When groundwater or stormwater enters dedicated sanitary sewer systems, inflow & infiltration is occurring. Most inflow & infiltration, or I&I, is caused by aging infrastructure that includes manholes and sewer pipes. While this may seem like a hopeless, inevitable problem, there are steps that communities can take to protect their wastewater systems.
Infiltration is when clean groundwater enters the sewer collection system. A great example of this is when heavy rain or snowmelt penetrates the ground and raises the groundwater level. That ground flow can infiltrate sewer pipes and manholes through cracks in the structure, missing or deteriorated gasket joints and leaky pipe penetrations. High water tables, or groundwater, is particularly common in coastal areas but also occurs in communities that are near lakes, rivers, ditches or wetlands.
While seeping groundwater infiltration may not sound like an immediate concern, it can become a costly long-term problem. For example, a single leaking manhole can allow thousands of gallons of clean water into the wastewater system daily. That clean groundwater must then be treated as dirty wastewater at the treatment plant. The longer infiltration issues are left unaddressed, the more costly it becomes to operate the wastewater treatment facility. (Read more about the efficiency of wastewater treatment facilities.)
Inflow is a more rapid flow of stormwater that enters the sewer collection system through defective manhole covers or frame seals, improperly connected roof or foundation drains, or combined sewer systems. Inflow is often the result of significant rainfall, snowfall, springs or snowmelt that contribute to excessive sewer flows.
In an effective water and wastewater system, this large volume of stormwater will either drain into the ground or be routed to the nearest storm drains. Unfortunately, aging infrastructure often suffers from leaking manholes or connections that allow stormwater to drain into the wastewater system. Unlike slower infiltration, peak inflow during heavy storms can cause wastewater systems to backup and flood basements, culverts and creeks.