The typical characteristics of the streams of domestic wastewater, shown in table 1 clearly characterize that yellow and brown water contain most of the nutrients discharged to sewers in the conventional sanitation. This means that they are generally wasted instead of being used as fertilizers (except the small portion of nutrients being contained in sludge which is used sometimes as fertilizer after sanitisation).

Due to pathogens, brown water poses high health risk, but it represents a very small volume flow in domestic wastewaters (only 50 litres are excreted per person per year). In conventional systems, this small volume is mixed with other streams of domestic wastewater with higher volume flows: yellow water (tenfold volume flow compared to faeces) and grey water. grey water volume flows depend on habits. That is why a wide range is given for grey water volume flow: 25,000 to 100,000 litres per person per year. Table 1 is related to Central European patterns. Of course, also extremely smaller grey water volume flows per person can be found, especially in regions with water scarcity. Additionally toilet flush water has to be taken into consideration (which might be up to 10 litres per toilet use).

Table 1: The typical characteristics of the streams of domestic wastewater
(Compiled from: Geigy, Wissenschaftliche Tabellen, Basel 1981, Vol.1, Larsen and Gu-jer, 1996 and Fittschen and Hahn, 1998)

It is very impressive that the large volume flow of grey water is accompanied by comparably small nutrient mass flows (about 3 % of the total nitrogen mass flow and 10 % of the total phosphorus mass flow (phosphorus concentration can still be lowered by using phosphorus free detergents) discharged with domestic wastewater). However, about one third of the potassium (which is also important for plant growth and a limited fossil fertilizer component) mass flow of domestic wastewater is contained in grey water. Because of the large volume flow of grey water (compared to yellow and brown water), its potassium concentration is quite low (commonly below 10 mg/l), however. Because of its low contribution to mass flow of the nitrogen and phosphorus in domestic wastewater and its high volume flow grey water turns out to belong to the water cycle and represents a splendid source for wastewater reuse. As grey water contains nearly half of the organic load of domestic wastewater, this is the main group of pollutants to be removed from grey water before its eventual reuse. Therefore, treatment of grey water is far cheaper than treatment of total domestic wastewater as there is no need of costly nitrification and denitrification processes mostly practiced in modern municipal wastewater treatment plants.

The scheme clearly demonstrates that greatest part of the nutrients nitrogen, phosphorus and potassium of domestic wastewater are contained in the comparably small volume flow of yellow water. Moreover, urine contains trace metals required for plant growth. Only about 10 % of the organics of domestic wastewater are urine borne. From these reasons, yellow water has to be taken into consideration as fertilizer, and is thus related to the food cycle rather than to the water cycle.

brown water contributes greatly to the phosphorus load of domestic wastewater and can thus also be considered as fertilizer. Moreover, the organic solids make brown water a splendid candidate as a soil conditioner after suitable treatment. Therefore, also brown water is belonging to the food cycle.