In countries practicing wastewater treatment, the disposal of sewage sludge is a frequently discussed topic. Some classify sewage sludge as “waste” and insist that it should not be utilized agriculturally in areas with high expectations of product quality. Considering the fact that sewage sludge consists of concentrated pollutants taken out of wastewater, this statement is understandable. However, it is also undeniable that this “waste” contains significant amounts of nutrients, including those among non-renewable resources, such as phosphorus. On the one hand these resources are wasted by disposing sewage sludge without any recovery. On the other hand the production of industrial fertilizer containing phosphorus requires the exploitation of geological sources which leads to a reduction of decomposable rock and therefore contradicts the rule of sustainability. The range of remaining resources is estimated at 100 years for phosphorus reserves and at another 370 years for the reserve bases (ATV-DVWK, 2003).
In Europe, about 79% of the phosphorus consumption is required for agriculture (Johnston and Steén, 2000). Implementing a good fertilization practice the demand of phosphorus for German agriculture would add up to 364.000 t P (an area-specific mean of 50 kg P2O5/(ha·a) is recommended) (UBA, 2001).