Which statement best describes the effect of flat sewer grades on solids in the lines?

Which statement best describes the effect of flat sewer grades on solids in the lines?

• A. Decomposable solids are allowed to settle
• B. Treatment time in lines increases
• C. Inorganic solids settle faster
• D. No effect on solids

Answer: (A)

Flat sewer grades slow down the flow in gravity sewers, so suspended solids have more time to settle to the pipe bottom. Most wastewater contains organic, biodegradable material, so these decomposable solids are the ones that tend to settle out when velocity is low. As they accumulate, they can form deposits, potentially causing blockages and odors in the sewer line. That’s why the best statement is that decomposable solids are allowed to settle. The idea isn’t that treatment time in the lines automatically increases, or that inorganic solids necessarily settle faster due to flat grading, and there is indeed an effect—solids deposition—that can lead to problems.

Flat sewer grades slow down the flow in gravity sewers, so suspended solids have more time to settle to the pipe bottom. Most wastewater contains organic, biodegradable material, so these decomposable solids are the ones that tend to settle out when velocity is low. As they accumulate, they can form deposits, potentially causing blockages and odors in the sewer line. That’s why the best statement is that decomposable solids are allowed to settle. The idea isn’t that treatment time in the lines automatically increases, or that inorganic solids necessarily settle faster due to flat grading, and there is indeed an effect—solids deposition—that can lead to problems.