Inflow refers to the entrance of non-sewage water — such as storm runoff or snowmelt — entering a sewer system. Inflow generally occurs during times of precipitation, often through means other than storm drains. Its entry into the sewer system carries biota and chemicals scoured from roads, yards, cellars and roof gutters.
Trenchlesspedia Explains Inflow
Inflow increases the cost of sewage treatment and the overall operation costs of treatment facilities. The overburden caused by the introduction of excessive water resulting from inflow can cause a sewer system to exceed its design capacity. This means the treatment facility is incapable of handling the amount of sewage and gray water presented for treatment. System failure can cause a sewers to back up, resulting in public health concerns and financial losses for municipalities.