Urban drainage systems mainly handle two types of flow:
- Wastewater and
- Storm water
Urban drainage systems are needed in an area because of interaction between human activity and natural water cycle. This interaction has two main forms;
- The abstraction of water from the natural cycle to provide water supply for human life;
- Covering of land with impermeable surfaces that divert rainwater away from the local natural system of drainage.
- If untreated wastewater is allowed to accumulate, the decomposition of the organic materials it contains can lead to the production of large quantities of gases causing unfavorable smell in atmosphere.
- Wastewater usually contains numerous pathogenic micro-organisms from human being, animals or from toxics from industrial wastes.
- The presence of nutrients in waste water can stimulate the growth of aquatic plants, which contain toxic materials.
- Wastewater contains dissolved material, solids originating from WCs (water closets) from washing, industry and other water uses.
- If rain water is not properly drained, it would cause inconvenience, damage, flooding and even health risks. It may also carry pollutants from air or from catchment areas.
Thus, the removal of wastewater from its source of generation, followed by treatment and disposal is not only desirable but also essential for a civilized society.