What is the primary purpose of a primary settling tank in wastewater treatment?

The primary purpose of a primary settling tank, also known as a primary clarifier, is to remove heavy solids from wastewater. During the initial stages of wastewater treatment, wastewater enters the primary settling tank where it is held still, allowing for the separation of solids and liquids based on their densities. The heavier solids settle to the bottom, forming a sludge layer, while the lighter materials, like oils and greases, may float to the surface, forming a scum layer. The clear liquid, or effluent, then flows to the next stage of treatment. This process is crucial in reducing the organic load and suspended solids that will later undergo further treatment, such as biological treatment processes. The effectiveness of this initial separation helps improve the overall efficiency of the treatment system by preventing excessive loading in subsequent treatment units.

The other options refer to different stages or processes within wastewater treatment. Aeration, for example, is related to biological treatment processes where oxygen is supplied to microorganisms to break down organic materials. Disinfection is a later stage where pathogens are eliminated, and nutrient removal typically refers to specific treatment processes designed to remove nitrogen and phosphorus from the effluent, often after primary settling has taken place.