What are the three main stages of wastewater treatment?

The three main stages of wastewater treatment are correctly identified in one of the options as preliminary, primary, and secondary treatment.

In the context of wastewater treatment, preliminary treatment involves the initial steps to remove larger debris and particles from the water, often through screening and grit removal processes. This stage is crucial for protecting downstream treatment equipment from damage and ensuring that the wastewater is prepared for more refined processing.

The primary treatment is the next stage, where the focus is on removing settleable organic and inorganic solids from the wastewater. This is typically done through sedimentation, allowing heavier materials to settle at the bottom of a tank. This stage is vital for reducing the overall load of pollutants that will enter the next treatment phase.

Following this is the secondary treatment, which is designed to biologically degrade the organic matter that remains in the effluent after primary treatment. This process often uses microorganisms to consume the organic pollutants, leading to a significant reduction in biochemical oxygen demand (BOD) and other harmful substances.

The other options present variations that either combine stages incorrectly or substitute terms that do not fit the standard classification used in wastewater treatment processes. Knowing these stages is essential for understanding how wastewater is processed and treated to protect public health and the environment.