ANS:C - 75 to 80%

Assuming a percentage of sewage discharge between 75% to 80% for the design of sewers implies that engineers are accounting for a higher proportion of the total water flow to be sewage. This assumption takes into consideration factors such as residential, commercial, and industrial water usage patterns, as well as infiltration and inflow into the sewer system. Here's an explanation:

• Residential Usage: In urban areas, residential properties typically contribute a significant portion of sewage discharge. This includes wastewater from toilets, sinks, showers, laundry, and other domestic activities.
• Commercial and Industrial Usage: Commercial and industrial establishments also generate sewage, often in larger quantities compared to residential properties. This includes wastewater from businesses, factories, restaurants, and other facilities.
• Infiltration and Inflow: Sewer systems can experience infiltration (seepage of groundwater into pipes through cracks or joints) and inflow (direct entry of stormwater into sewer systems from sources such as roof drains, yard drains, and manhole covers). Infiltration and inflow can significantly increase the volume of wastewater flowing through the sewer system.

By assuming a higher percentage of sewage discharge (between 75% to 80%), engineers ensure that the sewer system is adequately designed to accommodate the expected volume of wastewater while maintaining proper flow velocities, preventing backups, and meeting treatment capacity requirements. This assumption allows for a conservative approach in sewer system design, ensuring that the infrastructure can effectively manage the anticipated sewage load and any potential variations or surges in flow. It helps in minimizing the risk of overloading the system and preventing issues such as sewer backups or overflows.