Environmental Performance in the Production and Use of Recovered Fertilizers from Organic Wastes Treated by Anaerobic Digestion vs Synthetic Mineral Fertilizers.

Recovered fertilizers (RFs), in the form of digestate and digestate-derived ammonium sulfate, were produced from organic wastes by thermophilic anaerobic digestion (AD) at full scale. RFs were then used for crop production (maize), substituting synthetic mineral fertilizers (SFs). Environmental impacts due to both RF and SF production and use were studied by a life cycle assessment (LCA) approach using, as much as possible, data directly measured at full scale. The functional unit chosen was referred to as the fertilization of 1 ha of maize, as this paper intends to investigate the impacts of the use of RF (Scenario RF) for crop fertilization compared to that of SF (Scenario SF). Scenario RF showed better environmental performances than the system encompassing the production and use of urea and synthetic fertilizers (Scenario SF). In particular, for the Scenario RF, 11 of the 18 categories showed a lower impact than the Scenario SF, and 3 of the categories (ionizing radiation, fossil resource scarcity, and water consumption) showed net negative impacts in Scenario RF, getting the benefits from the credit for renewable energy production by AD. The LCA approach also allowed proposing precautions able to reduce further fertilizer impacts, resulting in total negative impacts in using RF for crop production. Anaerobic digestion represents the key to propose a sustainable approach in producing renewable fertilizers, thanks to both energy production and the modification that occurs to waste during a biological process, leaving a substrate (digestate) with high amending and fertilizing properties.

Thermophilic anaerobic digestion as suitable bioprocess producing organic and chemical renewable fertilizers: A full-scale approach.

This work reports a full-scale study in which organic wastes were transformed by high-solid thermophilic anaerobic digestion (HSAD), into N fertilizers and organic fertilizers, i.e. digestate. The produced fertilizers were characterized over 42 months and their properties were discussed in comparisons with literature data. HSAD coupled with N stripping technology led to ammonia sulphate production having high N concentration (74 ± 2 g kg-1 wet weight), neutral pH (6.8 ± 1.3) and low traces of other elements. Digestate showed both higher carbon (C) content (314 ± 30 g kg-1 on dry matter (DM) and biological stability than green composts, indicating good amendment properties. Digestate was also interesting for its N (77 ± 3.7 g kg-1 dry matter - DM) content, half of it in the ammonia form, and P content (28 ± 4.1 g kg-1 DM) that was 43% readily available as soluble P-orthophosphate. K content was low (6.5 ± 1.3 g kg-1 DM), indicating poor fertilizing ability of digestate for this element. All organic pollutants investigated were much lower than the limits required for agricultural use and levels of some of them were lower than the content revealed for other organic matrices such as agricultural and energy crop digestates and compost. Emerging pollutants (i.e., pharmaceuticals) were tested as markers and they were found to be below the detection limit (<0.01 mg kg-1 DM) indicating very low content. The results obtained showed that HSAD coupled with N stripping allowed transforming sewage sludge into fertilizers and soil improvers exploitable in agriculture.