[Consider this overview picture of a WWTP][1]:

[![][2]][2a]

In the back, you see two oval tanks with bridges and walls, separating an inner from an outer area. These are aeration tanks.  

Is there a specific reason to build them like this, instead of circular, or is there a specific process that calls for this shape? I'm aware of aeration tanks with an aerated zone and an anoxic zone, built concentrically with circular tanks.


  [1]: http://www.wver.de/index.php/abwassertechnik/109-abwassertechnik/klaeranlagenuebersicht/293-klaeranlage-heinsberg-kirchhoven
  [2]: https://i.sstatic.net/KL7nBt.jpg
  [2a]: https://i.sstatic.net/KL7nB.jpg