One of the most common causes of foaming is the surfactant-like molecules with hydrophobic and hydrophilic moieties. Artificially produced surfactants are the most efficient foam causing molecules, but naturally occurring polysaccharides and proteins also can act as inefficient surfactants since these also have hydrophobic and hydrophilic moieties. The hydrophobic moiety prefers to stick out to air while hydrophilic moiety roots into water. Surfactant molecules can cause foams at a sufficiently high concentration by aligning side by side. Microorganisms and particles with hydrophobic surface tend to accumulate on the thin film of foam particles and interfere water drainage from the foam, which eventually stabilizes foam.
Surfactant-like molecules and hydrophobic particles also tend to accumulate on rising bubble surfaces in aeration tank (or bubble column). The accumulated molecules and particles on the bubble surface interfere oxygen transfer by physically screening the air-liquid interface. This is how foaming and oxygen transfer efficiencies are interconnected in biological process.
Fig. 1 illustrate the boom and burst cycle of foaming and OTE.
- In normal condition (a), a thin foam layer exists at the level foaming and defoaming balance each other. The foam causing materials such as proteins, polysaccharides, filamentous microorganisms with hydrophobic surfaces, hydrophobic cell debris, etc. move up to the top and cause foaming. But, those hydrophobic materials return to the water when foams collapse.
- If the balances between foaming and defoaming are broken, hydrophobic materials start to accumulate in the foam layer as more hydrophobic materials move to the top than return to the bulk liquid. Even though the biology produces more hydrophobic molecules and particles in this stage, OTE in liquid may stay not affected since the hydrophobic materials that interfere oxygen transfer move to the foam layer. If the upward transportation of hydrophobic materials occurs effectively, OTE can even increase in this stage.
- If antifoam is dosed and the foam layer collapses, the large amount of accumulated hydrophobic materials in the foam layer return back to water phase within a short period of time. This can cause a dramatic OTE decrease by increasing hydrophobic material concentration in water. The low OTE decreases dissolved oxygen concentration and put a stress on microorganisms. The stressed microorganisms in turn produce more biopolymers and cell debris from dead microorganisms, which can causes second surge of foaming.
- As the dissolved hydrophobic materials are discharged from the bubble column, OTE gradually recovers. The rate of OTE recovery is depending on many factors, but the rate of hydrophobic materials discharge is dependant on HRT and SRT.
Fig. 1. Mechanism of oxygen transfer efficiency (OTE) collapse
There is no reliable real time OTE data obtained before and after foam layer collapse in public domain. However, the dramatic effect of foam layer collapse can be found in syngas fermentation, where mass-spectrometer tracks the mass transfer efficiency real time. As shown in Fig. 2, the initially stable mass transfer efficiency in stage 1 starts to increase in stage 2 as a consequence of the scavenge of hydrophobic materials from the liquid to the foam layer. However, the mass transfer efficiency sharply drops from 50% to 18% as soon as antifoam is added and foam layer collapses. Since the amount of the antifoam used is less than 1 mg/L based on the reactor volume, it is not likely that the antifoam itself causes the mass transfer drop. After approximately one cycle of SRT, i.e. 72 hours, mass transfer efficiency starts to increase again.
This observation suggests that sudden kill of foam layer may not be the best way to control foam layer. It is more desirable that the foam depth is reduced gradually with a smaller amount of antifoam while allowing hydrophobic materials to exit from the system with excess sludge.
Other logical consequence of foam layer collapse is that the hydrophobic materials returned to the liquid phase can accelerate membrane fouling in MBR. However, little has been known about this phenomenon with supporting data.
Fig. 2. Effect of foam layer collapse on mass transfer efficiency in syngas fermentation using bubble column at 72-hour HRT.
© Seong Hoon Yoon