Life cycle inventory of microalgae production in a real industrial plant

Microalgae are a valuable feedstock with great potential in a plethora of production routes, including valuable biochemicals (Braun and Colla, 2022), biofuels/bioenergy (Marangon et al., 2023), and biomaterials (Nanda and Bharadvaja, 2022). Microalgae-derived biochemical products could be applied in the nutraceutical, cosmetic, pharmaceutical, and feed/food sectors. However, the environmental sustainability of microalgal systems is uncertain. Indeed, life cycle assessments (LCAs) have been characterized by the adoption of heterogeneous methodologies. They are also affected by the use of hypothetical or lab-scale extrapolated data in most cases. Therefore, they have led to controversial results. This issue also regards the basic step of the analysis, i.e., the life cycle inventory (LCI). A few LCA studies have been performed so far with primary data from pilot- or industrial-scale plants. For example, Pérez-López et al. (2017) compared two photobioreactor (PBR) configurations (vertically stacked, Vst, and horizontal, Hor) and an open raceway pond (ORP) at pilot scale. Onorato and Rösch (2020) compared three commercial-scale PBR plants, i.e, Flat Panel Airlift (FPA), Green Wall Panel (GWP), and Unilayer Horizontal Tubular (UHT), with a cultivation volume of 93 m3, using primary data at that scale for the FPA and GWP PBRs. This work aims at compiling the LCI of an industrial-scale plant, thus taking a further step in the realistic evaluation of the environmental sustainability of microalgae production systems. Moreover, the results are compared with LCI data from the two abovementioned studies. The inventoried plant is installed in Caltagirone (Italy) within the facility of Plastica Alfa. Chlorella vulgaris is cultivated in a 42 m3 vertically stacked horizontal PBR system located in a greenhouse. The plant has a capacity of 1600 kgDW year–1 (DW = dry weight), corresponding to a productivity of ~0.1 gDW L–1 day–1. Microalgae are harvested via centrifugation at a concentration of 200 gDW L–1. Demineralized water, produced via reverse osmosis (RO) of tap water, is used for both cleaning and cultivation. The functional unit (FU) selected for all elementary input/output flows was 1 kgDW biomass. Energy, nutrients, water, chemicals, and infrastructure (PBR) materials were inventoried to characterize the plant operation and construction. The LCI was compiled by assuming 300 operating days per year, 10 years of lifetime for construction materials, and 5 complete operating cycles per year. The inventory was elaborated in a spreadsheet (Excel). Three subsystems were identified in the product system, i.e., reactor cleaning, cultivation (including inoculum), and harvesting. Results showed that the tap water consumption is 1.14 m3 kgDW–1, with a contribution of ~50% in both cleaning and cultivation. The main chemicals are citric acid (~4 kg kgDW–1) for cleaning and sodium bicarbonate (~1.7 kg kgDW–1) in the cultivation. The total energy consumption is 440 kWh kgDW–1, and its main contributions are pumping (210 kWh kgDW–1) and thermoregulation (163 kWh kgDW–1) in the cultivation phase, followed by lighting by LEDs (64 kWh kgDW–1). In contrast, cleaning and harvesting centrifugation play a marginal role in energy consumption. Plastic materials (mainly PMMA) amount to ~1.8 kg kgDW–1.