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The AB process is wastewater treatment process that works like this:

'A Stage': Wastewater is (after mechanical treatment) subjected to a high load activated sludge process where also a lot of contaminats are adsorbed on the sludge.

'B Stage': Low load activated sludge process with further BOD destruction and nitrification and denitrification in anoxic zones. A part of the effluient of the B vessels (before settlement tanks) is recirculated to the beginning.

Each stage has it's own set of settlement tanks and return sludge, so the biology is adapted to the specific conditions.

Now my question: I've seen the claim (and actuall don't doubt it) that the B-stage is more effective (Per volume available) when the effluent of the A-Stage is added in a cascading way: Say (in a rectangular, long tank) 50% of effluent at the inflow end, 50% in the middle. Return sludge is all added at the beginning of the B stage. A carbon source can be added if required.

What is the process reason why cascading is more effective? In a cascading low load reactor, is there still recirculation?

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