|Urban Wastewater Projects - A Layperson's Guide (EEA, 1998, 124 p.)|
|Chapter 2. An Introduction to Urban and Rural Wastewater Management|
2.2.1 What is urban wastewater?
The main component of urban wastewater is sewage from domestic dwellings, offices and other commercial premises. Liquid effluents from industrial processes and service industries such as laundries are also commonly discharged into urban sewers together with domestic sewage. Older sewer systems were also designed to receive rainwater, drainage from streets, roofs and other paved and impermeable areas. Although this practice continues today, it is increasingly common to provide a separate network for rainwater.
2.2.2 What is domestic sewage?
The liquid waste produced by domestic activities has two main components:
· Grey water; Water that has been used for baths, showers, in wash-basins and the washing of clothes and floors, and
· Black water; Water and waste from toilets and kitchen sinks. In turn, toilet waste in areas not served by sewer systems may be termed night soil, as it is separately stored and carted away from the house.
Normally both of these components, black water and grey water, are combined and discharged into a single drainage system and together are referred to as domestic sewage or simply sewage.
2.2.3 Why differentiate between grey water and black water?
Grey water contains very little solid material and under the right circumstances can be considered suitable for recycling.
If plumbing systems permit the separation of the two components, grey water can be used for watering gardens in times of drought. However, the adverse influence of increasing amount of detergents in grey water on gardening should be kept in mind. In very exceptional circumstances, i.e. in chronically water-short areas, grey water may be treated at the place of origin and reused for toilet flushing.
It should be emphasised that the recycling of grey water imposes not only significant additional costs on housing construction but also its treatment is subject to many problems. Reuse and recycling of grey water is not yet a common practice.