What issue is most likely caused by flatter sewer line grades, leading to added maintenance costs and odor nuisances?

Flatter sewer line grades often lead to a decrease in the velocity of wastewater flow. When the velocity is reduced, there is insufficient force to keep organic and inorganic solids suspended and moving through the sewer system. This settling of solids can result in blockages and backups, leading to increased maintenance costs as these issues must be addressed. Furthermore, when solids accumulate, they can create odor nuisances as organic materials decompose. Therefore, the association between flatter grades and the settling of solids is crucial in understanding the implications for sewer system maintenance and the potential for unpleasant odors in nearby areas.