Sustainable Microalgae Cultivation for Biogas Production: A Technical and Economic Analysis

Abstract

Microalgae cultivation using waste streams as nutrient source in industrial scale is an unconventional, and so far untapped, process pathway to produce microalgae biomass. The main advantage of this approach is the fact no resources are spent on nutrients, which make up for the major part of the conventional production costs. The main limitation is that culture medium is directly influenced by the available in-site waste streams, which limits the application of this to industrial processes with nutrient-rich and toxin free waste streams. There is also a degree of limitation with respect to specific nutrient concentration towards cultivation optimization. Optimization of biomass productivity implies knowledge about the behavior of each specific microalgae strain towards process parameters such as illumination, temperature, oxygen gas concentration in the culture medium, nutrient availability, pH value, hydraulic retention time, mixing and heavy metal presence. The use of the cultivated microalgae to produce methane through anaerobic digestion also put emphasis on the specific chemical composition of microalgae. This study was done in partnership with Biokraft AS and it integrated in the innovation projects in business and industry COMPLETE. The purpose of this study is to investigate the effect of the aforementioned parameters in microalgae cultivation and find out patterns to act as general guidelines for process optimization. It also an objective of the this work to evaluate laboratory and pilot-scale applications of conventional and wastewater-based microalgae cultivation towards the establishment of an industrial-scale cultivation model, whose profitability under a set of conditions will be calculated and evaluated. The produced work highlights that there is interest in the scientific community about the advantages of using wastewater-based microalgae cultivation systems, especially because of their potential as a biological wastewater treatment with reduce energetic consumption. It was also found that the cultivation technology that best suits the specific needs of this project is an internally illuminated photobioreactor that will keep production steady and independent from the variable solar irradiance in Norway. Furthermore, it was found that the most cost-effective solution for harvesting the cultivated biomass is electrocoagulation.