In what type of treatment is activated sludge used?

Activated sludge is primarily used in the secondary treatment phase of wastewater treatment. This method involves the use of aerated biological processes to break down organic matter in the wastewater. In secondary treatment, microorganisms, primarily bacteria, are introduced into the mixed liquor of wastewater to consume and metabolize the organic pollutants. The activated sludge process relies on this rich microbial population, which is cultivated and aerated to enhance the breakdown of waste.

During this stage, the process typically consists of two main components: aeration tanks, where the wastewater is mixed with the activated sludge under controlled conditions, and secondary clarifiers, where the biomass is settled out. The efficiency of activated sludge processes is crucial for meeting regulatory standards for wastewater discharge, as they significantly reduce the biochemical oxygen demand (BOD) and help in nitrification and denitrification, important for achieving nutrient removal.

Primary treatment, by contrast, focuses on the physical separation of solids from wastewater, while tertiary treatment is aimed at further polishing the effluent to remove remaining contaminants not addressed in previous stages. Preliminary treatment is geared towards the initial removal of large debris and grit. Therefore, secondary treatment is the correct context for the use of activated sludge, as this stage leverages microbial action to treat wastewater effectively.