What is inflow?

• A. Additional water, such as stormwater, that enters the sewer system, often unintentionally, contributing to overflows.
• B. Groundwater entering sewer lines.
• C. Water that remains within the sewer system during dry periods.
• D. Wastewater from households during base flow

Answer: (A)

Inflow is water that enters the sewer system from outside sources, most commonly stormwater runoff that finds its way into the sewers, often during rain. This extra water isn’t part of the usual wastewater from households or businesses, but it adds to the flow in the pipes, which can overwhelm the system and cause overflows or backups, especially in combined sewer setups.

This differs from infiltration, which is groundwater seeping into the pipes through cracks or joints and tends to add to flow regardless of rainfall. Water that stays in the sewer system during dry periods describes base flow or dry-weather flow, which is typically the normal wastewater component, not inflow. Wastewater from households during base flow is just the regular sanitary sewage, not inflow.