Due to severe intensification of agriculture in the EU-27, a very linear use pattern of fertilizer has become the norm, which has become painfully clear through three major issues. First of all, a large amount of synthetic fertilizers is necessary to supply the fertilizer lost due to harvesting, poor management of soils and lack of fertilizer recovery from waste streams to recycle these fertilizers back to the fields. As these synthetic fertilizers are produced through very energy-demanding processes (1-2% of global energy demand) or mining of finite fossil deposits (located outside of the EU-27), such a position has become unsustainable, both from an environmental, economic and political point of view. Secondly, fertilizer losses due to poor soil management and improper treatment of fertilizer-rich wastes lead to eutrophication (e.g. algal blooms in both fresh and seawater resources), ecosystem degradation (as many natural ecosystems cannot cope with high fertilizer concentrations) and even global warming (due to N2O emissions from denitrification processes). Finally, fertilizer-rich wastes which are captured before discharge are treated with energy-intensive processes which destroy rather than recover the fertilizer value in them. These issues have spurred research to look the other way – policy and agricultural measures to improve soil management and fertilizer recovery rather than removal from waste streams. In this thesis, the latter is investigated.

Synthetic fertilizer is mostly composed of three major elements, also known as macronutrients: nitrogen (N), phosphorus (P) and potassium (K). For these elements, the loss processes, sources, sinks and flows are relatively well known. Next to food waste, one of the major flows containing these nutrients is wastewater. In domestic wastewater, up to 80% of N, 50% of P and 70 % of K comes from human urine, which takes up less than 1% of the total volumetric flow. As the concentrations of these nutrients in urine range in the order of gram per litre, human urine is a concentrated source of fertilizer, and therefore an excellent target for nutrient recovery. In this Ph.D. thesis, a novel, cost and energy-efficient treatment technique for nutrient recovery from urine is investigated: SATURN, or Solar-Assisted Treatment of Urine with Recovery of Nutrients. This technique is situated around two core technologies: membrane distillation/stripping for recovery of N (as ammonia) and potassium struvite precipitation for recovery of P and K (as a crystalline material: KMgPO4.6H2O).