I completely concur and ...

I completely concur and saved me doing a lot of writing.  This answer below is spot-on.  The only thing that I would add is a microscopic analysis of the sludge to tell you the predominance of different filamentous bacteria.  Then you can go to a decision tree to see what matches your situation (what you are doing wrong, i.e. O/G high, low DO, pH variations, temperatures, toxic or recalcitrant compounds, etc.)  The black as stated below is almost certainly septicity.  Increase your recycle rate first to get rid of the bottom portion of the clarifiers.  You may be running too high of bed levels, which give low DO environments and proliferate anaerobic bacteria which take oxygen from first nitrate and then sulfate in that order.)  There has to be float on the secondary clarifiers, unless you are running an MBR system.  Below is what I agree with whole-heartedly:

Hi Arun,

As people ​have already ​stated in ​the  ​comments, the ​foaming is ​normally caused ​by filamentous ​bacteria. There ​are conditions ​which create an ​ideal ​environment for ​the proliferation ​of these ​bacteria. Think ​of it like ​survival of the ​fittest, the ​filamentous ​bacteria are ​bigger than the ​other bacteria ​and will ​dominate in ​situations such ​as:

Low F/M ratios.

Low DO

Insufficient nutrients.

The black ​MLSS is ​normally a sign ​of septicity, ​once again low ​F/M, you will ​need to reduce ​sludge age ​ and WAS more ​frequently.​ 

Thanks, Sean L. Roop - Senior Water and Wastewater Consultant - Industrial and Engineering Consultants, LLC.