Which types of industrial discharges may contribute to the plugging of sewers?

The correct choice identifies fibrous materials and heavy solids as a significant contributor to sewer plugging. When these types of discharges enter the sewer system, they can accumulate and combine with other materials, leading to blockages. Fibrous materials, such as those found in certain waste products, can entangle with debris, while heavy solids can settle in the sewage pipes, restricting flow and creating clogs.

In contrast, liquid fuels tend to remain in a fluid state and can flow through the sewer system without causing obstruction. Semi-fluid substances might behave similarly, flowing through pipes but having less impact on clogging compared to heavier or fibrous materials. Evaporative residues typically consist of gases or light solids that do not contribute to physical blockages in the way that more substantial materials do. Thus, recognizing the physical characteristics of various discharges is essential for managing and preventing sewer system issues effectively.