t is undeniable that phosphorus is vital for life – literally. It is an essential nutrient for all living forms since it plays an important role in DNA/RNA molecules, and is also related to cellular energy transport via ATP (i.e. the P in ATP – the molecule that carries energy around cells) . Phosphorus is not found free in nature, due to its reactivity to air and many other oxygen-containing substances, but it is widely available in different minerals. Phosphorus rocks are a non-renewable resource originated from igneous rocks and marine sedimentary deposits. The widely used approach to obtain these rocks which contain a high content of phosphorus is by mining [2, 3].
Phosphorus rocks are used worldwide for many industrial applications such as detergents, food and drinks, metallurgy. However, the most important use of phosphorus rocks is for the production of phosphate fertilizers . Agriculture is by large the main user of phosphorus globally, accounting for between 80-90% of the total world demand . Phosphorus is one of the three macro-nutrients needed by plants to develop and grow. Together with nitrogen (N) and potassium (K) nutrients, phosphate (P) plays a leading-role in NPK fertilizers which are essential for optimal growth of crops . Unfortunately, a shortfall of phosphorus in soils will result in a reduction of crop yield and can affect food production worldwide.
Phosphorus rocks reserves are being mined at growing rates due to increasing demand. According to the United Nations Food and Agriculture Organisation (FAO), the world demand for fertilisers was estimated to increase around 3 million tonnes every year . The potential threat of phosphorus global limitation has been discussed since the quality of the existing rocks is declining, making the extraction more expensive. Indeed, in 2014, the European Commission declared phosphorus rock as one of the 20 critical resources from the European Union . Estimation of the remaining time phosphate reserves will last are variable and contested, ranging between 100 to 600 years at current production rates . Some researchers have applied Hubbert’s concept of “peak oil” to phosphorus rock mining and named as “peak phosphorus” [10, 11]. Peak phosphorus refers to the moment when production of phosphorus from mining reaches a maximum (its peak); followed by the decrease in quality of the remaining reserves, making it harder to access. Then, mining and processing will be more expensive, which will result in the supply decline and rapidly increase of the prices . Additionally, phosphorus rocks reserves come from a limited number of countries, with large parts of the world, including Europe, being almost totally dependent on imports. As shown in Figure 1, Morocco holds the vast majority of global supplies of this resource, approximately 73%. China is currently in second place with only 4% . This is geopolitically sensitive as Morocco currently occupies Western Sahara and controls its phosphate rock reserves, which could present significant food security risks.
In order to ensure phosphorus remains available for food production to future generations, the development of novel phosphorus recovery and reutilization initiatives through more sustainable wastewater systems is needed. In particular, human waste from households contributes largely to the amount of nutrients found in waste streams. Approximately, ~50% of the phosphate mass load in municipal wastewater treatment plants comes from human urine. Therefore, urine could play the new leading-role in food production worldwide and help current global demand for phosphorus. Thus, a new perspective that re-evaluates human urine as a reusable and eco-friendly resource should be established. Phosphorus recovery from human urine represents a new promising avenue, since the recaptured phosphorus from waste streams can be utilized for fertilizer production. Additionally, the development of new wastewater systems can offer a sustainable form of sanitation for 2.5 billion people in developing countries that still have no access to proper sanitation . From the extraction and reuse of phosphate from wastewaters, it promotes a circular and sustainable closed-loop of nutrients. And, at the same time, it could increase phosphate availability for fertilizer production worldwide.
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