Residual oxidizer in process waters, rinse waters or after cyanide treatment may cause a floating floc problem. You can determine this one of two ways: check the ORP to see if you have a high positive value or add a reducer (sodium bisulfite or metabisulfite) to your jar test. If the floc sinks to the bottom it would indicate residual oxidizer. Oxidizers that may affect floc include: peroxide, permanganate, nitrates, nitrites, perchlorates, bleach, etc. On occasion, floc will also rise to the top with time.
Floating floc can also be a consequence of over-adding your flocculant polymer. Typically flocculant polymers are made up at 0.1%/wt in a day tank and dosed 5-10 ppm for metals and slightly higher for a cationic flocculant. The floc size you look for is typically a medium size floc that settles to the bottom (unless you have a DAF system). A large floc maybe an indication your feed rate is too high and too much polymer is entering the tank. This can be adjusted for your water flow until you see the turnover floc changes to the size that settles the best for your clarifier.
Bacteria or Induced Air
Growth or carryover of bacteria in the waste water generates oxygen and will float any solids to the top. If there is a pocket of old solids on the bottom of the system this can lead to bacterial growth along with shutting a system down in the middle of operation. This allows any bacteria present to utilize nutrients in the water to feed on and outgas. Any source of induced air can latch on to the floc. Foam on top of the premix tank can also latch on to floc. This is why air lines are not used for mixing in wastewater systems.
Need For Weightier Floc
The addition of inorganic salts followed by a flocculant may not correct a floating floc problem, even after massing together. This situation may require the use of a heavier coagulant or the use of a different flocculant. These two situations need to be jar tested to determine the best course of action.