Floating solids on secondary clarifiers

Floating solids on a secondary clarifier are usually a byproduct of influent composition or biological processes that have drifted from ideal operations. While some floating solids are not a big problem, heavy solids conentrations can increase effluent TSS resulting in uninteded biological solids wasting. Here are the  most common casues of floating secondary clarifier solids:

• Excess EPS
  While EPS in normal concentrations is needed for floc formation, at times we have excess EPS that creates non-filamentous or Zoogleal type bulking. Non-filamentous bulking sludge is less dense and can trap bubbles of air & nitrogen that acts to float the sludge. 
• Fats, Oil, & Grease (FOG)
  FOG compounds are adsorbed by the floc acting as a sponge. The FOG being less dense than water can disrupt gravity separation and if you have too much FOG acts to float the sludge.
• Denitrification
  If you have a system with nitrification, soilds can use nitrite/nitrate in resiration while in the anoxic clarifier bed. Denitrification appears as small bubbles released when you disturb the floating solids. Water spray and increaseing recycle pump rate usually solves this problem.
• Nocardia & M. parvicella filament foams
  Both Nocardia and M. parvicella produce thick foam on the aeration basin. This foam can continue into the secondary clarifiers and create high effluent TSS. 

Once you know the cause of the floating solids, you can select among various control options. Key is to identify the source of the floating solids, use a quick control strategy to maintain effluent compliance, and then take longer term steps to reduce likelihood of more floating sludge events.