What does secondary treatment in a POTW convert?

Secondary treatment in a publicly owned treatment works (POTW) primarily focuses on the biological processes used to treat wastewater. The correct answer highlights that this treatment method converts oxygen-demanding compounds, such as organic matter and pathogens, into more stable forms. This process typically involves microorganisms that break down the organic material, resulting in a significant reduction in biochemical oxygen demand (BOD), which measures the amount of oxygen that microorganisms will consume while decomposing organic matter.

Through secondary treatment, the biodegradable substances are transformed into inert and more stable components, making them less harmful to the environment. This stability is crucial for the efficient functioning of subsequent treatment processes and for the overall protection of water bodies into which treated effluent will be discharged. The conversion of oxygen-demanding compounds ensures that the treated water has a lower impact on aquatic life when released.

The other options provided do not accurately describe the primary function of secondary treatment in a POTW. Liquid waste is not converted into solid waste in this treatment stage; instead, it focuses on the removal of organic materials. While some toxic materials may be mitigated through different stages of treatment, secondary treatment is not specifically designed to turn these harmful substances into reusable resources. The treatment process also does not directly convert excessive nutrients into harmless