Sewage is wastewater from homes, businesses and pre-treated industry. Nature can handle small amounts of waste remarkably well, but the volumes a modern city produces every day would overwhelm it.
That is why sewage must be treated before it reaches rivers, seas or groundwater. Treatment combines physical steps that remove solids, biological steps in which microorganisms break down organic matter, and sometimes chemical steps that remove what remains.
Treated water can then be returned safely to the environment or reused for irrigation and industry, which matters greatly in a water-scarce country like Saudi Arabia.
Related solution: Environmental and water