The EU is currently relying on phosphorous ore imports and the energy intensive production of ammonia to produce necessary fertilizers. These fertilizers are needed in agriculture to ensure a sufficient food production. The recovery of valuable nutrients (phosphate and ammonia) from waste streams will help to overcome future shortages and reduce the need for phosphorous ore imports and energy intensive ammonia production. Phosphorus is an essential nutrient; it is applied as phosphate salts (fertilizers) to the soil. Phosphorus is a scarce and non-replaceable resource; “once depleted: there is no substitution” (J. L. Barnard 2009). Based on proven reserves it is estimated that these phosphorus reserves will be depleted within 50 to 100 years. At same time the quality of the phosphorous ore is decreasing, due to heavy metal contamination (Driver et al., 1999, Cordell et al., 2009). Therefore, there is a great need to develop sustainable processes which close the phosphorus cycle. Nitrogen, on the other hand, can be recovered as ammonia from nitrogen gas (N2) present in the atmosphere. However, this ammonia recovery process is energy intensive and currently heavily depending on fossil fuels.
Phosphate and ammonia in urban wastewater are largely lost during treatment in conventional wastewater treatment plants (WWTPs). Nitrogen (N) is mainly removed by sequential biological nitrification and de-nitrifcation, after which it is lost as N2-gas to the atmosphere. The energy consumption of this nitrogen removal process is 45 kJ/gN-removed. The Haber-Bosch process is applied globally for the recovery of nitrogen from the atmosphere in the form of ammonia (NH3). The energy consumption of the Haber-Bosch process is 37 kJ/gN-recoverd. In WWTPs, phosphorus (P) is immobilized in the sludge by precipitation with iron or by the Enhanced Biological P Removal (EBPR) process, the latter requires 28 kJ/gP-recovered (Maurer et al., 2003). However, the sludge from WWTP is not suitable for direct re-use due to the large volume and contaminants also immobilized in the sludge. The sludge can be digested, dewatered and incinerated to reduce its volume and remove organic contaminants. The remaining ash is rich in P, but also in heavy metals. Moreover, the P is mainly bound to iron; therefore its solubility is too low to be used as a fertilizer.