First floating sludge is ...

First floating sludge is most often caused by:

• Denitrification  – small nitrogen gas bubbles float the sludge in the clarifier creating floating sludge chunks with small bubbles entrapped

•   Fats, Oils & Grease  – simply put, FOG floats on water. When entrapped in floc, excessive grease or oil can cause floating biomass. This appears as a scum blanket that can cover the entire clarifier.

• Viscous bulking  or billowing sludge – viscous bulking can sometimes create floating sludge (more often it is just billowing over the weir versus floating). This is often caused by nutrient deficiencies (normally low phosphate) in industrial waters.

There are several ways to address the problem based on the cause. As suggested by Amit, bacteria growth can help or in some cases skimming of sludge will be helpful. 

https://www.biologicalwasteexpert.com/blog/why-are-solids-are-floating-on-my-secondary-clarifier#:~:text=Denitrification%20%E2%80%93%20small%20nitrogen%20gas%20bubbles,put%2C%20FOG%20floats%20on%20water.&text=This%20is%20often%20caused%20by,low%20phosphate)%20in%20industrial%20waters.