**1. Introduction**

Phosphorus (P) is an essential nutrient for all forms of life. Specifically, P is an important nutrient element in agriculture and a major limiting factor for plant growth and the entire food production chain. Currently, both P and nitrogen (N) are the basic components of mineral fertilizers viewed as an irreplaceable part of modern agriculture. P is a resource obtained mainly from phosphate rocks located in a few regions of the world [1]. More than 87% of all mined and processed phosphate rock is turned into fertilizers while few phosphate rock is used for additives in livestock feed and food.

There are evidence that P reserves used for mineral phosphate fertilizers as a primary source of P input to agricultural lands are steadily decreasing with time, with the expectation to be depleted in a few centuries or less [2–4]. More striking is the view of the United States Geological Survey according to which the phosphate deposits will last about 50 years at the current rate of extraction [5]. Such a perspective can

**Figure 1.** *Global production of phosphate rock (blue) coupled with world population (red) in time [6].*

be reasonably explained by the continuously growing population and rising global demand for food [6]. Global production of phosphate rock coupled increased world populace is shown in **Figure 1**.

The expected P rocks deficiency is supported by the current monetary evaluation of the global phosphate rock market. It shows an annual growth rate higher than 5% in recent years [7] reaching \$25.49 billion in 2023.

Recognizing that phosphate rock is a limited resource for mineral fertilizers production, the necessity of identifying P-rich waste streams and finding technical ways for P extraction into valuable products is of extreme importance. Part of the increased pressure on P resources could be alleviated by recycling P contained in various agricultural, industrial, and urban wastes [8, 9]. Excess P during fertilization and the non-proper treatment of wastes, ground an imbalance of the biosphere following severe negative environmental effects. While previous efforts have been directed toward removing P from the wastewater discharged into surface water, current efforts are directed to recover P as useful species [10]. The potential of the waste P is high as the global mass balance shows that up to 20% of the world's P-production (about 3Mt P/year) is currently lost [11]. On the other hand, resource recovery from waste streams is increasingly seen as one option to improve the economics of wastewater treatment. Domestic sewage, industrial wastewater, and manure are no longer considered waste but just a resource of different origins [12]. The green line in **Figure 2** (below) outlines the recovery route in addressing the P-resource recycling in the framework of the circular economy concept.
