Inflow and infiltration (or "I and I") are two leading causes of environmental risk stemming from urban wastewater networks.
Inflow refers to both stormwater entering the wastewater network and wastewater entering the stormwater network. Inflow occurs mainly through illegal cross-connections between the two pipe networks.
In the case of the wastewater network, excess stormwater overloads the system and can result in wastewater that has not been treated fully entering the environment.
Wastewater cross-connections into the stormwater network result in the discharge of untreated waste into streams and harbours at stormwater discharge points.
In urban areas, this can pose a health risk, as these discharge points can be near coastal recreational or food gathering areas.
Infiltration describes the entry of ground-water, including sea-water, into the networks, mainly through faults such as cracked and broken pipes. Again this contributes to overloading pipe capacities and improper discharges.