What is inflow and infiltration (I&I)?
Inflow and infiltration: I&I: clean water entering sanitary sewers
In short
Inflow and infiltration (I&I, also written I/I) is clean water that enters a sanitary sewer and takes up capacity meant for sewage. Infiltration is groundwater seeping in through cracked pipes, leaking joints and manhole walls. Inflow is stormwater entering directly through roof drains, sump pumps, cross-connections and manhole covers. Both raise treatment costs and cause overflows in wet weather.
Reducing it is the purpose of most sewer rehabilitation: Klarifi tracks 2,270 sewer rehabilitation and I&I projects in published US city capital improvement plans.
Why I&I matters
Every gallon of rain or groundwater in a sewer is pumped at the lift stations and treated at the plant as if it were sewage. In a leaky system, peak wet-weather flow can be several times the dry-weather flow. The consequences are sanitary sewer overflows and basement backups, biological treatment washed out by storm flows, permit violations, and capacity built for water that never needed treatment. Regulators answer chronic overflows with consent decrees that commit a utility to a multi-year I&I programme.
How utilities find it
Flow monitoring compares dry-weather and wet-weather flow in each sewer basin, to rank the basins. Inside the worst ones, smoke testing reveals roof drains, area drains and broken laterals, dye testing confirms cross-connections, CCTV inspection grades the condition of each pipe, and manhole inspections find leaking walls and low-lying covers. A large share of I&I typically comes from private laterals, which makes it a legal as well as a technical problem.
How it is fixed
Cured-in-place pipe (CIPP) lining renews a sewer without excavation and is the dominant method; pipe bursting and open-cut replacement are used where a pipe has collapsed or is too small. Manholes are lined, grouted or fitted with inserts and sealed covers. Disconnecting roof drains and sump pumps, and lining or replacing private laterals, takes ordinances and often cost-sharing programmes. Where removing the water is not cost-effective, utilities add equalization storage or wet-weather treatment capacity instead.