Separate collection of yellow water is possible with sorting toilet (see figure 7 and 8). Among the flows of wastewater, yellow water contains most of the nutrients (table 1). One person produces on average 3.92 kg of nitrogen, 0.38 kg of phosphorous and 0.97 kg of potassium per year. These nutrients, such as nitrogen in the form of urea, phosphorus as super phosphate and potassium as an ion, are in a form which are ideal for uptake by plants (Esrey et al.; 1998). Beneficially, urine contains very low levels of heavy metals and pathogens. These heavy metal concentrations are much lower than those of most chemical fertiliser. In Sweden, for instance, urine contains less than 3.2 mg cadmium per kg of phosphorus compared to 26 mg Cd/kg of phosphorus in commercial fertiliser and 55 mg Cd/kg of phosphorous in sludge (Esrey, 2000).

In the conventional wastewater treatment systems, instead of utilising the yellow water for plant nutrition, it is wasted. In modern municipal wastewater treatment plants, nitrogen compounds (most of them originating from yellow water) is removed with the costly nitrification and denitrification process. Even with the high inputs of money, a reasonable amount of nitrogen compounds (especially nitrate) escapes with the effluents of treatment plants and causes eutrophication in water bodies. In high-tech treatment plants, most of the nitrogen compounds are converted to N₂, which is itself a raw material for high energy consuming nitrogen fertilizer synthesis (e.g. in natural-gas based ammonia plants).

Even the best affordable treatment plants discharge over 20 % of nitrogen, over 5 % of phosphorus and more than 90 % of potassium to the aquatic Environment where they are lost for ever and cause severe problems (Otterpohl et al., 1997). Those nutrients, which are captured in sludge are often contaminated with heavy metals such as Cadmium (Cd) and organic compounds such as PCB (polychlorinated Biphenyle), which pose potential toxic risks to plants, animals and humans (Metcalf and Eddy, 1991). Therefore, large amounts of Sewage sludge are disposed of in landfills or incinerated. Only a smaller part is applied to agricultural land.

Due to following reasons wasting of yellow water is not sustainable:

1st: Production of fertiliser is energy intensive, draws on very limited fossil resources and causes environmental problems. Generating nitrogen (N) from air requires a considerable amount of energy. Mining and refining the raw materials for phosphate production generates a huge amount of hazardous wastes. Reserves of phosphate (P) and potassium (K) are definitely limited on a time scale of only a couple of human generations especially with regard to economic constraints. Moreover, also sulphur is a limited fossil resource and is required by plants. Also sulphur is contained in yellow water. Therefore, we should use the human resource "Anthropogenic Nutrient Solution" (ANS; Larsen and Gujer, 1996) yellow water as fertiliser which contains reasonable amounts of these nutrients.

2nd: If yellow water is wasted (i.e. it is added to municipal wastewater), it will either contribute to eutrophication of surface waters because of its nutrient content or it will even lead to groundwater contamination (high nitrate concentrations can be generated by transformation of ammonia, and sometimes high nitrate concentrations are detected in ground waters contaminated with domestic wastewater - they can lead to lethal methemo­globinemia of babies drinking water from such contaminated sources).

3rd: Even if domestic wastewater is treated in modern treatment plants, adding yellow water to wastewater is disadvantageous Residual nitrogen compounds (mainly nitrate) escape with the effluents even of very efficient wastewater treatment plants and contribute to surface water eutrophication. But the main disadvantage of adding yellow water to municipal wastewater is that a great deal of energy consumption for operating activated sludge tanks is required for ammonia removal (aeration for nitrification). Removal of phosphate from municipal wastewater requires an additional biological stage and/or precipitation stages (addition of iron or aluminium salts). It is assumed that nutrient removal requires about 50 % of energy consumption in German wastewater treatment plants (Rakelmann 2002). It is clear that production of electric power is contributing to the greenhouse effect and leads to emission of air pollutants.

4th: Because of its content of nutrients, yellow water can substitute a reasonable amount of synthetic fertilizers. It is assumed that about 50 % of crops needed by human beings can be fertilized with human excreta. When yellow water is wasted, this means additional energy consumption for fertilizer production.

(See definitions of “sustainable development� under: http://www.gdrc.org/sustdev/definitions.html )