Microalgae cultivation in a biogas plant: Environmental and economic assessment through a life cycle approach

Using liquid digestate and flue gases from a microalgae cultivation in a biogas plant, enables nutrient and carbon uptake reducing the costs for nutrients for microalgae cultivation and reducing problems associated with the overproduction of digestate. The present study aims to examine the environmental impacts and economic aspects of a novel technology for microalgal cultivation. In particular, a combined LCA and LCC from a novel microalgae cultivation pilot system coupled with an operating biogas plant is proposed. The technological concept investigated aims to evaluate the potential benefits and drawbacks of the use of digestate and CHP flue gases from biogas a plant to sustain algal growth. Two scenarios were compared: i) a business -as -usual (i.e. baseline scenario) defined as a traditional biogas plant without microalgae cultivation, ii) a scenario with a scaled -up microalgae cultivation technology (Scenario 2) to highlight differences and evaluate the overall environmental and economic feasibility of a commercial scale. The comparison showed that the baseline scenario is slightly more advantageous in environmental performances than that associated with microalgae cultivation. The techno-economic assessment shows an economic feasibility with a positive NPV value (i.e. 2 573 935 euro) and good IRR (i.e. 14). The implementation of Scenario 2 with microalgae cultivation is found primarily in its relatively low operational costs as the thermal and electrical energy, digestate, and CO2 required for operation and algal growth come from the associated biogas plant. However, technical improvements should be considered to reduce the capital costs associated with the construction of microalgae cultivation plant.