Grit Chamber:
A grit chamber is a primary treatment unit used in wastewater treatment plants to remove heavy inorganic solids such as sand, gravel, and grit from the wastewater stream. It is typically the first unit process in the treatment train and plays a crucial role in protecting downstream equipment and processes from abrasive wear and damage.

Sedimentation Tank:
A sedimentation tank, also known as a clarifier or settling tank, is a primary or secondary treatment unit used in wastewater treatment plants to remove suspended solids from the wastewater stream. The tank allows the solids to settle to the bottom, forming a sludge layer, while the clarified water overflows from the top. The detention period in a sedimentation tank is longer compared to a grit chamber as it is designed to promote the settling of finer particles.

Oxidation Ditch:
An oxidation ditch is an extended aeration activated sludge process used for secondary treatment of wastewater. It consists of a long, narrow channel or ditch where wastewater and activated sludge are mixed and aerated to promote the biological oxidation of organic matter. The detention period in an oxidation ditch is typically longer than that of a grit chamber but shorter than a sedimentation tank.

Oxidation Pond:
An oxidation pond, also known as a stabilization pond or lagoon, is a natural or man-made wastewater treatment system that uses sunlight, algae, and bacteria to promote the oxidation and stabilization of organic matter. It consists of a shallow, open-air pond where wastewater undergoes biological and physical processes. The detention period in an oxidation pond can vary depending on the design and hydraulic loading rate.

Minimum Detention Period:
The minimum detention period refers to the shortest time required for the wastewater to stay in a particular treatment unit for effective treatment. In this case, the grit chamber has the minimum detention period among the given options.

The grit chamber primarily aims to remove heavy inorganic solids, which settle quickly, from the wastewater stream. It does not involve significant biological or chemical processes. Therefore, the detention period in a grit chamber is relatively short, typically around 30 seconds to 2 minutes. This short detention period allows for the separation of grit and other heavy materials without promoting significant settling of organic matter.

In contrast, sedimentation tanks, oxidation ditches, and oxidation ponds involve more complex treatment processes that require longer detention periods. Sedimentation tanks allow for the settling of suspended solids, including finer particles, and require detention periods ranging from several hours to a few days. Oxidation ditches and oxidation ponds rely on biological processes to degrade organic matter and typically have detention periods ranging from several days to weeks.

Overall, the minimum detention period is observed in a grit chamber due to its specific purpose of separating heavy inorganic solids from wastewater without significant settling of organic matter.