Dorrco aerator in ASP

The Dorrco aerator is a mechanical-cum-compressed-air aeration device (supplied by Dorr-Oliver) used in the activated sludge process.

Operation

   motor
    |
  +-+-+  rotating agitator
  | | |   (hollow shaft)
~~|~~~|~~~~~~~~~~~~~~~ water level
  |   |  <- mixed liquor circulates
  | air supply pipe
  |___|_____ impeller/sparger plate ___
  tank bottom  (sludge kept in suspension)

• A vertical shaft carries a rotating agitator (impeller) in the aeration tank. Compressed air is supplied to the base of the agitator through a pipe or the hollow shaft.
• The rotating blades break the air into fine bubbles and throw the liquid outward and upward. This produces a circulation of the mixed liquor, so the bubbles are in contact with the sewage for a long time.
• The sludge is kept in suspension by the stirring action and the dissolved oxygen is supplied, so organic matter is oxidised by the micro-organisms.

Advantages

• Good oxygen transfer, since fine bubbles are produced by shearing and are retained in the liquid for a long time.
• Efficient mixing of sewage and return sludge; there is no deposit of solids in the tank, so clogging of diffusers does not occur.
• Aeration and mixing can be controlled independently, so the unit works for varying loads.
• Tanks can be deeper or of any shape, and the system suits small plants.
• Needs less compressed air than a plain diffuser system, and the pipe work is simple.