## 4 System and Stages in Digestate Production
### Using LCA to Improve Digestate Production Systems  
LCA is a key tool for optimizing digestate production by identifying environmental impacts, improving efficiency, and guiding the transition to a circular economy through sustainable waste management and high-value product generation. This text consolidates the remarks and possible improvements highlighted by the LCA methodology in the reviewed papers. LCA identifies critical \"hotspots\" in the system, such as fugitive emissions during feedstock handling, digestate post-treatment, storage, land application, anaerobic digestion, and long-distance transport, which significantly contribute to energy consumption and greenhouse gas (GHG) emissions [33; 93; 101; 104; 106]. These emissions often go unaccounted for but considerably impact Global Warming Potential. LCA also quantifies resource consumption, acidification, eutrophication, and ecosystem toxicity, identifying unit processes with the highest contributions. Digestate-derived fertilizers tend to have high acidification and eutrophication impacts due to ammonia and nitrate volatilization and runoff during  
Figure 3: Diagram of digestate production systems. land application [52; 99; 119]. If not properly managed with efficient application and storage techniques, these impacts can surpass those of inorganic fertilizers [113]. Many studies highlight anaerobic digestion (AD) as key in reducing GWP and fossil fuel resource depletion, with biogas production providing positive environmental credit. Increasing biogas yield is the most direct way to reduce environmental impact [98; 102; 105; 107; 110; 120], while fertilizer displacement also plays a role [89]. LCA facilitates comparisons between liquid digestate and solid fertilizers, assessing transport, handling, and fertilizing potential. A previous study [112] found that while manures reduce emissions in storage, municipal organic waste has greater fertilizing potential and displaces more inorganic fertilizers. Another study [89] found lower emissions when digestate is dried and separated before storage. The results in [96] showed that transforming digestate into hydrochar reduces GWP by storing carbon in the final product. The authors in [100] reported that burning digestate instead of using it as fertilizer can further lower GWP by reducing external energy demand. LCA identifies key environmental impacts and informs sustainable system improvements, fostering a circular economy through fertilizer and bioproduct valorization. Electricity consumption is a major contributor to digestate production's environmental footprint, accounting for 50-80% of the total impact [90]. Adopting renewable energy sources and biofilters can mitigate this burden [93]. Storage and land application are primary GHG emissions, acidification, and eutrophication sources. Storage alone contributes up to 65% of direct GHG emissions [103], while land application is responsible for 99% of eutrophication potential due to nitrogen runoff [89]. Improved sealing of digestate storage can reduce fugitive methane emissions by up to 165% [106]. Transportation distance plays a crucial role in digestate feasibility. When exceeding 10-15 km, drying and transport emissions can outweigh economic benefits [96]. The maximum break-even distance for achieving GWP neutrality is 211 km [92]. Composing helps mitigate ammonia volatilization and nitrate leaching by stabilizing organic matter [88]. Digestate's total solid (TS) content influences emissions, with higher TS reducing ammonia losses but increasing ozone depletion [99]. Phase separation is crucial for digestate management. Mechanical separation methods, such as centrifugation and pressing, are widely used due to their efficiency and lower [97]. Filtration techniques, like reverse osmosis, reduce GWP by 9% but require higher energy input [97]. Anaerobic digestion provides fossil fuel substitution credits, lowering overall environmental impact [107]. Electro-digestion improves energy efficiency by 35.9% but increases initial energy demand [110].