## 2 Digestate as Fertilizer
### Challenges of Digestate as Fertilizer  
Despite its potential as a sustainable fertilizer, digestate faces several environmental, economic, and logistical challenges that hinder its widespread adoption. One of the main concerns is the risk of soil, air, and water pollution. The organic nature of digestate means it may contain pathogens and heavy metals, depending on the feedstock and anaerobic digestion conditions. Pathogen contamination is a critical issue, as food waste, sludges, and contaminated plant materials may introduce microorganisms that persist through digestion and transfer to soils [69]. While certain bacteria and fungi contribute to soil health by fixing nitrogen and solubilizing phosphates [70; 71], others may pose health risks if not properly managed [72]. Further research is needed to refine treatment conditions and post-processing methods to neutralize pathogens. Heavy metal (HM) accumulation in soils is another concern, as these elements do not degrade during digestion and may transfer from feedstock to digestate, eventually contaminating crops and ecosystems [73; 74]. The highest HM concentrations are typically found in animal slurries and manures, as HMs are commonly added to livestock feed to promote growth and microbial resistance [74; 75]. Repeated applications of HM-containing digestate can lead to long-term soil accumulation, requiring careful feedstock selection and post-treatment strategies to reduce toxicity and bioavailability [76; 77]. Nutrient runoff and volatilization also pose environmental risks, affecting both soil fertility and water quality. Nutrients such as phosphorus and nitrogen can leach from fertilized soils into water bodies, leading to eutrophication--a process that depletes oxygen levels and disrupts aquatic ecosystems [78]. Nitrates, due to their high solubility, are particularly prone to leaching [75]. Factors such as soil type, rainfall, and application method influence runoff potential, requiring optimized fertilization strategies [58; 75; 79]. Ammonia volatilization is another challenge, as anaerobic digestion increases the concentration of soluble inorganic nitrogen, primarily in the form of ammonium (NH\({}_{4}\)\({}^{+}\)). Under alkaline conditions, ammonium is converted into ammonia (NH\({}_{3}\)) [80], which is released into the atmosphere, contributing to soil acidification and phytotoxicity [75; 81; 82; 83]. This volatilization not only reduces the fertilizing efficiency of digestate but also exacerbates environmental degradation. Beyond environmental concerns, digestate adoption is hindered by economic and logistical barriers. Its viability as an alternative to chemical fertilizers depends on overcoming feedstock variability, storage challenges, and market uncertainty. Unlike synthetic fertilizers, digestate composition is influenced by feedstock type, seasonality, and livestock diet [51; 83]. This variability affects nutrient content and fertilization performance, making it difficult to standardize digestate as a reliable agricultural input. Storage and co-digestion strategies have been proposed to mitigate seasonal and supply-chain fluctuations. Proper storage provides a buffer against supply inconsistencies, while the co-digestion of multiple feedstocks can improve digestate uniformity and enhance biogas production [6; 84]. However, digestate storage and handling present additional logistical challenges, particularly due to its high-water content, which increases transport costs and risks of leakage [6; 73]. Current approaches to address these issues include moisture reduction through physical and biological post-treatments, although further optimization is needed to improve cost-effectiveness. The economic value of digestate remains uncertain, limiting its commercial potential. Pricing is influenced by nutrient content, regional demand, and competing synthetic fertilizers [84; 85].