## 1 Introduction  
Organic waste generation has been inherent to human activity since its beginnings. As the world's population grows, so does the amount of organic waste it creates daily. In this context, and to ensure correct management and disposal, the European Commission has designed an organic waste management hierarchy [1] based on prevention, reuse, recycling, and recovery, only allowing disposal as a last resort. Among recovery strategies, anaerobic digestion (AD) has emerged as a widely adopted solution enabling waste reduction and energy generation. AD is a microbial-driven process in which a diverse community of microorganisms collaborates to break down complex organic substances [2] without oxygen, generating an energy-rich gas composed mainly of methane and carbon dioxide [3]. This renewable energy source, widely known as biogas, can be directly used as fuel or further upgraded into biomethane, which has a higher energy density after removing residual gases from the digester. Due to its organic nature, biogas and its derivatives are considered sustainable fuels with versatile applications. Moreover, it has been demonstrated that replacing conventional energy sources with biogas in electricity generation, natural gas substitution, and transportation fuels leads to a reduction in global warming potential (GWP) [4; 5]. Given its role in the energy transition, AD emerges as a key waste-to-energy (WTE) system, facilitating the imperative shift towards sustainable energy generation. In addition to biogas, AD generates digestate, the residual output composed of solid and liquid organic matter that is not decomposed in the anaerobic fermentation process [6]. Digestate is rich in organic matter (OM) and essential nutrients such as nitrogen, phosphorus, and potassium, as well as other trace elements, which depend significantly on the feedstock type and the specific conditions of the anaerobic digester [7; 8; 9]. Depending on the feedstock used in AD, digestate can be classified into four broad categories [10]: municipal sludge digestate, food waste (FW) digestate, municipal solid waste (MSW) digestate, and agricultural digestate (comprising both plant-based and livestock manure feedstocks). Digestate composition varies significantly within these categories. For instance, study [11] classifies FW digestate as having the lowest nitrogen content while being richer in phosphorus compared to digestates from different feedstocks, such as sewage sludge. Furthermore, digestate composition can vary within the same feedstock category, as total nitrogen (N) content and the carbon-to-nitrogen (C:N) ratio may fluctuate even among similar substrates [12]. Digestate flows out of the digester in a solid-liquid state and is typically subjected to a separation process before any further treatment [13]. This separation reduces the total volume for easier transportation and disposal [14], with the liquid fraction (LF) retaining most of the dry matter (DM), phosphorus, and ammonia, while the solid fraction (SF) presents higher concentrations of total organic carbon (TOC), total solids (TS), and volatile solids (VS) [15; 16]. Initially, digestate separation served primarily as a waste disposal method to minimize landfill and incineration inputs. However, as research on digestate composition advanced, its perception shifted from a mere waste byproduct to a valuable resource [17]. A better understanding of its composition has revealed that digestate, rich in nutrients and OM, holds agronomic value as a fertilizer and supports various applications depending on its specific properties [18]. One of the most promising applications of digestate is its use as a fertilizer. Its high nitrogen availability for plants and lower environmental impact than synthetic fertilizers [19] make it an efficient and cost-effective option for organic waste management [10].