Infiltration and inflow (I&I) are terms used in wastewater and stormwater management to describe the ways in which groundwater and stormwater enter sewer systems, respectively.

• Infiltration refers to the entry of groundwater into sanitary sewers through cracks, joints, or other openings in the sewer pipes. This can occur when the water table is higher than the sewer pipe, or when there are structural issues in the pipes. Infiltration can overload the sewer system, leading to increased flow and potential sewage backups. It can also contribute to the wear and deterioration of sewer infrastructure.

• Inflow, on the other hand, is the direct entry of stormwater into the sewer system. This can happen through various means, such as roof drains, yard drains, or cross-connections between storm sewers and sanitary sewers. Inflow can occur during heavy rainfall or snowmelt when stormwater runoff finds its way into the sewer system. Like infiltration, excessive inflow can overwhelm the sewer system, leading to overflows and basement flooding.

Both infiltration and inflow contribute to increased flow in sewage treatment plants, which can strain the system's capacity and result in the discharge of untreated or partially treated sewage into water bodies, posing environmental and public health risks.

Municipalities and wastewater utilities employ various strategies to identify and mitigate infiltration and inflow, such as regular inspections, repairs and rehabilitation of sewer infrastructure at the Point of Sale, and the implementation of stormwater management practices to reduce inflow during heavy rainfall events. These efforts help in maintaining the integrity of sewer systems and ensuring efficient wastewater treatment and drainage processes.