Totally Different Condition of Diffusers in SBR1 and SBR2 of the Same WWTP

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Totally Different Condition of Diffusers in SBR1 and SBR2 of the Same WWTP

This week I had a job of supervising the process of cleaning a WWTP, namely one aerated Equalization basin and two SBRs. The WWTP is for wastewater from a company that manufactures pharmaceuticals. The wastewater is not aggressive whatsoever.
When the basins were emptied we discovered that the membrane plate diffusers in Equalization tank and SBR2 were in great condition: no ruptures, no cloggings, membranes were soft, etc. On the other hand, diffusers in SBR1 were in a very bad condition: 6 out of 40 had ruptured membrane, and all the rest had some kind of hardened material underneath the membrane, and when I put pressure with my hand it crumbles. Another strange thing is that it looks as if the aeration in SBR1 is just fine. There is no significant increase of backpressure to the blowers and the target DO of 2 mg/l is being easily achieved.

Do you have any idea how is this possible? Just to make it clear, same wastewater goes alternately from equalization basin through SBR1 or SBR2, depending on the cycle.

The operator said that he did not put any additives to the SBR1, but the fact is that he is not to be trusted, because there are rumors that in the past he tried to sabotage the biological process.

There are couple of pictures attached, for your reference.

Thank you in advance for your answers.