soil pipes only directly connected to manholes. waste pipes why we are connecting through gully?
There are gases in the sewer system that you don't want in the house. The floor trap provides a water seal to prevent these smelly guys where they belong. If a fixture is used infrequently, the water seal can dry out, losing the seal.
Because of the law's. Just because you see all that junk being washed into the catch basin's doesn,t mean its not used again. imagine recycling acid which has drained into a basin like it does in our rivers and oceans, it may make it thru the recycle process but it also becomes a part of our drinking water. Acid in your coffee,tea the water you boil your spaghetti in and who knows what else.Every living thing needs water to grow and stay alive. contamination kills and destroys.
