Foam in aeration tanks - Water and Wastewater.com Help Forum

Water and Wastewater.com Help Forum - Foam in aeration tanks

Question by Manfredo:
We have a BNR Plant in New Zealand. A description of it can be found in our website http://www.kapiticoast.govt.nz/

Part of the last upgrade included the replacement of the surface aerator with submersed fine bubble membrane diffusers. The aerators look like a DAF. While the oxygen transfer and control was improved, there is a thick fresh foam layer on the top of the Aeration Tanks 24 hours a day.

It is not a bulking problem as the sludge settles normally in the clarifiers. However, foam reaches the clarifiers and it is not a good aesthetic appearance. Plus seems that the WAS contains foam too.

Any suggestions? Any other similar experiences with the membrane diffusers?

Answer
From the description it seems that only domestic wastewater is treated.
Anyway, the source of foaming substances is either the influent (surfactants, proteins) or the microflora of your treatment system. You might check the influent streams foaming properties.
On the other hand, there is no foaming without air bubbles. Therefore, eventually the air input can be reduced until the treatment efficiency gets insufficient.
Addition of defoamers or sludge ballasting/increasing the sludge concentration leading to sorption of foaming substances is another option.
Eventually the pH can be reduced if higher than necessary.

Answer by jetland
Yes we have played with membrane diffusers in our activated sludge technology. When installed to just genereate fine bubbles and allowing these bubbles to migrate to the surface without control of the pattern, surface foam occurred. A note is that the foaming was more predominent when the temperature increased.

I think you have answered your own question. If it looks like a DAF, quakes like a DAF, it will function as a DAF. Not an efficient DAF but a DAF.

MichaelJ

answer by cgillen
This type of foaming is very common in municipal plants with diffused aeration. The typical cause is due to biological foams from Nocardia or Microthrix. These feed on the fats, oils and grease in the influent and produce polysaccharides which produce very stable, thick foams - especially with fine bubble diffused air where it can resemble a mousse!
Is this foam thick and sticky? What colour is it?
If you take a sample of the foam and look at it under the microscope you should see typical Nocardia profiles.
This can be controlled using biological cultures.
For more information please contact me on cgillen@biofuture.ie

comment by jamko
I am just starting up a wastewater treatment plant at a fast-food place here in Jamaica. We use fine bubble Venturi Aeration, which is excellent, however the foam is a real problem. Ambient temperature is 28-29 C, which seems ideal for foam formation. Also I would think that we carry quite a bit of fat over and not to forget detergents from restaurant washing every night. All in all ideal conditions for foaming.
What do you suggest?
In this context I am not familiar with the term DAF, can someone explain that.
Does intermitting of aeration help with the excessive foam formation, and if so which time intervals would be recommenndable with respect to the high levels of ambient air temperature (I would be hesitant with more than 30' off - times).
Are there any chemicals/biochemicals known, which might be dosed?

answer by Victor Santa Cruz
DAF=Dissolved Air Floatation In simple terms, it is a system in which air is infused into the liquid making solids float to the top and the floating scum is scraped off.
Foaming can be attributed to several factors and they include, but not limited to, bacterial interaction (several filamentous bacteria have the capacity to create foaming havoc and they are Nocardia (Gordonia) and Microthrix parvicella, surfactants, and overaeration. Your job will be to determine which of these is the root cause of your foaming.

comment by jamko
we are presently working at the biological part. We think it has to do with detergents, fat as well as proteins in conjunction with water temperature around 85F, since its a fast food place.
From our observation we know that the foam decomposes after we turn off aeration for not more than 30'. Therefore we consider over-aeration. We will install timer to vary aeration from permanent to a one hour plus 15' off - cycle as first measure.

answer
as mentioned in the previous post there is a range of defoamers on the market, which can be applied successfuly as a first aid measure or as part of the process.
Intermitting aeration seems an option to consider if overall reduction of aeration is not feasible. Please share your experience with the help forum!
It seems that foaming in wastewater treatment would be a topic to consider for the Ask Tom column.