When sewer line grades are too flat, the odor problem is most likely caused by decreased velocity, which result is produced?

When sewer line grades are too flat, the odor problem is most likely caused by decreased velocity, which result is produced?

• A. Decreases the treatment time in the lines
• B. Allows decomposable solids to settle
• C. Increases the treatment time of inorganic solids
• D. None of the above

Answer: (B)

Low line grades reduce flow velocity. When wastewater moves slowly, heavier solids begin to settle to the bottom of the pipe. This accumulation creates anaerobic conditions where organic matter is decomposed by bacteria without oxygen, producing odorous gases such as hydrogen sulfide. So the odor problem stems from the settling and subsequent anaerobic decomposition of decomposable solids in the pipe. The other options don’t fit because odor in sewers isn’t caused by shorter treatment time in the lines, and increasing treatment time of inorganic solids isn’t the mechanism behind odors in this context.

Low line grades reduce flow velocity. When wastewater moves slowly, heavier solids begin to settle to the bottom of the pipe. This accumulation creates anaerobic conditions where organic matter is decomposed by bacteria without oxygen, producing odorous gases such as hydrogen sulfide. So the odor problem stems from the settling and subsequent anaerobic decomposition of decomposable solids in the pipe. The other options don’t fit because odor in sewers isn’t caused by shorter treatment time in the lines, and increasing treatment time of inorganic solids isn’t the mechanism behind odors in this context.