What is the Difference Between Activated Sludge and Primary Sludge

The key difference between activated sludge and primary sludge is that activated sludge contains a high content of microorganisms, whereas primary sludge does not contain microorganisms.

Sludge and slurry can be defined as outcomes of industrial processes. They are different in their texture due to their components. However, sludge and slurries can be made from similar processes. The term activated sludge refers to the process of having a high concentration of microorganisms, including bacteria, protozoa, and fungi, that occurs as a loose, clumped mass of fine particles. Primary sludge is a type of sludge generated from chemical precipitation, sedimentation, and other primary processes.

1. Overview and Key Difference
2. What is Activated Sludge
3. What is Primary Sludge
4. Activated Sludge vs Primary Sludge in Tabular Form
5. Summary – Activated Sludge vs Primary Sludge

What is Activated Sludge?

The term activated sludge refers to a process having a high concentration of microorganisms, including bacteria, protozoa, and fungi, as a loose clumped mass of fine particles. This sludge is kept in suspension through stirring, which is helpful in removing organic matter from wastewater.

The activated sludge process can be described as a type of wastewater treatment process used for the treatment of sewage or industrial wastewaters. This process involves aeration and a biological floc consisting of bacteria and protozoa.

Activated Sludge vs Primary Sludge in Tabular Form

Figure 01: An Activated Sludge Tank

The components in an activated sludge include an aeration tank and a settling tank. The aeration tank is where the air or oxygen is injected into the mixed liquor. The settling tank is where the biological flocs settle; therefore, separation of the biological sludge from the clear treated water occurs at this tank. The settling tank is also known as the final clarifier or the secondary settling tank. The biological flocs that form in this tank are known as the sludge blanket.

Typically, in a sewage treatment plant, the activated sludge is the biological process in which one or several of the following steps take place:

Oxidizing carbonaceous biological matter
Oxidizing nitrogenous matter such as ammonium and nitrogen in biological matter
Removing nutrients

What is Primary Sludge?

Primary sludge is a type of sludge generated from chemical precipitation, sedimentation, and other primary processes. It mainly consists of organic matter such as feces, vegetables, fruits, textiles, paper, etc. It is a consistent sludge that occurs as a thick fluid with a water level of around 93% and 97%.

Activated Sludge and Primary Sludge - Side by Side Comparison

Figure 02: Primary Sludge Drying Bed

Unlike activated sludge, primary sludge does not require aeration, and a lot of decomposition occurs during the formation of primary sludge. Primary sludge forms during the primary sewage treatment, and it does not possess flocs of decomposer microbes. It occurs as a concentrated suspension of particles in water. The primary goal of this step is the separation of readily-removable suspended solids and biological matter.

What is the Difference Between Activated Sludge and Primary Sludge?

Sludge and slurry are outcomes of industrial processes. They are different in their texture due to their components. The key difference between activated sludge and primary sludge is that activated sludge contains a high content of microorganisms, whereas primary sludge does not contain microorganisms.

The below infographic presents the differences between activated sludge and primary sludge in tabular form for side by side comparison.

Summary – Activated Sludge vs Primary Sludge

The term activated sludge refers to a process having a high concentration of microorganisms, including bacteria, protozoa, and fungi, which are present as a loose, clumped mass of fine particles. Primary sludge is a type of sludge generated from chemical precipitation, sedimentation, and other primary processes. The key difference between activated sludge and primary sludge is that activated sludge contains a high content of microorganisms, whereas primary sludge does not contain microorganisms.