Ph may be off? Or polymer/coagulant dosage? Several factors depends on what type of process you run
We've had this problem with sludge. We increased our sludge pump frequency and length of time in which it operates. The nitrogen cycle was floating our sludge to the surface.
What is the time your sludge remains in the clarifier? It could well be that the sludge sits too long and may lead to the growth of filamentous bacteria.
There are many causes of sludge floating. We usually identify probably cause by looking at the floating sludge:
(1) rising "chunks" of sludge with fine bubbles is usually denitrification and can be fixed by increasing recycle pumps/lowering bed depth
(2) Filamentous bacteria usually have a thick foam in the cases of nocardia or micothrix
(4) Oils & Grease - tends to float as a scum - (also seen in excess microbial slime in petrochemical wastewater)
(5) Toxic or shock - floating solids are part of a loss of floc formation. The SV30 will increase markedly.