Foaming/Frothing | Activated Sludge Process | Wastewater Treatment Plant

 Foaming/Frothing

A buildup of foam or froth on the surface of the aeration tank is referred to as foaming/frothing. Foam might be thick, dark brown foam, light, white foam, unstable foams that come and go and are easy to treat, or stable foams that are more difficult to treat.

Causes

• A low MLSS results in light, white foam.
• Instable foams can be caused by nutrient inadequacies, particles recycled from dewatering processes, the presence of surfactants (detergents) in the plant's influent, excessive aeration, and polymer overdose.
• Nocardia foam refers to persistent, stable foams caused by the filamentous bacteria Nocardia. A low F/M ratio, high MLSS (high MCRT), and insufficient wasting and reaerating activated sludge generate Nocardia foam, which is a thick, dark brown foam. The high air flow requirements required to maintain high MLSS tend to expand Nocardia foam and exacerbate the problem.

Solutions

1. For control of detergent foam:

• Boost the MLSS concentration.
• Reduce the air supply during low flow periods while maintaining the residual DO level.
• During periods of low flow, slowly return digester supernatant to the aeration tank.

2. For control of Nocardia foam

• Lower the MCRT.
• Reduce the flow of air to reduce the thickness of the foam layer.
• Add a selector tank before the aeration tank to provide conditions that are unfavorable for Nocardia growth, such as a high F/M ratio, dissolved oxygen levels between 2 and 5 mg/L, and MCRT less than 5 days.
• Add a biological foam control agent made up of bacterial cultures that will inhibit the production of bio surfactants by Nocardia.
• Return sludge should be chlorinated.
• Directly spray a chlorine solution or sprinkle calcium hypochlorite on the surface of the foam.
• Reduce the pH of the mixed liquor by adding chemicals or starting nitrification.