What is the most likely cause of odor problems when sewer line grades are too flat?

The correct response highlights that when sewer line grades are too flat, it creates conditions that allow decomposable solids to settle within the pipeline. This settling can lead to the accumulation of organic matter, which, as it decomposes, generates foul odors. Typically, sewer lines are designed with a specific slope to ensure that wastewater and solids flow smoothly toward the treatment facilities. When the grade is insufficiently steep, it impedes the flow, allows solids to settle, and can result in stagnant areas where anaerobic conditions prevail. In these stagnant zones, bacteria break down the organic material, leading to the production of malodorous gases such as hydrogen sulfide.

In contrast, the other options do not adequately address the issue of odors in the context of flat sewer line grades. For instance, decreasing or increasing treatment times pertains more to the efficiency of the treatment processes in a sewer system rather than the physical conditions within the lines causing odors. The treatment of inorganic solids is typically not a direct concern related to odor problems that arise specifically from settled decomposable solids. Similarly, allowing decomposable solids to be suspended would likely mitigate odor issues, as it would prevent the buildup of solids that could produce gases when decomposing. Thus, the key issue presented in the question is