Tell us what the total and dissolved portions of the influent BOD are.  Any type of physical treatment equipment is not going to remove dissolved BOD.

I am familiar with "dissolved" air flotation, but not suspended air flotation.  Hopefully, Dan will jump in with his great expertise in the technology.

Can you tell us more about what the plant processes are, both the production facility and the waste treatment system?

I have not had experience with this technology, but I did some research last month.
The tiny bubbles required for floatation are generated by aerating a surfactant to form microbubbles then releasing this into the water.
I imagine that any residual surfactant would add to BOD rather than remove it.  
Also I agree that there would be very little reason for it to remove soluble BOD.
Some form of caogulant or flocculant addition before treatment may be able to reduce some of the soluble BOD?

I do have some experience with SAF.  The technology involves generating small "soap" bubbles with a variety of surfactants having either a negative or a positive charge, depending on the application.  Assuming all the potential issues of rise rate, equipment sizing, surfactant selection and so forth are correctly addressed, it boils down to the BOD value of the suspended solids that are separated in the water.

If I recall correctly, your company is involved in processing and packaging frozen fruit primarily.  In that arena, the majority of the "five day" BOD is soluble (dissolved) and will not be removed by this type of equipment.  However, if the soluble BOD is contained inside suspended unruptured clusters of fruit cells, it can be removed with properly sized and applied SAF or DAF, but only the portion that is contained in the suspended solids that are removed.

To find the potential for removing BOD in this type equipment, run a simple soluble BOD lab test.  Anything not removed in the filter is soluble and will not be removed in a SAF/DAF.

In fruit processing, peeling is often done with a hot caustic bath followed by machinery that strips off the loosened peel.  This process introduces a tremendous amount of fruit particulate into the caustic bath which also tends to burst the individual fruit cells, releasing the dissolved fruit sugars into the water.  The caustic chemicals used in the peel process can make subsequent coagulation and flocculation very difficult.  You may need to neutralize those chemicals in order for coagulation to take place.

The driving factor in the fruit and vegetable processing projects I've worked with was removal of suspended solids that tended to blanket and blind the soil where the water was applied for irrigation of crops, grown to consume the BOD.  This would cause standing water to occur and then the fiber content would float to the surface, cutting off oxygen to the standing high BOD water, causing anaerobic conditions.  The pretreatment removed those solids, allowing the land application of the high BOD water to be more effective.

If I were looking at your situation in person, I would:
  1.  Perform soluble BOD tests
  2.  Determine if proper coagulation is taking place and proper chemicals/pH modification is being used
  3.  Determine if you are getting good flocculation prior to the SAF
  4.  Determine if your SAF equipment is properly sized for your unique wastewater situation.

Let me know what of those items have been addressed and I'll help you along with your analysis.  I can be more detailed about your specific equipment in private correspondence.  I try not to promote or criticize specific equipment brands on the forum.

Dan Keys

For what it's worth, I was able to get 40 to 50 percent BOD reduction in peach processing water with DAF, but it required neutralizing the water with acid or carbondioxide followed by some very agressive coagulant and a polymer flocculant.  It is NOT easy water to work with.

I would almost bet the effluent in your SAF looks about as "dirty" as the water going in.  If everything is working correctly and the equipment is properly sized, the discharge should be clear water with a light yellow tint.  If the water is cloudy or opaque, either the chemistry or the equipment or both are not working correctly.

For a more agressive program for BOD reduction, I would recommend finding a way of isolating the overflow from the peeling process for separate disposal as a long term solution.

A properly sized DAF or "SAF" should never need a pre-settling component unless the water contains heavy particulates that do not float.  Including such an extra and costly step in the process would simply be a "fix" for bad system engineering.

There are several manufacturers who overstate the size/throughput capability of their equipment based on one or two installations that operate in unusual circumstances.  This allows them to offer lower prices and the unsuspecting purchaser pays dearly in the end when it just doesn't perform.  This is an inexcusable practice, but we run into it way too often.

With DAF, I have been able to increase flow capacity when the unit is undersized for the job by applying our split influent aeration approach, eliminating the capacity used for recycle.  But this is not possible for the SAF because there is no significant recycle flow in the design.  Sadly, the unit is probably too small for the job and/or the coagulation/flocculation process is not working.