This has occurred because the ...

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This has occurred because the plant has become overloaded and you have lost oxygen.  From the colour it sounds like it has become partially anaerobic and possible anoxic.  When plants lose oxygen, the organisms, which are able to grow at lower oxygen levels, or anoxically these grow more rapidly and don't become included in the main floc.  When the plant operates anoxically, the ultimate result is the conversion of Nitrate to Nitrogen gas and this causes the floc to separate.  If it is possible, increase the oxygen levels and add a high charge, high molecular weight cationic polymer to the aeration system.  This will help draw the sludge together.  The amount needed will depend on the size of your aeration system. The important thing is to get the aeration up and keep it up till the plant recovers.

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To add on to Don's accurate assessment, remember when the sludge does not settle and massive surface accumulation occurs, you will lose some to carryover from the secondary clarifier, most likely why you have a drop in MLSS concentration. 

Physical removal of accumulating surface scum or temporary high pressure jetting tends to speed up the recovery, by breaking the surface scum you will encourage re-settling that will be aided by the addition of the floc.

In the event that filamentous bacteria tend to dominate colonisation, you may want to consider applying chlorine - be aware you run the risk of complete process failure if your chlorination is too high as you will destroy too many, or all of your biomass.

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