What Is "Sewage"?

Water that is discharged after home or industrial use, and cleared away via a system of pipes leading to a waste water treatment plant.
 
For what purpose are sewerage systems required in the local authority area?
 
To prevent pollution and hazards that potentially can cause diseases, and to prevent ground water pollution.

As opposed to closed water systems that are not accessible to the public, except  whenopening the tap. The sewerage systems are open and allow the entry of various objects and materials via lavatories and other systems for discharging water after home or industrial use.
 

These objects and materials that are transferred together with the sewage to the treatment plants, make the treatment work difficult and cause substantial disruptions in the process and affect the effluent quality.