Operator terms · Wastewater
What is infiltration and inflow (I&I)?
Infiltration and inflow, or I&I, is clean groundwater and storm water that leaks into a sewer system where it does not belong. Infiltration seeps in through cracks and bad joints; inflow pours in through improper connections. Both overload the collection system and treatment plant.
Last reviewed 2026-08-03
The difference between the two
Infiltration is groundwater seeping into pipes through cracks, bad joints, and worn seals, often steady after wet weather. Inflow is storm water entering directly through wrong connections such as roof drains or leaky manhole covers, spiking fast during rain.
Why it is a problem
Extra clean water takes up pipe and plant capacity meant for wastewater. Heavy I&I can cause backups and overflows and forces the plant to treat water that never needed treating, raising cost and risk.
Common questions
- What is the difference between infiltration and inflow?
- Infiltration seeps in through cracks; inflow enters directly through wrong connections.
- Why is I&I bad?
- It uses up capacity and can cause backups and overflows during wet weather.
- When is I&I worst?
- During and after rain, when groundwater is high and storm water is running.