When anything is flushed down the toilet it goes into the sewer and then into a sewerage treatment plant. This is not a water reservoir. A water reservoir is a holding area, frequently a lake, for future drinking water. It is not sewerage. The two types of water are treated entirely differently to different standards with different chemicals. In some cities the drains along the streets are not part of the sewer system and are not treated but empty directly into canals or rivers.