Although domestic wastewater is a pollutant, it can be utilised after proper treatment as a resource for fertiliser,
water and energy. Grey water, which is the wastewater generated in the household excluding toilet wastewater (black water), represents the major volume of the domestic wastewater (60–75%) with low content of nutrients and pathogens. Most of treatment plants of the grey water include a one or two-stepseptic tank system for pre-treatment.
The grey water treatment needs both physical and biological processes for removal of particles and dissolved organic
matters. Recently, grey water treatment has been studied either by application of high-rate aerobic systems, such as rotating biological contactor, fluidised bed, aerobic filter, membrane bioreactor, or by application of low-rate systems, such as slow sand filter and vertical flow constructed wetlands.