Why in a same infrastructure, water infiltration system and sewage system both are present?
Do you mean that there are separate foul and storm water systems in modern infrastructure? In older infrastructure, there is often a combined sewer system, where water from inside buildings (toilets, sinks, showers etc.) is combined with water from outside buildings (rainwater from roofs, run-off from roads, etc.). In the UK at least, both foul and storm water would go to a sewage wastewater treatment works where they are combined, treated, and then discharged to surface water such as a river or the sea. The issue is what happens during storms: if there is a lot of water flowing through the system, the sewage treatment works cannot cope and the water has to be discharged untreated to surface water. In a combined sewer system, this means discharging both slightly dirty rainwater and very dirty sewage directly to a river or to the sea. With separate foul and storm water systems, it is possible to discharge the rainwater to surface water but continue to treat the foul water in the treatme
