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Andres

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DAF+flocculant?
May 10^th, 2012, 6:41am

Dear all,

I got two designs for treating my rainwater+factory wash water that has a bit of free oil in it. One of the designs suggested using DAF with addition of flocculant, the other offered some "oil separator" and an flotation chamber, where flocculant will be added.

I was wondering, how common it is to add polymers to DAF system? May I have understood correctly, that usually just air bubbles are used?

What would be your oppinion (if you have enough info?): Would it be OK to dose polymer to a DAF system or will be do more harm as the flocs might be broken by small bubbles, for example?

Best regards,
Andres

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The sludge judge

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Re: DAF+flocculant?
Reply #1 - May 10^th, 2012, 8:34am

I would really look at your influent TSS, BOD, and oil and grease concentrations and determine to what level reduction you need. If the only thing that needs to be removed is the free oil then your seperator may be effective, if you want higher removal I'd imagine DAF would be needed. Run jar tests and get an idea of how much reduction you will get chemically.

You may or may not have to feed a coagulant prior to the flocculant in the DAF. Also with washwater especially (detergents/ surfactants) you want to do extensive jar testing before choosing the ideal program. In some cases at my plants we have a couple chemical programs and need to change frequently due to CIPs and changing influent ww characteristics etc.

In a DAF the air bubbles don't hurt the process/ they attach to the flocs and float them to the top where their skimmed off. The trick is getting the right chemistry beforehand to achieve nice flocs.

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Tom Keenan - nesa

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Re: DAF+flocculant?
Reply #2 - May 10^th, 2012, 11:41am

It should not be necessary to treat rainwater in a DAF.
If, as you say, "it has a bit of free oil in it" then you should first work to ensure that the surface from which the rainwater is draining is kept as clean as possible and as free of oil as possible.
Then you should pass this rain water through an "interceptor" where the "bit of free oil" can be separated and recovered periodically.
Finally you should make sure that uncontaminated rain water is not allowed to drain to oil contaminated drains or to pass through the interceptor.

With regard to adding Poly-electrolyte to a DAF.. this is a common and successful practice. The quality of the water entering the DAF and the emission limit values that you need to achieve will effectively determine whether you need a DAF or not and whether a DAF will be appropriate for this duty or not.

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grrun

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Re: DAF+flocculant?
Reply #3 - May 10^th, 2012, 4:49pm

If the oil is emulsified (which it usually is in the presence of surfactants), then a DAF is usually required. My experience has been that both oil and suspended solids are associated together when in wastewater. If there are not enough suspended solids already present in the water, some solids such as Al(OH)₃ or Ca(OH)₂ may be added to the wastewater to destabilize the emulsions and to provide the TSSs necessary to combine with the oil. Flocculants may then be added to form a floc. The microscopic air bubbles released in the DAF then attach themselves to the floc to float the whole mess to the surface of the DAF where it is skimmed off.

The whole point that I'm trying to make here is that chemicals are usually added to DAFs. Seldom is the DAF sucessful in removing contaminants without chemical help.

grrun

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Andres YaBB Newbies Offline I love Water and Wastewater.com Posts: 40 Gender: Company or Organization: Tallinn University of Technol.	Re: DAF+flocculant? Reply #4 - May 14^th, 2012, 2:05am Thank you both for your answers. They are very helpful.
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