It can be seen in a map (figure 2) provided by the FAO that Soil degradation caused by human activities is alarming worldwide. About 38 % of globally used agricultural land is degraded- mostly in Asia, Africa and South and Central America (Esrey, 2000). The main causes of soil degradation are: erosion, fertility decline, overcropping and use of synthetic fertiliser. Since synthetic fertiliser does not contain organic matter which prevents soil erosion, reuse of brown water as soil conditioner plays important role to reduce the soil degradation as brown water contains most of the organic solids in domestic wastewater (see table 1).

Like yellow water, separate collection of brown water is possible with sorting toilet (See figure 7 and 8). As faeces are the predominant source of pathogens of all streams of domestic wastewater, they have to be sanitised prior to their use as soil conditioner. However, killing of pathogens is facilitated when faeces are collected separately. Possible treatment techniques are dehydration (utilization of solar energy) or composting. Together with organic solids helping to prevent soils from desertification, also nutrients are transferred to agricultural fields with processed brown water.

Figure 2: Map of global status of human-Induced soil degradation (Source: UNEP World Food Summit 1996)