Activated Sludge

Activated sludge is doubtless the biological treatment process with more worldwide installations for sewage and industrial wastewater treatment facilities, for situations where a high quality of the effluent is required and there is limited space available. Nevertheless, these systems when compared to others as lagoons require more associated equipment, a thorough operation and higher energy consumption.


Activated sludge is also used in many facilities as a “polishing phase” of the aerobic or anaerobic systems, which cannot reach rollover requirements and demands by themselves. These settings have the advantage of less energy consumption than a 100% activated sludge system; they have less sludge generation and similar quality of the effluent.

Activated sludge consists of:

  • Aeration tank or Reactor
  • Aeration system
  • Decanter or secondary clarifier
  • Sludge recirculation
  • Sludge excess waste

Biochemical reactions associated with organic material removal, and under certain circumstances, also nitrogenous material, have a place in the biological reactor or aerations basins. Biomass or microbiology develops from thesubstrate existing in the raw effluent. Solids settling (biomass) takes place in the secondary clarifier and tends to clear the treated effluent. A significant part of the solids settled at the bottom of the clarifier are recirculated (sludge recirculation), in order to keep certain amount of biomass in the reactor which is responsible of the high efficiency of these systems. The other part of the solids (sludge excess) is wasted to keep balance.


Biomass is separated at secondary clarifier due to its flocculation and settling characteristics. This is produced by the generation of jelly-like material that favours bacterial clumping, protozoa and other microorganisms responsible for organic load removal in macroscopic flocs.

As a result of sludge recirculation, the suspended solids concentration in an activated sludge reactor is very high. At these processes, the HTR is very short, just hours, which is often accompanied by a small size reactor. Nevertheless, solids (biomass) stay in the system for a longer time than in the hydraulic system due to recirculation. Celular Residence Time, or sludge age or time of sludge retention is determined thus as the ratio between the solids mass in the reactor and the solids removed from the system.

The prolonged stay of solids in the system guarantees high efficiency of the activated sludge, since biomass has the time required to metabolise almost all the organic load of the effluent.

Lodo activado 3Because of the continuous income of substrate (BOD of the effluent) inside the reactor, microorganisms are continually growing and reproducing. If they were allowed to an endless growth, they would reach such concentration in the aeration tank that it would reduce oxygen transfer to all the cells. Besides, decanter would be beyond its capacity of handling solids, hampering the settling and part of the solids could leave with the treated  effluent, deteriorating the quality of the final effluent. In order to keep the optimum balance of the system, the removal of the same amount of generated biomass is required. This is commonly known as “sludge excess”, and can be removed directly from the reactor or from sludge recirculation line.

Changes to an activated sludge are made for the biological removal of nutrients such as nitrogen and phosphorus.