Introduction:
In the field of wastewater treatment, sludge refers to the solid residue that is generated during the treatment process. Sludge is a mixture of water and solids, and it needs to be treated before disposal or further use. There are different types of sludge depending on its origin and treatment process. In this case, we need to determine the type of sludge based on the information provided.

Explanation:
To determine the type of sludge, we need to understand the various types of sludge and how they are generated.

1. Primary Sludge:
- Primary sludge is the solid residue that is obtained from the primary clarifiers or sedimentation tanks.
- It consists of heavy organic solids, such as fecal matter, grit, and other suspended solids.
- Primary sludge has a high concentration of solids, typically around 5-7%, and is usually treated separately from other types of sludge.

2. Secondary Sludge:
- Secondary sludge is generated during the biological treatment process in wastewater treatment plants.
- It mainly consists of microbial biomass, which includes bacteria, fungi, and other microorganisms.
- Secondary sludge is obtained from secondary clarifiers or sedimentation tanks after the activated sludge process.
- It has a lower solids concentration compared to primary sludge, typically around 0.5-2%.

3. Tertiary Sludge:
- Tertiary sludge refers to the sludge generated during the tertiary treatment process.
- Tertiary treatment is an advanced treatment process that is applied to the effluent from secondary treatment to further remove pollutants.
- Tertiary sludge mainly consists of chemical precipitates, such as metal hydroxides or carbonates, and other fine solids.
- The solids concentration of tertiary sludge can vary depending on the specific treatment process.

4. Activated Sludge:
- Activated sludge is a type of secondary sludge that results from the activated sludge process.
- The activated sludge process involves the biological treatment of wastewater using microorganisms in aeration tanks.
- The microorganisms consume organic matter in the wastewater, and the resulting sludge is separated in the secondary clarifiers.
- Activated sludge typically has a solids concentration of around 0.5-2%.

Conclusion:
Based on the given information that 7% of solids concentration of thickened sludge is obtained from gravity thickener, the type of sludge is activated sludge. This is because activated sludge is a type of secondary sludge that is generated during the activated sludge process, and it has a solids concentration similar to the value provided.