How is wastewater containing no dissolved oxygen characterized?

Wastewater containing no dissolved oxygen is typically characterized by a dark gray to black color and a strong disagreeable odor. When oxygen is absent in water, anaerobic bacteria start to break down organic matter. This process leads to the production of sulfide and other gases that contribute to the characteristic dark coloration and foul smell associated with stagnant or heavily polluted water.

In contrast, other choices do not accurately describe the conditions present in wastewater devoid of oxygen. Clear and odorless water suggests high levels of oxygen and minimal contaminants, which is the opposite of what would be found in oxygen-depleted wastewater. Bright blue color and a sweet smell might indicate the presence of certain chemicals or non-polluted water, again inconsistent with the characteristics of oxygen-less wastewater. Lastly, brown with an earthy odor could relate to organic matter, but it does not capture the specific conditions caused by an absence of dissolved oxygen, which typically yields darker colors and more offensive odors.