Activated sludge is a process in which organic matter of wastewater is digested/ oxidized using micro-organisms, enzymes in presence of oxygen or in absence of oxygen. The activated sludge or biological flocs mix with the waste stream, oxidize the organic substances in the wastewater in the presence of oxygen for bio-oxidation and nitrification reactions, or in the absence of oxygen for denitrification reaction. In this post we will discuss the details of ASP system.

Activated sludge process is secondary treatment of wastewater for the use of biological oxidation to remove organic content. Activated sludge consists of biological flocs that are matrices of micro-organisms, nonliving organic matter, and inorganic materials. Microorganisms are either bacteria, fungi, protozoa. An activated sludge process can be defined as a system in which biological flocs are continuously circulated to come into contact and to oxidize the organic substances in the presence of oxygen.

The activated sludge is used to remove maximum organic substance with shortest time, and to produce conditioned biological flocs having good settling, which is essential for secondary effluent quality.

Typical Activated sludge process flow sheet is shown in drawing,

ASP Schematic

In aeriation tank biodegradable complex organic matter is converted in to simple inorganic molecule. In the biological oxidation of wastewater, both synthesis and oxidation occur. Many groups of activated sludge microorganisms take part in carrying out the process. The important group of microorganism is as follows.

  1. Bacteria: Bacillus subtilis, Alcaligenes, Chromobacterium, Pseudomonas, Achromobacterium, Spirillum, Sphuerotilus natans, etc.
  2. Fungi
  3. Protozoa
  4. Metazoa

If activated sludge systemis operated properly, biological flocs are produced that incorporate most of the important group of microorganisms.

Process Control

The Activated sludge process health is depend on sludge blanket level, sludge volume index (SVI), mean cell residence time (MCRT), food to microorganism ratio (F/M), dissolved oxygen (DO), BOD and COD.

  • Sludge blanket is measured from the bottom of the clarifier to the level of settled solids in the clarifier’s water column.
  • SVI is the volume of settled sludge in milliliters occupied by 1 gram of dry sludge solids after 30 minutes of settling in a 1000 milliliter graduated cylinder.
  • MCRT is the total mass of mixed liquor suspended solids in the aerator and clarifier divided by the mass flow rate of mixed liquor suspended solids leaving as WAS and final effluent.
  • F/M is the ratio of food fed to the microorganisms each day to the mass of microorganisms held under aeration.

In Activated sludge system, aeration process plays a vital role to maintain dissolved oxygen (DO) level. There are three different types of aeration process

  1. Diffused aeration
  2. Surface aeration
  3. Pure oxygen aeration

All the above method is used to maintain minimum dissolved oxygen (DO) level of 2 mg/L.

By Krunal

Krunal Bhosale is crazy about new gadgets and try them as soon as they are available in market. You can trust him because he uses those products and write reviews about products. He is a Water and Wastewater treatment expert from Pune, India. He received his Chemical Engineering from University of Pune. You can contact him by email krunal (at)

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