For design purposes, the normal rate of infiltration of ground water into the sewer, is

ANS:D - 2000 litres/km/cm

A rate of 2000 litres/km/cm for infiltration of groundwater into the sewer signifies a higher level of groundwater entering the sewer system per kilometer length of the sewer per centimeter of diameter over a 24-hour period. Here's an explanation:

1. Infiltration Rate: The infiltration rate of 2000 litres/km/cm indicates that for every kilometer length of the sewer and for each centimeter of diameter, approximately 2000 liters of groundwater can seep into the sewer system within a 24-hour period.
2. Implications: A higher infiltration rate suggests that the sewer system is more susceptible to groundwater intrusion. Excessive infiltration can lead to hydraulic overloading of the sewer network, which can strain treatment facilities and increase operational costs. It may also contribute to structural issues such as corrosion, erosion, and sewer overflow events.
3. Design Considerations: When designing sewer systems, engineers must account for the expected rate of groundwater infiltration to ensure the system can handle the anticipated flow rates without compromising its performance or integrity. Higher infiltration rates may require additional measures such as improved pipe materials, better sealing techniques, or increased capacity to manage the excess groundwater.
4. Mitigation Strategies: To mitigate the impacts of excessive infiltration, sewer systems may incorporate various strategies such as rehabilitation of aging infrastructure, installation of liners or seals to prevent groundwater intrusion, and regular maintenance to identify and address infiltration points.