Why separate pipes are used for sewage and rainwater?

Separate pipes are used for sewage and rainwater to prevent contamination of stormwater with pollutants from wastewater, which can harm the environment and public health. This separation allows for more effective treatment of sewage before it is released into water bodies, while rainwater can be managed through drainage systems to mitigate flooding and reduce urban runoff. Additionally, using separate systems can enhance the efficiency of wastewater treatment plants and promote sustainable water management practices.