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DAF question
May 03^rd, 2012, 12:51am

Hi All,

I'm running a DAF that feeds off an equalization tank. The sludge gets fed to a decanter that is not able to handle the total volume of sludge, hence some of the decanter's product is returned to the eq tank.

The problem is that the DAF get a lot of carry over into the weir. I have done extensive jar tests and the chemical programme works well. There is excellent flocculation when a sample is drawn from the pipe flocculator, even so that the sludge rises without the aid of DAF (in the jar).

The COD's of the incoming water to the DAF is between 12000mg/L (yes three zero's) and 20000mg/L.

There is a stage 1 DAF that removes FOG's without any acidification and then the effluent of DAF 1 goes to DAF 2 with a chemical programme (Al salt + poly)

The DAF is cleaned every weekend, but on Tuesday/Wednesday the carry over normally starts. I know throwing the sludge back would have a snowball effect on COD, chemical usage etc.

Any advice/recommendations??

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Tom Keenan - nesa God Member Offline Environmental Consultant - contact info@nesa.ie Posts: 2103 Gender: Company or Organization: nesa environmental consultants	Re: DAF question Reply #1 - May 3^rd, 2012, 6:58am Have you tried cleaning the chemical DAF more frequently, or at least drawing off bottom sludge every day from this DAF.?
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we_trading YaBB Newbies Offline I love Water and Wastewater.com Posts: 28 Company or Organization: Consultant	Re: DAF question Reply #2 - May 3^rd, 2012, 7:02am Thanks Tom, yes the DAF is cleaned every weekend, there is an automated de-sludging valve at the bottom of the DAF that opens every 40 second to remove bottom sludge.
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Tom Keenan - nesa

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Re: DAF question
Reply #3 - May 3^rd, 2012, 7:08am

Automatic de-sludging from the bottom of the DAF every 40 seconds sound absolutely excessive. Is there a problem with sludge accumulating on the bottom of the DAF?
If bottom sludge is not the issue then you may need to increase the air flow to the DAF to increase the air:solids ratio in the flotation section.
You could also check that air pressure in the saturation vessel is operating at manufacturers recommended level.

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Jeff Naumann God Member Offline I love Water and Wastewater.com Posts: 1133 Torrance, California Gender: Company or Organization: Jeff Naumann & Associates	Re: DAF question Reply #4 - May 3^rd, 2012, 9:46am What is the source of the waste water that you are treating? It sounds like the DAF system and decanter are under-sized for the amount of waste you are receiving. Source control in the plant that produces the waste is probably needed.
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we_trading YaBB Newbies Offline I love Water and Wastewater.com Posts: 28 Company or Organization: Consultant	Re: DAF question Reply #5 - May 3^rd, 2012, 11:56am @Tom My mistake 40 minutes @Jeff Poultry effluent. We know the decanter is undersized and apparently the DAF has run at this high COD before with less caryover
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Tom Keenan - nesa God Member Offline Environmental Consultant - contact info@nesa.ie Posts: 2103 Gender: Company or Organization: nesa environmental consultants	Re: DAF question Reply #6 - May 3^rd, 2012, 12:10pm Thanks for that clarification. Even opening the bottom sludge draw off every 40 minutes seems excessive. If the DAF is floating the sludge then there should be very little bottom sludge generated in 40 minutes. If the DAF is not floating the sludge then that aspect needs to be addressed.
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Jeff Naumann

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Re: DAF question
Reply #7 - May 3^rd, 2012, 12:34pm

we_trading wrote on May 3^rd, 2012, 11:56am:

@Tom
My mistake 40 minutes

@Jeff
Poultry effluent. We know the decanter is undersized and apparently the DAF has run at this high COD before with less caryover

It's no the COD per se that is your problem.........it's the FOG (fats, oils and grease) and solids. And, possibly the flow rate. Hopefully, Dan Keys will pipe in as our resident DAF expert.

Is there a "meal plant" or rendering plant in the area, that could possibly use the waste solids from the plant?

What do you do with the captured sludge from the DAF and decanter?

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Re: DAF question
Reply #8 - May 3^rd, 2012, 1:21pm

It sounds to me like the chemistry is changing considerably due to adding the solids back to the EQ tank. It's likely this is changing the chemistry of the water depending on the EQ tank level (may be more noticeable if it's lower)

Either that or during these periods the DAF is hydraulically overloaded?

As for the decanter can you describe how this process works?. If you have the proper equipment and chemistry you should be able to get a relatively thick sludge off the DAF (maybe 6% or so) without any additional thickening. Are you decanting the clear liquid from the bottom (does the sludge float due to the oil and grease). One possibility is slowing down the skimmer and sending less sludge (but thicker) into the decanter.

best regards,

Ryan

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Eric Raj

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Re: DAF question
Reply #9 - May 4^th, 2012, 10:06am

It sounds like the unit is simply not being operated properly. Reaeration rates need to be optimized along with verifying that the water is indeed being saturated with air. How fast you operate the top flights helps dictate how thick the float solids will become. But running the flights too slow allows the float to build up too great depth-wise and carryover can then occur.

Influent Feed rate must be properly matched by polymer feed rate and the unit can't be operated beyond its design specs. This decanter is the hopper into which the DAF float solids are emptied into? If it can't hold all of the solids then you are running the DAF feed rate too high and are overloading the unit. Slow the feed rate down.

Good luck!

~Eric

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noj

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Re: DAF question
Reply #10 - May 7^th, 2012, 1:19pm

Did you try to work with reduced recycle flow?
Also you can try to inject some saturated water into the flocculator, after chemical injection.

What do you mean with 'stage 1 DAF that removes FOG's without any acidification '?

Do you use additional chemicals on the dewatering decanter?
The sludge you feed back to the eq tank, is that with/without chemicals?

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aditya_2281 YaBB Newbies Offline I love Water and Wastewater.com Posts: 10 Company or Organization: World Water Works Inc.,	Re: DAF question Reply #11 - May 15^th, 2012, 1:01pm It sounds like there is a problem with the aeration pump ? You may need to increase the air flow to increase the A/S ratio which is suggested by Tom Keenan - nesa. What is the influent flow to the DAF ?. Thank you.
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