Reasons for foaming in sewage treatment plants
There are several main reasons for foaming in sewage treatment plants:
1. Chemical factors
The presence of surfactants
Surfactants are one of the common causes of foaming. Domestic sewage contains a large amount of detergent ingredients, such as sodium alkylbenzene sulfonate. The molecular structure of these surfactants is amphiphilic, with a hydrophilic group at one end and a lipophilic group at the other end.
When they enter the sewage, the hydrophilic groups will attract each other with water molecules, and the lipophilic groups will face the air, forming a directional arrangement at the air-liquid interface, reducing the surface tension of the liquid. This makes it easier for sewage to produce foam, and the foam is relatively stable and not easy to break.
For example, after laundry wastewater is discharged into the sewage plant, if the detergent content is high, a large amount of white foam will be generated on the surface of the sewage.
High concentration of organic matter
Sewage contains a high concentration of soluble organic matter, such as protein, oil, etc. These organic matter will produce some intermediate products during the decomposition process of microorganisms, and they also have certain surface activity.
Take protein as an example. In sewage, protein molecules may be hydrolyzed to produce small molecules such as amino acids. These small molecules contain functional groups such as carboxyl and amino groups, which can change the surface properties of the liquid and promote the formation of foam. In the treatment of food processing wastewater, foam is easily generated due to the large amount of protein and oil.
… This impact will cause the gas in the sewage to be drawn into the liquid, forming foam.
For example, when sewage flows rapidly from a sedimentation tank to an aeration tank, foam may be generated due to the sudden change in the drop and water flow rate.
III. Biological factors
Microbial growth and reproduction
Microorganisms play a key role in sewage treatment. When the nutrients in the sewage are sufficient and the environmental conditions are suitable, microorganisms will multiply in large numbers. Some microorganisms will secrete some extracellular polymers (EPS) during their growth.
EPS is a high molecular polymer that is sticky and surface active. These microorganisms and the EPS they secrete will adsorb on the surface of the bubbles, making the bubbles more stable and less likely to break. For example, in some systems that use biofilms to treat sewage, microorganisms in the biofilm will produce EPS, leading to the generation of foam.
Filamentous expansion
Filamentous bacteria are a type of microorganism. Under normal circumstances, filamentous bacteria and flocculent bacteria maintain a certain balance. However, when the nutrient ratio in the sewage is unbalanced (such as insufficient nitrogen and phosphorus content) and the dissolved oxygen concentration is low, filamentous bacteria will multiply in large numbers and filamentous expansion will occur.
The massive growth of filamentous bacteria will loosen the structure of activated sludge and form large flocs. These flocs are easily adsorbed on bubbles, making it difficult for the bubbles to break, thus producing a large amount of foam. Moreover, this foam is usually viscous and dark in color, sometimes appearing black or brown.