Hello @Mirko. We have seen ...

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Hello @Mirko.
We have seen such issues in the past usually caused by membrane breaking or popping up. The grid loses pressure, bubble pattern goes uneven. At first anaerobic cycle (and SBR has them) , the system depressurises and water + active sludge enter the grid. This happens in one group, usually associated with one dropper. At next start, sludge clogs the whole grid from inside and makes that layer you mentioned. Backpressure may be ok as the membranes popped out. Aeration still happens with bad pattern and coarser bubbles. All 40 diffusers on same dropping pipe? 

I fully agree with Mr. @Martins and Mr @Qiao. 

Hardened membranes usually happen when softeners in EPDM get lost. This is ageing or exposure to the UV radiation (sometimes in summer tank was drained, no water on top of diffusers for some weeks... only for that tank). Check the logs of the plant.

Replacing all membranes and backwashing the grids might be a solution. Condensate removal to be implemented.