Inflow and Infiltration Control
Inflow and infiltration (I/I) are extraneous water that enters the collection system. Inflow is direct, rapid entry such as stormwater through manhole covers, roof drains, and illegal connections. Infiltration is slower, indirect entry through defective pipes, joints, and manholes, often groundwater-driven. Distinguishing the two matters because they require different remedies.
Key concepts: flow monitoring and rainfall correlation identify I/I sources. Sanitary sewer overflows often follow wet weather when capacity is exceeded. Smoke testing finds inflow connections; dye testing confirms specific sources; CCTV finds infiltration defects. Rehabilitation methods include lining, grouting, sealing, and manhole rehabilitation.
Traps
A common trap is treating all wet-weather flow as infiltration. Rapid peaks during and shortly after rain point to inflow; sustained elevated base flow in wet seasons points to infiltration. Another trap is assuming a single fix solves I/I: source removal, rehabilitation, and capacity improvements are complementary.
Rule of thumb
Compare dry-weather and wet-weather flow. A large, sharp rainfall response signals inflow; a slow seasonal rise signals infiltration. Target the dominant source first.
Worked example
A basin shows flow spiking within an hour of rainfall and falling quickly. That pattern indicates inflow. Smoke testing reveals roof drain and sump connections; removing them reduces the peak. A separate basin shows elevated flow for weeks after rain, indicating infiltration, so CCTV and lining target pipe defects.