Wherever people live in buildings, wastewater is generated, which can be divided into two categories: polluted water and rainwater. The contaminated water is usually discharged directly into the sewage system, then it is treated at the treatment plant, after which it goes back to the water circulation cycle. Rainwater can be separated from contaminated water if a separate sewerage system is used, or drained along with it if a common sewerage system is used. Developers and operators are advised to always involve designers in the development of a water drainage scheme in order to avoid unpleasant surprises in the future.