Hydrothermal carbonization of Spirulina platensis and Chlorella vulgaris combined with protein isolation and struvite production

Abstract Microalgae have high energy potential, however, it is expensive to use them only for energy production. To improve the sustainability of microalgae utilization, in this study, hydrothermal carbonization was combined with the protein isolation and nutrient recovery, so that added-value co-products were generated together with the hydrochar. Through pH shifting, up to 63% of the microalgae protein was isolated. The protein removal resulted in an increase in the hydrochar and the carbon recovery yield, as well as a reduction in the nitrogen content of the hydrochar. This increases the fuel quality of the char. Moreover, the protein isolation tends to be more effective towards to Chlorella vulgaris than to Spirulina platensis . After the hydrothermal conversion, the majority of the phosphate‑phosphorus in microalgae was released into the process water, from which up to 85% of the total phosphorus was recovered as struvite.