Biological treatment - Activated Sludge

Biological treatment using suspended biomass is currently the most common treatment solution used to treat urban wastewaters. The general principle is to accelerate the oxidation process of the organic matter, which occurs naturally within the receiving waters. This is usually achieved by using the activated sludge technique.

Activated sludge

ARDEN and LOCKETT were the first people to recognize the possibility of using activated sludge. In 1914 they noticed that bacterial binding material forms naturally during prolonged aeration of urban wastewaters.

Part of the organic matter is converted into biomass and the remainder is mineralized. The bacterial binding material can then be separated from the treated water using simple settling techniques.

At first the aeration and sedimentation phases were carried out in the same tank.
Later, the sedimentation phase was isolated in a second tank to allow for a continuous operating process.

The activated sludge technique was born.

The technique consists of 3 different systems :

  • a biological reactor - using a suitable biomass for settling,
  • an aeration system to provide the necessary oxygen for the biomass,
  • a separation facility, where the purified water is separated from the biomass through settling.

Review of the treatment options


The treatment of urban wastewater using the activated sludge process basically consists of the following stages :

  • pretreatment - using a screen to retain fibers and generally any other particles that are larger than one cm, followed by the use of a combined tank to remove :
    - sand and coarse matter, which settles at the bottom of the tank
    - surface materials such as fats and oils or other components of vegetable or animal origin.
    The relatively small volume of waste recuperated in this way is stored in skips for disposal. However, it is possible to incinerate them or to treat them (e.g. by using the Biolix™ process for removing fats and oils).
  • settling - retaining a large part of the suspended matter that forms the primary sludge. At this stage, a coagulant (usually a ferric salt) is added to assist with the settling of colloids that do not readily settle. This results in a physical-chemical sludge. This process also has the advantage of precipitating a large part of the phosphorus.
  • activated sludge tank - the water, now depleted of a lot of its suspended solids, is brought into contact with the active biomass. The remaining dissolved pollution is then oxidized and re-released into the atmosphere in the form of CO2 or transformed into biomass. In addition, the operator of the treatment plant ensures that this biomass forms into a good floc which settles well and can thus be separated in a second sedimentation tank called a clarifier. For operational reasons, a large part of the sludge stored in the clarifier is recycled in the biological tank, and the excess sludge is extracted to the sludge treatment system.






Depending on the size of the tanks and aeration conditions, this biological stage is not only able to degrade a large part of the carbonate pollution (DCO and DBO5) but also the nitrogen (which can be released as N2 gas) and the phosphorus (retained within the sludge, possibly through the addition of a coagulant).


The above example is but one of a variety of treatment options available. For example :


  • the pretreated raw water may be sent directly to the biological tanks, with no primary treatment stage.

  • the wastewater treatment plants can simply use a primary settling tank (using a coagulant to precipitate the colloids and the phosphorus) if the receiving water is not too sensitive (i.e. a fast flowing river, only slightly polluted and in a rural area).


It should also be pointed out that the water treatment process removes the majority of pathogenic bacteria contained in the raw water either by killing them or by removing them in the sludge.


However, in some wastewater treatment plants the water treatment process may be refined further with a polishing stage.


Which aims to, for example, reduce any residual suspended solids and phosphorus (through the use of filtration and post-precipitation techniques) and/or disinfect the treated water with chlorine or UV for its possible agricultural reuse or release into a highly sensitive receiving water (shellfish farming).