Rainwater does not belong in the sanitary sewer system, but it can still find its way in. Sewer professionals usually describe that extra water in two categories: inflow and infiltration.
Inflow is water entering directly
Inflow happens when rainwater or surface water has a direct path into the sanitary sewer. Examples can include improperly connected downspouts, sump pumps, yard drains, foundation drains, or other drainage systems that discharge into a sanitary line.
Because inflow can add a large amount of water very quickly during a storm, it can cause sudden spikes in flow.
Infiltration comes through defects
Infiltration is groundwater that seeps into sewer pipes through cracks, deteriorated joints, damaged manholes, or other openings. Instead of entering through a direct connection, the water works its way into the system from the surrounding soil.
Infiltration may increase gradually as the ground becomes saturated after extended rain or snowmelt.
Why both matter
The sanitary sewer system is designed to carry wastewater from homes and businesses. When rainwater or groundwater enters that system, the additional volume still has to be transported and treated. That can increase operating costs and reduce available capacity during wet weather.
How problems are found
Crews may use sewer cameras, manhole inspections, flow monitoring, smoke testing, dye testing, and other methods to locate sources of unwanted water. Sometimes the problem is in a public sewer main. Other times it is an improper private connection or a defect in a building sewer.
Homeowners can help
Make sure sump pumps and downspouts discharge to an approved location rather than into the sanitary sewer. Keep cleanouts capped and in good condition. If you suspect a broken or leaking sewer line, have it inspected before the problem grows.
Keeping stormwater and sanitary wastewater separate helps the entire system work more efficiently. For a broader overview, see Understanding the Difference Between Sanitary Sewer and Stormwater Systems.
