Environmental and economic benefits of using microalgae grown in wastewater as biofertilizer for lettuce cultivation

Mineral fertilizers play a significant role in environmental pollution of soil and water bodies, prompting the scientific community to search for more sustainable alternatives. However, environmentally-friendly organic options use to be more expensive. The objective of this study was to assess the potential environmental and economic impacts of biofertilizer from microalgae grown in wastewater, in a circular economy approach. To achieve this, a Life Cycle Assessment (LCA) was conducted to compare three scenarios: (i) lettuce cultivation using microalgae grown in wastewater as a biofertilizer; (ii) lettuce cultivation using mineral fertilizer and; (iii) lettuce cultivation using a combination of microalgae and mineral fertilizer. All scenarios considered the following steps: fertilizer production, transportation, fertilizer application. The scenario involving the wastewater grown microalgae reported better results in 10 out of 11 impact categories. Specifically, microalgae fertilizer only reported higher values for freshwater eutrophication, which could be reduced by improving the nutrients removal efficiency of the wastewater treatment facility. Moreover, an economic assessment was carried out to evaluate and compare the economic impact of the aforementioned scenarios. Results showed that the production and use of microalgal biofertilizers are an economically competitive alternative compared to the use of inorganic fertilizers, i.e. approximately 30% cheaper. On the whole, according to the results obtained in this study, wastewater grown microalgal biomass appears as a sustainable alternative to mineral fertilizers that may contribute to a greener agriculture.