The stormwater system has been built to collect only rainwater. In towns and cities, another system of pipes collects the waste water from the kitchen, bathroom, laundry and toilet. These are the sewerage pipes. This waste water is called sewage and it flows and is pumped to a sewage treatment plant. It must never be allowed to flow into waterways without being treated.
There are several ways of treating sewage. After the sewage is treated it is released into the sea, rivers or wetlands. Some sewage treatment plants can clean the water so well that it is cleaner than drinking water.
Other sewage plants clean the water so it does not cause any pollution when it is released. However, there are still some sewage plants that do not clean the water well enough and it causes some water pollution.
Grey water is the name given to the water that has been used in your bath, shower, kitchen and laundry. Instead of going into the sewerage system, it can be used to water your garden. Black water is the name given to the waste from your toilet. It must be treated so it does not pollute the environment.