Conference Dates

June 5 – 10, 2022


The digestate produced by anaerobic digestion process is a source of nutrients (N, P, K) and organic matter (C content), generally used as spreading in culture to ideally substitute for the use of mineral fertilizers. This type of digestate recovery presents some weaknesses as for example the poor effective substitution due to the relative low content of bio-available nutrients. Thenceforth, a higher benefit for the recovery of digestate could be thought to increase the effective use of nutrients. Instead of direct spreading, a higher quantity of the digestate and the CO2 contained in the biogas could be used in a micro-algae culture. To this end, the BIOMSA system is based on the coupling of anaerobic digestion and micro-algae culture (Figure 1). To assist the eco-design of BIOMSA, the life cycle assessment (LCA) methodology is used. The first system (A) was studied to identify the environmental hotspots according to the functional unit, defined as the anaerobic digestion of 25 000 tones/year of agricultural residues and the production of micro-algae from digestate, in order to highlight the performances of the whole system. The data collection and the inventory building are based on experts’ knowledge and experiments. As the first results show that the digestate spreading is the most impacting step, one of the eco-design options is to send more nutrients and CO2 in micro-algae culture to maximize the recovery through the micro-algae and not through direct spreading of digestate (liquid and solid phases). The system B is based on these modifications (Figure 1). So both systems (A and B) provide different functions and especially different quantities of micro-algae produced. In order, to allow a comparison between both systems, we studied several allocation rules based on the economic value of the outputs, their LHV (Low Heating Value) or based on their compositions in C, N, P. The novelty of our approach is the management of nutrients from digestate and C from biogas to increase the micro-algae production and to improve the effectiveness of the nutrients recovery through a biogas plant. The discussion of results focuses on the difficulty of overcoming the comparability, thanks to LCA, of both systems. The allocation rules used are difficult to perform because biogas, digestate and micro-algae are very different types of products with different C contents, nutrients contents, ICP and economic values. The combination of allocation rules could be a solution.

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