Fighting Foam

Just like a poorly poured pint of beer can wreak havoc on your coffee table, excessive foam formation in aeration tanks can create operational challenges for both municipal and industrial treatment plant operators. In municipal plants, foam formation is common during secondary treatment startup, as the young microbiology is unable to breakdown surfactants associated with soap, shampoo, etc. This phenomenon is typically short- term and goes away as the mean cell residence time increases. Filamentous organisms can cause foaming issues in aeration tanks and this viscous, brown biological foam can create issues with solids separation/inventory, and reactor volume decreasing effluent quality. In aerobic and anaerobic digestion processes the breakdown of microbial cells can create a significant, long-term foaming challenge.

In industrial treatment plants, foaming issues in aeration tanks are often associated with the contents of the production plant effluent including the presence of surfactants, the breakdown of protein, or both. In nitrifying systems, excess foam can also be due to filaments or the accumulation of EPS (extracellular polymer substances) especially in MBR processes. Nitrification reactors are particularly susceptible as a build-up of a thick, insulating foam blanket can disrupt biological heat dissipation and negatively impact the performance of nitrifying microorganisms. In all of the above cases, if not attended to, excess foam can spill over onto the ground, platforms, and walkways creating safety issues and costly clean-ups.

There are many ways to battle foam and for severe foaming conditions the most commonly used are mechanical devices, modifications to the aeration system operating strategy, or the use of antifoam or defoamer chemical agents.

Mechanical:

There are a number of specialty mixing devices in use today for reducing foam volume. These foam “cutters” are located above the tank liquid level and run as the foam layer rises. A specialty designed blade cuts the foam, reducing bubble size which collapses and creates denser foam, thus becoming less voluminous. The use of jet nozzles aimed at impingement plates has shown to be very effective in managing foam. For nuisance events, spray nozzles located along the circumference of the aeration tank has proven to be effective. These mechanical systems add to the capital and operational cost of biological treatment plants, and companies make every effort to try to effectively battle foam using the least amount of energy.

Operational Strategies:
Modifications to operating strategy often involve the reduction in the amount air input to the aeration tank, and is only a viable option when the plant loading allows for airflow reduction while not seriously impacting dissolved oxygen levels. In jet or injector type aeration systems the ability to control both the air and liquid flow using variable frequency drives (a.k.a. change the gas/liquid ratio) offers a significant advantage, as it is possible to shift the ratio without negatively impacting oxygen levels. This strategy can work because the reduction in the liquid flow through the jet/injector will create larger gas bubbles which tend to more rapidly coalesce, thus reducing foaming.

Antifoams and Defoamer:
The chemical additives used to combat foam are surfactants and are classified as anionic, cationic, and non-ionic. Defoamers and antifoams are used differently. Defoamers are typically added during and after a foaming event; destroying the existing foam and preventing further foam production for a period of time. Antifoams are added prior to the aeration tank and inhibit foam production by preventing the formation of fine bubbles and accelerating the coalescence of the bubbles as they exit the aerator and enter the entrainment process and vertical bubble column. A secondary effect of these chemicals, even at concentrations as low as 3 mg/l, is the depression of the oxygen transfer rate (KLa ). Surfactant chemistry is powerful and we have observed full-scale plants experiencing > 50 % reduction in the system oxygen transfer capacity.

From KLa Systems perspective, antifoams/defoamers are rarely ever recommended to combat foaming events as it adds another operating expense and the negative impact on oxygen transfer and the biological process far outweigh the benefits. However, if chemical addition is under consideration there are a few products available in the marketplace that perform well without significantly depressing oxygen transfer. KLa Systems has a data base of a variety of antifoams/defoamers which can be categorized by their major component and include; hydrocarbon, vegetable oil, organic acid, silicone oil, or alcohol. We have tested them and will work with our customers to find suitable products that mitigate foaming, while not adversely affecting oxygen transfer.

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