What describes sewage that is dark in color and lacking in oxygen?

The condition described—sewage that is dark in color and lacking in oxygen—best fits the term "septic." Septic sewage typically indicates a state of decomposition where organic matter is breaking down anaerobically, which means without oxygen. This process often results in the production of darker, more turbid wastewater and the characteristic odors associated with a lack of aeration.

In septic conditions, the breakdown of materials can lead to the presence of hydrogen sulfide and other gases that contribute to the foul smell often associated with such sewage. This contrasts with fresh sewage, which would typically appear lighter in color and contain higher levels of dissolved oxygen, indicating that it hasn’t yet undergone significant decomposition. Aerated sewage would have undergone treatment processes to increase oxygen levels, improving quality and reducing odors. Thus, the identifying features of dark color and low oxygen levels clearly point to septic sewage as the correct description.