Which of the following terms refers to the unwanted substances found in sewage?

The term that best describes the unwanted substances found in sewage is "contaminants." Contaminants refer to any harmful or undesirable substances that are present in a particular environment, in this case, sewage. These can include bacteria, chemicals, heavy metals, and various organic matter that degrade water quality and pose risks to health and the environment.

While "pollutants" can also refer to harmful substances in the environment, it is a broader term often used for substances that cause pollution in various contexts, not limited specifically to sewage. "Effluents" specifically denote the outflow from a sewage treatment facility or industrial discharge, which may contain contaminants but does not itself refer to the unwanted substances alone. "Composts" are organic materials that have decomposed, commonly used in gardening and agriculture, and therefore do not relate to unwanted substances in sewage.

Understanding these definitions clarifies why "contaminants" is the most accurate choice when identifying unwanted substances in sewage.