Applications and properties of defoamers in the treatment of various sewage and wastewater streams

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Papermaking water treatment defoamers are primarily used in the pulping and papermaking processes of the paper industry. The main components in this type of wastewater are lignin, cellulose, volatile organic acids, etc. It is characterized by high concentrations of pollutants, large discharge volumes, difficulty in degradation, and poor biodegradability, making it one of the more difficult industrial wastewaters to treat. The commonly used treatment method for papermaking wastewater is coagulation. This method is adaptable, requires simple equipment and is easy to operate, but it has disadvantages such as high costs and the generation of large amounts of sludge ； Another method is the biological approach, which features no secondary pollution and low treatment costs, but it cannot completely reduce the level of organic pollutants. In recent years, scholars at home and abroad have conducted research on the use of talcum powder as a coagulant aid, coagulant, and filter aid, as well as a defoamer in the treatment of papermaking wastewater. Antifoam agents used in the treatment of oily wastewater are primarily applied to cooling and lubricating fluids originating from the petroleum industry, steel manufacturing, gas production facilities, and the machinery industry. The oil layer on the surface of such wastewater prevents the water from having its natural self-purification ability, disrupting the ecological balance in the water. This not only poses a threat to human health but also affects crop cultivation. Currently, air flotation is the main method for treating oily wastewater, but it has high energy consumption, requires large amounts of flocculants, and occupies a lot of space. Other methods such as electrochemical and adsorption methods also have drawbacks such as high operating costs and limited applicability. In recent years, research by some scholars on antifoaming powder as an oil absorbent has provided a new approach for treating oily wastewater. Yu Renhuan et al. investigated the effects of two factors—talc dosage and particle size—on the purification of oily wastewater by air-assisted cyclones. The results showed that when the fineness of talc powder (150 g/m3 water) was 0.074 mm and it accounted for 81%, the oil removal rate reached 78.30%. Talc powder had a significant effect on oil removal in the aerated cyclone; compared with the case without adding talc powder, the oil removal rate increased by about 5%. Some researchers have also examined the effectiveness of talc in treating oily wastewater under different conditions. The results showed that at 20°C, with a talc addition of 20 g/L and slow stirring for 15 minutes, the COD removal rate for gasoline was 97.8%, while that for diesel was 81.5%. Talc is a natural hydrophobic and lipophilic mineral that can facilitate the separation of hydrophobic and lipophilic particles in oil-water mixtures; therefore, talc powder can be used as an oil absorbent to treat oily wastewater. Treating dye wastewater: Dye wastewater has a complex composition and high color intensity, causing severe pollution to the environment. Cationic dye wastewater is difficult to biodegrade and decolorize due to the presence of complex aromatic groups, and it has poor biodegradability; as a result, such wastewater is hard to treat. Traditional treatment methods for dye wastewater (such as filtration and coagulation) only convert pollutants from the liquid phase to the solid or gas phase; they do not completely remove the pollutants, and may cause secondary pollution. Therefore, treatment technologies that are cost-effective, highly effective, and environmentally friendly are being continuously developed. Treatment of aromatic organic wastewater: With the development of industries such as petrochemicals and plastics, more and more wastewater containing aromatic compounds is being discharged by factories. These pollutants have a stable structure and are difficult to degrade. Currently, the main methods for treating aromatic organic wastewater are advanced oxidation processes using reagents and liquid membrane separation techniques. Although the former method is convenient to operate and rapid in reaction, it has high operating costs, which is why it is less used in China ； The liquid film used in the latter tends to age quickly, and it also faces issues such as film fouling and high costs. The effectiveness of defoaming powder in treating wastewater depends significantly on the structure and properties of the wastewater: (1) a unique pore structure and large specific surface area ； (2) The surface functional groups OH—, H—O—H, and Si—O are active ； (3) Natural hydrophobic lipophilicity ； (4) Chemical stability. The main modification methods for antifoaming powder used in water treatment include surface coating modification, mechanical crushing modification, ultrasonic treatment, and acidification modification. After modification, the antifoaming powder has an increased specific surface area, enhanced surface activity, and improved adsorption capacity, which enables it to be used for wastewater treatment. The author suggests that future work should focus on the following aspects: (1) There are few modification methods for antifoam powders as water treatment agents; based on their inherent properties, research should be expanded into local activity modification methods or outer membrane layer modification methods to enhance the selectivity of modified talc in adsorbing specific pollutants. In addition, the issue of acid discharge from acidified talc powder also deserves careful consideration ； (2) The wastewater treated with defoaming powder is usually laboratory-scale simulated wastewater or wastewater containing a single type of pollutant; it remains to be determined whether defoaming powder can be used on a large scale in actual production processes ； (3) Studies have shown that the mixture resulting from the hybridization of SN (octadecyldimethyloxyethylquaternary ammonium) and magnesium silicate can adsorb anions in dye wastewater. The interlayer structure of magnesium silicate is similar to that of talc; therefore, the hybridization of talc and SN can be considered for the removal of anions from dye wastewater ； (4) Utilizing the lubricating and adsorptive properties of talc, the filtering aid role of antifoaming agents in membrane separation technology can be investigated ； (5) Research on methods such as the recovery and reuse of defoaming agents should be strengthened to provide a theoretical foundation for further developing the application of defoaming agents in water treatment. With further research into the use of antifoaming agents to purify water pollution, as well as ongoing advancements in the processing and modification techniques for talc, significant breakthroughs in the application of antifoaming agents in water treatment are expected.