Runoff is carried above or below the ground and led into a layer or shaft packed with gravel. From there, the rainwater is allowed to infiltrate into the ground, either directly or by using a perforated horizontal pipe wrapped with geotextile (reverse drainage / IT sewage system, Infiltration and Transport ). Such systems are used next to paved surfaces or next to unpaved surfaces that do not offer sufficient room for infiltration ditches or where the ground has an insufficient permeability factor. In some cases, precipitation drains from roofs, overflows from rainwater ponds and rainwater reservoirs can also be connected to these systems. Obviously again in this case only clean rainwater may be infiltrated. Polluted rainwater must first be treated through, for example, a ground filter with vegetation. At ground level these systems require virtually no space. After a heavy rainfall the IT sewage system can end up being an emergency overflow measure on surface water or the rainwater sewage system. As with all infiltration systems, the IT sewage system helps to prevent desiccation. The IT sewage system can also be used to infiltrate surface water in areas facing desiccation. If and how such infiltration measures can be implemented is determined by the municipal sewage plan. Contrary to above-ground infiltration systems such as bioswales, a disadvantage of underground systems is that problems such as pollution are not easily visible.