## 3 LCA as a Tool in Digestate Use
### LCA Applied to Digestate Production and Uses--A Survey of Recent Studies
Most of these studies only specify the amount of waste to be processed and its main composition (food waste, manure, OFMSW); however, in most of the reviewed studies (18 out of the 28 reviewed papers), the physicochemical characteristics of the substrates were not analyzed. Most papers clearly indicate the system's boundaries, specifying the stages included and excluded from the study. However, as seen in the table, variations in the scope are considered, depending on its goal. The table clearly outlines the approach used for the life cycle analysis, categorizing it into the following groups: cradle to cradle (CtC), cradle to grave (CtGr), cradle to gate (CtGa), gate to grave (GatGr), and finally, gate to gate (GatGa). In thirteen of the reviewed papers, the system boundaries are defined from the cradle to the grave, which includes the stages of digestate production, processing, and agricultural use. In six studies, the scope is cradle to gate, which concludes once the product is finished, excluding distribution and usage. Four articles follow the gate to grave approach, excluding the prior production stages. For the other scopes, there are two gate to gate and one cradle to cradle. The Life Cycle Impact Assessment (LCIA) utilizes Life Cycle Inventory (LCI) data to determine environmental impact indicators based on a functional unit. While ISO standards suggest impact categories, they do not prescribe a specific LCIA method, allowing for varied methodological choices that can significantly affect the analysis results. Key global LCIA methods like CML-IA [115], Recipe 2016 [116], IMPACT 2002+ [117], and ILCD [118] generally focus on mid-point impacts and differ in scope and detail. CML-IA, for example, offers two versions: a baseline with 11 impact categories and an extended version with 50 categories. The Recipe 2016 method encompasses 18 impact categories at the midpoint and 3 protection areas at the endpoints. The IMPACT 2002+ method [117] is damage-oriented and uses 15 impact categories at the midpoint level and 4 damage categories. The ILCD 2011 method uses 16 impact categories at the midpoint level. Out of all the reviewed papers, 10 of them use the ReCipe midpoint method and 3 of them also include the endpoints. Most used the 2016 version, except for one, which uses the 2008 version. Eight papers used the CLM-IA method in different versions. The following evaluation methods, Impact 2002+, TRACI, and ILCD, have each been used in two papers. Table 2 also indicates whether the paper has conducted a sensitivity analysis and an economic study. In 17 of the reviewed articles, a sensitivity analysis is included to identify the variables that most significantly influence the study's outcomes. Meanwhile, only two of them conducted an economic evaluation. The objectives of the reviewed studies vary based on the functional unit, system boundaries, raw materials, and products obtained, among other factors. Some studies seek to evaluate and compare the environmental impacts of applying different treatments to digestate for agricultural use, either as a soil conditioner or biofertilizer, including the use of vermifilters [88], phase separations, and treatments thereof [89], and the production of fertilizer in different physical forms [59]. In other cases, the direct use of digestate is compared with the use of biofertilizer derived from treated digestate [91], direct composting substrate versus composting digestate [93], and co-digestion with management methods [101; 110]. Furthermore, other studies compare waste treatment by AD with other types of treatment, such as incineration, landfill composting, etc. [100; 102]. Finally, some studies compare the products obtained from digestate treatment with the same products obtained by conventional methods [94]. Some of these works also evaluate the impacts of AD on waste, including pre-treatment to improve biogas production yield and, subsequently, treatment of the digestate for land application [96; 98].