8.1 What is infiltration, and what is its purpose?

Infiltration is the process of water, wastewater or treated effluent penetrating from the ground surface into the soil. It involves the controlled and gradual percolation, both horizontally and vertically, of treated effluent into the subsurface soil.

Infiltration is to be considered when the conditions of safe reuse are not in place. It allows a final treatment step for effluent following primary and secondary (and tertiary) treatment. As treated effluent percolates through the soil, various physical, chemical, and biological processes can remove remaining contaminants and further improve effluent quality.

Following percolation, the treated effluent will reach the groundwater reserves. It can recharge aquifers and maintain a sustainable supply of groundwater. Safe infiltration assumes that the treated effluent will thereby not pose any risks to human health or the environment.