Flatter sewer line grades may cause added sewer maintenance expense and odor nuisance. The problem is most likely caused by

• A. A decrease in velocity which increases the treatment time in the lines.
• B. An increase in velocity which decreases the treatment time in the lines.
• C. A decrease in velocity allowing gases to be released from the wastewater.
• D. A decrease in velocity allowing organic and inorganic solids to settle out.

Answer: (C)

When a sewer line has a flatter grade, the flow slows down. That reduced velocity allows gases dissolved in the wastewater to come out of solution and escape into the atmosphere. In sewer systems, this often means hydrogen sulfide and other odorous gases are released, creating the noticeable odor nuisance and can lead to more frequent maintenance as the gas and related deposits form. While slower flow can also let solids settle, the odor issue shown here is best explained by gas release due to decreased velocity.