## 1 Introduction  
The development of the global economy has resulted in unsustainable overuse of the natural resources available on Earth [1]. The situation is worsening as the world population approaches 9 billion. In the countries of the Global North, consumer excess accompanied by great wastage has occurred since the 1970s [2]. In recent years, Brazil, Russia, India and China have contributed significantly to this growing consumption [3]. Moreover, the consequences of resource use in terms of impact on the environment may induce serious damage that goes beyond the carrying capacity of the environment [4]. In September 2015, the United Nations General Assembly formally adopted a set of 17 Sustainable Development Goals (SDGs), which are to be implemented and achieved in every country from the year 2016 to 2030 [5]. The circular economy (CE) is considered an important means to meet the challenges of SDGs [6]. This concept entails gradually decoupling economic activity from the consumption of finite resources and is based on three principles: design out waste and pollution, keep products and materials in use and regenerate natural systems. It is forecast that a transition to CE approaches will provide more than U.S. $1 trillion globally in material cost savings by 2025 [7]. Phosphorus (P) recovery and recycling is considered a possible CE pilot, in other words, a potential case to demonstrate that CE principles work in practice [8]. P is an element of great biological importance [9]. The availability of P for crops ensures the proper growth of plant roots, good condition of the stem, adequate formation of flowers and fruits, timely ripening, appropriate volume and quality of yields, intensive N\({}_{2}\) fixation by leguminous plants and a stronger resistance of all plants to biotic and abiotic stress factors [9; 10]. The natural resources of phytoavailable P in arable soils do not fully satisfy the nutritional requirements of field plant communities [10], which must therefore receive some of this element from fertilisers [11]. The production of mineral phosphorus fertilisers almost completely relies on phosphate rock [12]. Although the P resources and stocks in the world are still relatively large [13], it is not disputed that they are limited and non-renewable. [PERSON] et al. [14] indicate that without proper management, phosphate rock will be depleted within the next 70-140 years. In addition, phosphate rock resources are unevenly distributed across the globe: most are located in Africa (71.4% of the P resources are controlled by Morocco and Western Sahara) [13]. This makes the European Union (EU), for example, largely dependent on imported phosphorites. Phosphate rock was included on the EU list of 20 critical resources in 2014 [15] and is still indicated on the updated list in 2017 [16]. The limited P resources can be compensated for by recycling used materials [17]. It is indicated that waste recovery at approximately 50% may defer the phosphate rock depletion time by 50 years [14]. A major step in this direction has been taken in some European countries, such as Switzerland, Germany and Austria, which have made the recycling of P from sewage sludge and slaughterhouse waste mandatory [18]. In recent years, many scientific centres have been involved in exploring suitable P substitutes among secondary raw materials and developing new methods of P recovery for fertiliser industry purposes [11; 19; 20; 21; 22; 23]. Sewage sludge ash (SSA) is claimed to combine the most promising P source and recovery technologies [24]. Methods based on sewage sludge biomass incineration eliminate organic pollutants, microorganisms and pathogens [17]. The final form of mineral ash contains 7-12% P [17]. The problem of toxic elements residues in SSA is also proving to be solvable [11; 20; 22; 23]. Slaughter industry by-products are also a valuable P source. Animal bones have been used for fertiliser purposes since biblical times [25]. In poultry bones, for example, the P content is 5-9% [26; 27].