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by carter-admin 22 Jul 2019

The activated sludge (AS) process was developed around 1912-1914 and it involves the treatment of industrial wastewater and sewage. The plant design of the activated sludge varies. However, all AS plants comprises three main components, including:

  • An aeration tank: This serves as a bio reactor
  • Settling tank: Also known as the clarifier, this equipment is used to separate AS solids and treat wastewater
  • Return activated sludge: This equipment transfers settled activated sludge from the clarifier (settling tank) to the aeration tank’s influent

How the Activated Sludge Process Works

Atmospheric air or oxygen is added to a mixture of industrial wastewater or screened sewage combined with organisms to create a biological floc. This is known as “Activated Sludge”. The mixture of biological mass and raw sewage is known as Mixed Liquor. Once the sewage or wastewater has been “bubbled” enough in the aeration tank, the mixed liquor is then discharged into the settling tank where dead bacteria rises to the top forming a crust with clear liquid in the middle, and live bacteria settles to the bottom.

The clean water is then discharged using a pump into a soakaway or a watercourse. The remaining live bacteria in the settling tank are then returned to the aeration tank through the return activated sludge to re-seed the next wave of raw wastewater or sewage entering the tank. The dead bacteria crust has to be removed on a regular basis by experienced pump technician.

Advantages of the Activated Sludge Process Over Other Types

  • No need to top up with extra bacteria
  • No moving parts within the plant
  • Inexpensive servicing
  • Simple process
  • Very reliable
  • They re-seed themselves with beneficial bacteria
  • They are odorless throughout the process
  • Less bad sludge to remove
  • Long emptying intervals
  • Deep mixing

The Purpose of the Activated Sludge Process

In industrial wastewater or sewage plant, the process is chosen for the following purposes:

  • Generating a liquor low in suspended or dissolved material
  • Generating a biological floc that settles easily
  • Driving off entrained gases: nitrogen, ammonia, carbon dioxide, etc.
  • Removing phosphate
  • Oxidizing nitrogenous matter, i.e. mainly nitrogen and ammonium in biological materials

Types Of Activated Sludge Plants

  • Package plants: These types of activated sludge plants often serve industrial plants that specialize in hybrid treatment processes or small communities without a dedicated operational staff. The process involves using aerobic sludge to treat incoming wastewater. However, the settlement stage of treatment can be omitted.

  • Oxidation ditch: This type of AS plant is better suited where more land is available. The wastewater is treated in large oval or round ditches with horizontal aerators driving the mixed liquor around the ditches and providing aeration. They are relatively easy to maintain than other types of plants.

  • Deep shaft: This type of plant is built when there is limited space or the land available is not able to accommodate a fully activated sludge plant. Wastewater is treated by injecting oxygen into a columnar tank that is deeply buried in the ground through a pressured return sludge stream. The cost of constructing this type of plant is high.