When sewer line grades are too flat, an odor problem is likely to result. The most likely cause of the odor problem is deceased velocity, which

When sewer line grades are too flat, an odor problem is likely to result. The most likely cause of the odor problem is deceased velocity, which

• A. allows decomposable solids to settle
• B. decreases the treatment time in the lines
• C. increases the treatment time of inorganic solids
• D. none of the above

Answer: (A)

Low flow velocity in a sewer line with too-flat a grade means there isn’t enough turbulence to keep solids suspended. Decomposable organic solids then settle to the bottom, forming layers where oxygen is scarce. In these settled zones, anaerobic bacteria break down the organics, producing foul gases such as hydrogen sulfide that cause odors. This is why the idea that it allows decomposable solids to settle is the best explanation: the odor stems from the buildup and anaerobic digestion of settled organic matter. The other options aren’t the primary cause—odor isn’t mainly about shortening or lengthening treatment time in the lines, and inorganic solids don’t drive this odor mechanism in the same way.

Low flow velocity in a sewer line with too-flat a grade means there isn’t enough turbulence to keep solids suspended. Decomposable organic solids then settle to the bottom, forming layers where oxygen is scarce. In these settled zones, anaerobic bacteria break down the organics, producing foul gases such as hydrogen sulfide that cause odors. This is why the idea that it allows decomposable solids to settle is the best explanation: the odor stems from the buildup and anaerobic digestion of settled organic matter. The other options aren’t the primary cause—odor isn’t mainly about shortening or lengthening treatment time in the lines, and inorganic solids don’t drive this odor mechanism in the same way.