A circular economy platform for treatment of wastewater by blue green microalgae

Water pollutions and increasing atmospheric CO2 levels are two critical environmental issues causing significant harmful effects on the environment and human health. Biological wastewater treatment using microalgae has been intensively studied in recent years. However, its implementation has been hindered due to challenges associated with the high costs of harvesting and drying steps. Therefore we aimed to design an integrated platform utilizing microalgae with the minimized cost of harvesting without any drying step. In our designed system, we will harness the power of blue-green microalgae due to their high capacity to sequester CO2, digest wastewater, and accumulate high concentrations of pigment (phycocyanin, a blue pigment). We will encapsulate cells in the first reactor to protect the cells from high concentrations of pollutants, while in the secondary reactor, filamentous microalgae can treat partly treated wastewater and then be flocculated by adjusting mixing speed. The growth of microalgae will be monitored by a respirometer, which estimates the algal growth kinetics based on the oxygen production and proton consumption rate. Subsequently, the flocculated biomass will be harvested and will undergo pigment extraction in water, as phycocyanin is a water-soluble pigment. The high phycocyanin price (approximately 221 Euro/kg) adds additional economic viability. The wet pigment-free biomass still contains compounds that can be anaerobically digested to biogas. The generated biogas can be utilized to provide energy for the system, while the emitted CO2 can be recycled back for microalgae cultivation. The digestate of microalgal biomass still contains carbonic material, which can be further transformed into high-value compounds such as bio-crude oil, biochar, and liquid phase through hydrothermal liquefaction. Thus, this project offers a circular economy platform that offers conversion of waste (wastewater and CO2 emissions) to high-value products.