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The answer is "it depends". Dissolved air flotation systems are great where
there are a lot of contaminants that float and some that tend to settle.
Floaters include scum, solids with entrained air, oils, greases and fats.
Those that tend to settle are suspended solids. If your pet food company
has both and particularly if the former is present in greater quantities
than the latter, they are an excellent candidate for DAF. If they have very
few floaters, it makes no sense to try to make contaminants that tend to
settle float so you can skim them off of the top of the liquid instead of
off the bottom.

If a DAF system is applicable to them, there may be enough fats oils and
greases (FOG) in the contaminants to send the waste to a rendering service
where the FOG can be reclaimed for commercial use. If the value of the FOG
outweighs the renderer's cost to process and dispose of the solids, they
may pay the dog food mfgr for the waste.

What about cost? DAF systems can get very expensive to install and operate
when the volume of wastewater exceeds 100,000 gpd. There are a lot of
chemicals to buy and use (acids, caustics, polymers, perhaps even alum) and
lots of tanks, pumps and test equipment. Systems in excess of 50,000 gpd
may need a full time operator-larger systems may need operators on two or
three shifts. Thus, as you can see, they get expensive fast. Small systems
are more economical. For the right application, they are the best way to
go, but they are not a panacea.

Also, if the facility is under the gun to reduce dissolved solids, a DAF
will remove virtually none of them (neither will a plate and frame
clarifier designed for settleable solids removal). Thus, you could install
a system that takes out many of the contaminants and still have high sewer
surcharges or even be in violation of your pre-treatment permit.

Finally, as with all pre-treatment systems, once in and operating, they can
become a trap. Employees start to slack off on good operating practices so
more waste goes onto the floor and down the drain. The squeegees get
propped in the corner in favor of the hose (with or without flow restrictor
nozzles). The attitude becomes "the wastewater treatment system will take
care of it". This is "anti-P2" and can overload the DAF system if not
designed for extra capacity. Now your system is expected to remove more
solids and FOG from more wastewater-an exponential problem. It gets even
worse if the FOG content of the solids waste has value so that they get
paid for it by the rendering service. They can then look at their system as
a "profit center" where maximizing the payment of shipments of waste
becomes a secondary goal.

One of my clients fell into this trap. One year they received $90,000 for
the waste solids from the rendering service that was taking their DAF
wastes. They were proud of the fact that their wastewater treatment system
allowed them to set up a new profit center at their plant. What they didn't
realize was that they were paying $1.75 million annually for the raw
materials that they were sending down the drain to the DAF system-and
"only" getting $90,000 for. Upon implementing the changes that our Penn
State team recommended, they saved $962,000 the first year. They also
eliminated the threat of the local sewer authority shutting them down for
exceedences on their operating permit. Of course, the rendering service no
longer paid them for the waste FOG and they had to invest $40,000 to obtain
these savings (a payback of .042 years-about 15 days), but now more than
half of what they were putting down the drain is now going into their
products and they are being paid for them (instead of having to pay sewer
surcharges and buy treatment chemicals). This was such an eye-opener to
them that they thanked us for saving their plant and 250 jobs. (They didn't
even know how much danger they were in).

Hope this is helpful.


>>Forwarded on behalf of Sherry Davis
>>Dear P2techers:
>>I have a client that called for help with installation of a system to
>>separate oil and grease from their wastewater.  They are a pet food
>>facility, with a daily flow of about 130,000gal/day.  I have three
>>questions: Would a DAF be the most appropriate type system for him to
>>What would be the best method to dry this sludge as much as possible?
>>there a re-use for the recovered solids? Any input would be
>>Sherry J. Davis, CHMM
>>Industrial P2 Specialist
>>133 Ward Hall,KSU
>>Manhattan, KS  66506-2508
>>Fax: 785-532-6952
>>Phone: 1-800-578-8898
>Jini Cook
>List Manager
>217.244-6553     jcook@wmrc.uiuc.edu