genera associated with biological foam that are widely recognized worldwide include [5]: ① Actinomycetes, including Nocardia amarae, a Gram-positive, branched mycelium ； Nocardia pinesis, Gram-positive, pine needle-shaped ； Rhodococcus sp., Gram-positive, branched mycelium. ②Filamentous fungi, including: Microthrix parvicella, Gram-positive, filamentous, unsheathed and unbranched ； Eikelboom type 0675, Gram-positive, sheathed and unbranched ； Eikelboom type 0092, Gram-negative, unencapsulated and unbranched. Among the aforementioned species, the most common are Nocardia amarae and Microthrix parvicella (see Figures 1, 2). 1.3 Main factors in the formation of biological foam ① Sludge retention time. Since foam-forming microorganisms generally have a low growth rate and long growth cycles (see Table 1), a long sludge retention time (SRT) is beneficial for their growth. If delayed aeration is used, foam formation is likely to occur. Once foam is formed, the biological retention time within the foam layer becomes independent of the sludge retention time in the aeration tank, resulting in the formation of stable and persistent foam [7]. Table 1 Growth cycle and growth temperature of microorganisms. Fungi: Growth cycle (d), Growth temperature (°C), Optimal temperature (°C): 2–4, 10–40. Rhodococcus sp.: 4–7, 23–37, 28. Nocardia amarae: 6–10, 8–35, 25. Microthrix parvicella: 10–21, 15–31, 18–25. Nocardia pinesis: Type 1863, 30. ② pH value. Reports indicate that reducing the pH value from 7.0 to 5.0–5.6 can effectively reduce foam formation. The growth of Nocardia amarae is highly sensitive to pH; the optimal pH value is 7.8, and its growth can be effectively controlled at a pH of 5.0. The optimal pH value for Microthrix parvicella is 7.7–8.0. ③Dissolved oxygen (DO). Nocardia is a strictly aerobic bacterium; it does not grow well under hypoxic or anaerobic conditions, but it does not die either. Microthrix parvicella, however, can tolerate hypoxic conditions [5]. ④Temperature. Fungi associated with the formation of biological foam each have their own suitable growth temperatures and optimal temperatures [2] (see Table 1); foam formation can occur when the environment or water temperature is favorable for their growth. ⑤Hydrophobic substances. Although the principle is not very clear, experiments have shown that insoluble or hydrophobic substances (such as oils and lipids) are conducive to the growth of actinomycetes. ⑥Aeration method. It has been observed that different aeration methods produce different types of bubbles; microbubbles or small bubbles are more conducive to the formation of biological foam, and the foam layer tends to concentrate in areas with lower aeration intensity. 2 Who controls biological foam? ① Sprinkle water. This is one of the most commonly used physical methods. Reduce foam by spraying water streams or droplets to break the bubbles on the surface of the water. Some of the dispersed sludge particles regain their sedimentation properties, but filamental bacteria still remain in the mixture; therefore, the foam phenomenon cannot be completely eliminated. ②Add a defoamer. Bactericides with strong oxidizing properties, such as chlorine, ozone, and peroxides, can be used. There are also commercially available agents produced using polyethylene glycol and silicon, as well as mixed agents of ferric chloride and copper acid pickling solutions. The effect of the chemical is only to reduce the growth of foam, but it cannot eliminate its formation. The widely used biocides generally have adverse effects, as excessive use or improper application can significantly reduce the number of flocculating bacteria and the total biomass in the reaction tank [2]. ③Reduce the sludge age. Generally, reducing the retention time of sludge in the aeration tank is used to suppress the growth of actinomycetes with a longer growth period. Practice has shown that when the sludge retention time is 5–6 days, the growth of Nocardia species can be effectively controlled to avoid the foam problems caused by them [8, 9]. However, reducing the sludge age also has many disadvantages: when nitrification is required, the sludge retention time must be at least 6 days during cold seasons, which contradicts the use of this method ； Furthermore, Microthrix parvicella and some filaments are not affected by changes in sludge age. ④Supernatant from the recirculating anaerobic digester. Tests have shown that recycling the supernatant from anaerobic digesters back to the aeration tank can control bubble formation on the surface of the aeration tank. The main function of the supernatant from anaerobic digesters is to inhibit Rhodococcus bacteria, but when this method was applied in several wastewater treatment plants in practice, it was not as successful as in the laboratory. Since the supernatant from anaerobic digesters contains high concentrations of aerobic substrates and ammonia nitrogen, both of which can affect the quality of the final effluent [5], its use should be approached with caution. ⑤Add special microorganisms. Some studies have suggested that certain specific microorganisms can inactivate Nocardia bacteria, including protozoa such as Renella. Additionally, increasing predatory and antagonistic microorganisms has a controlling effect on some foam-forming bacteria [5]. ⑥Selector. Selectors work by creating various reaction environments (such as oxygen, organic load, or sludge concentration) to select microorganisms that grow preferentially and eliminate others. Studies have shown that aerobic selectors can control M. parvicella to a certain extent, but have little effect on the Nocardia genus ； The hypoxia selector has a regulatory effect on the genus Nocardia, but no effect on M. parvicella [10]. 3 Case Analysis Although various studies and practices indicate that bubble problems share fundamental common principles, there are many factors that cause bubble phenomena, and the control methods vary (see Table 2). Table 2 Some foam control methods in wastewater treatment plants and their success rates [2,5] Control Method Statistic (1) Statistic (2) Statistic (3) Wastewater Treatment Plant Success Rate (%) Wastewater Treatment Plant Success Rate (%) Wastewater Treatment Plant Success Rate (%) Water spraying 58 88 　 　 46 28 Reducing sludge age 44 73 　 　 46 57 Biocides 48 58 9 66 46 20 Antifoam agents 35 20 7 57 　 　 Selectors 　 　 11 73 　 　 Reducing aeration time 5 60 　 　 46 33 Based on the results of foam control, careful consideration should be given to the use of various methods; for example, although selectors are a convenient option, they still require targeted application [11]. Therefore, the primary causes of the foam problem should be identified first, followed by the introduction of practical and feasible solutions. Daguan Sewage Treatment Plant is a sewage treatment plant for residential areas in Hangzhou, with a treatment capacity of 4,000 m3/day, and it uses a combined oxidation ditch system. Through observations from 1995 to 1999, the patterns of its foaming phenomenon were identified, which are mainly related to climate (temperature, water temperature, and atmospheric pressure). Severe foaming phenomena (see Figure 3) do not occur in the hot summers or cold winters; they appear every year during the transitions between spring and summer, as well as autumn and winter. That is, in an environment where temperatures, water temperatures, and air pressure vary. Analyzing the statistical data from 1999, the periods when bubble phenomena occurred were: ① when the water temperature switched from being higher than the air temperature to being lower than it (from late March to mid-April), and when the water temperature switched from being lower than the air temperature to being higher than it (from late October to mid-November). ②A period of fluctuating air pressure and temperature. Obviously, changes in the ecological environment have led to alterations in the growth and composition of microorganisms. Experience from past operations has shown that, without changing other conditions, the foaming phenomenon gradually disappears after a period of time (10–20 days), and the wastewater treatment system repairs itself automatically. Through microscopic examination, it was found that the foam that appeared during the transition between spring and summer was mainly caused by an outbreak of filamental fungi, which grew in large numbers and spread out ； During the transition between autumn and winter, the lifeless filaments become enclosed within similarly lifeless aggregates, forming floating foam. The underlying mechanism still requires further study. It is generally believed that microorganisms are affected when seasons (temperature, pressure) change, but filaments have greater adaptability than some flocculent bacteria; for example, Microthrix parvicella can grow at temperatures ranging from 8 to 35 °C, and it thrives better in low-temperature environments. When the environment is unfavorable for microbial growth, the hyphae of filaments extend from the mycelial aggregates to increase their surface area for nutrient uptake, allowing them to grow at a faster rate than other microorganisms. When spring gives way to summer, the activity of sludge decreases. A large amount of synthetic detergents and oils in domestic wastewater cannot be degraded, while certain filamental bacteria remain active; they use these substances as food and grow rapidly, leading to outbreaks of such bacteria and the formation of foam. During the transition between autumn and winter, what forms primarily is floating sludge (which is different from the former); it is difficult to find filamentous bacteria in this floating sludge and foam. Under a microscope, tiny bubbles can be seen enclosed within the floating sludge. It is estimated that when the environment changes, the bacterial aggregates become dispersed and smaller; by combining with the aeration bubbles, their density decreases, causing them to float to the surface. After identifying the patterns of foam formation, it is beneficial to implement control measures; for example, mechanical cleaning and scraping can be used to deal with foam that appears during the transition between spring and summer. Since these foams contain a large amount of filaments, they should not be left in the mixture to avoid causing foam formation again. Additionally, the use of bactericides can have a certain control effect, but it should be used with caution. As for the floating sludge and foam that appear during the transition from autumn to winter, this issue can be alleviated by using high-pressure water guns to spray water, since most of the particles in the floating sludge are still in the form of flocs; once broken apart, they can return to the mixture. These methods have achieved a certain degree of control, but further observation, analysis, and research are still needed. Furthermore, comparative tests were conducted between activated sludge and carriers during the experimental study of airlift loop reactors; it was found that when only activated sludge was used in the reactor, sludge bulking and severe foaming often occurred during the wastewater treatment process up to the nitrification stage ； Under the same conditions, after adding a carrier (slag) to the reactor and lifting it with gas, it was found that a large number of filamental bacteria adhered to the surface of the carrier and grew there. There was no sludge bulking or severe foaming; although the suspended solids (SS) in the effluent increased, both the treatment efficiency and stability of the system improved [12]. Therefore, in some activated sludge systems, mobile or fixed fillers are added to enable the fixed growth of microorganisms that tend to cause sludge bulking and foaming; this not only improves treatment efficiency but also reduces or controls foam formation.

One thing that needs to be noted is that the aerobic process here is different from that used in wastewater treatment. In wastewater treatment, the aim of the aerobic process is primarily to degrade COD. Here, however, the main goal is to oxidize sulfides into elemental sulfur, with attention being paid to controlling the redox potential. I wonder if anyone has encountered such a system before – excessive foam can have adverse effects on subsequent processes; equipment such as fans end up carrying a lot of water, and tanks and containers experience severe overfilling.

The simplest method: add some nozzles along the edge of the reflecting pool and spray water there – that’s it

It is a circular vertical reactor, with pipes at the top designed to collect the aerated exhaust gases, which are then removed using fans. It is also equipped with circulating spray pipes and nozzles. Under normal conditions, it can hold back the foam. It’s not possible now; a large amount of foam has been drawn out by the exhaust fan. It left foam everywhere. The fundamental issue is to understand the mechanism behind foam formation in order to resolve it at its root.