Environmental sustainability of microalgae-based production systems: Roadmap and challenges towards the industrial implementation

Microalgae and cyanobacteria are a precious source for the production of biofuels/bioenergy, biomaterials and valuable biochemicals. Beyond photosynthetic CO2 conversion, microalgal systems can involve the valorisation of waste streams and the implementation of green chemistry, industrial symbiosis, and circular bioeconomy approaches. However, their sustainability is uncertain, thus their large-scale application is hindered. The numerous life cycle assessments (LCAs) performed so far are mostly based on data extrapolated from lab-scale experiments or the literature, leading to qualitative and controversial results. This paper reviews primary data-based LCA studies on microalgal pilot to industrial-scale plants. Sixteen studies satisfied the selection criteria, despite they used primary data almost exclusively for cultivation and harvesting. The outlined current status (methodology, inventory, energy performance and environmental impacts) highlighted the lack of uniformity in the applied methods and the presentation of results, as well as some lack of transparency. Nevertheless, the review concluded that electricity consumption and infrastructure are major hotspots. Therefore, the use of renewable energy for supplying the process and of sunlight for biomass photosynthesis should be preferred. The upstream processes produce large impacts. Thus, a suitable reactor, geographic location, and harvesting method should be selected. Biofuels are not competitive in most cases, but some promising multi-product biorefinery scenarios have been presented. To improve the environmental profile of microalgal high-value compounds (e.g., astaxanthin or biostimulants), co product valorisation, waste stream utilization, renewable energy deployment, and compound productivity should be enhanced. More efforts on LCA of large-scale plants are required, especially looking at integrated biorefinery concepts, to take a crucial step towards the implementation of sustainable commercial systems.