The Complete Mechanism Behind the Sudden Increase in Foam When Rainwater Dilutes High-Salinity Wastewater

I. High salinity inherently has a foam-suppressing effect (basic premise)

Large amounts of electrolytes such as Na⁺, Ca²⁺, and Cl⁻ in wastewater cause salt precipitation:

In ultra-high-salt environments, the double layer of anionic surfactants (the most common type in industrial wastewater) is compressed by a large number of salt ions, causing hydrophobic chains to aggregate and precipitate, leading to inactivation. As a result, they cannot stably adsorb at the gas-liquid interface, and their foaming capacity decreases significantly;

High salt concentrations accelerate the drainage of liquid from the foam film, causing the film to rapidly thin and rupture, resulting in reduced foam volume and shorter lifespan;

Under high-salt conditions, colloids and large organic molecules are precipitated by salting-out, reducing the amount of free foam-stabilizing substances in the water and naturally inhibiting foaming.

II. After Rainwater Dilution, Salinity Decreases and Surfactants Fully Regain Their Foaming Activity (Core Reason)

After rainwater flushes in, salinity drops significantly, the salt precipitation effect disappears, and three major changes occur:

Surfactants are fully redissolved

Detergents, emulsifiers, and organic fatty acids—which were previously forced out by high salinity and adsorbed onto salt sludge, tank walls, and precipitated impurities—are all re-dissolved into the water. The concentration of free foam-stabilizing substances in the liquid phase surges, providing a substantial foundation for foaming.

Restoration of Adsorption Capacity at the Gas-Liquid Interface

In a low-salt environment, surfactant molecules are evenly distributed across the bubble surfaces, significantly reducing surface tension. This facilitates the formation of tiny bubbles. The charged bubble membranes create electrostatic repulsion, preventing bubbles from coalescing and rupturing, resulting in fine, dense, and stable foam. Experiments confirm that, with the same surfactant concentration, the foam height in a low-salt, clean water system is significantly higher than in a high-salt, concentrated water system.

Destabilization of Colloids and Organic Suspended Solids, Acting as a Foam Scaffold

In high-salt conditions, colloids, proteins, and polysaccharides settle; upon dilution, the colloids redisperse and suspend, interweaving between the bubble membranes to act as a scaffold, making the foam more resilient and difficult to deflate.

III. Rainwater Introduces Additional Turbulence, Continuously Generating New Bubbles

Large volumes of rainwater rush into the storage tank instantaneously; the impact of the water flow and the agitation from the circulation pump draw in air, continuously generating a large number of microbubbles;

The lower temperature of the rainwater causes the overall temperature of the process liquid to drop, increasing the viscosity of the foam liquid film and slowing the discharge rate, resulting in foam accumulation that does not dissipate;

Rainwater washes away salt sludge deposited at the tank bottom, releasing large amounts of surfactants and foaming organic matter trapped within the sludge, further intensifying foaming.

IV. Foam Carryover Amplified by Evaporation Vacuum Conditions

The boiling point of the diluted feedstock decreases, leading to more vigorous boiling under the same heating conditions and an increase in secondary steam velocity; fine, lightweight foam is easily carried by high-speed steam, penetrating the demister, resulting in rampant foaming and persistent fluctuations in vacuum.