Microalgae Cultivation as a Method for Nutrient Removal and Valorisation of Liquid Digestate from Biogas

Liquid digestate from biogas production represents a nutrient-rich by-product that poses logistical and environmental challenges. This study investigated the suitability of liquid digestate derived from fish sludge as a growth medium for the freshwater microalga Chlorella sorokiniana (UTEX B2805), with the aim of evaluating its tolerance to high ammonium concentrations, nutritional value, and nutrient removal capacity. Small-scale screening experiments were conducted using different digestate concentrations and light intensities, followed by cultivation in 1 L photobioreactors in 50 % and 100 % digestate at high light intensity. The results showed that C. sorokiniana was able to grow in undiluted digestate containing very high ammonium concentrations, although high digestate concentrations caused prolonged lag phases and reduced photosynthetic efficiency during the initial cultivation period. The photosynthetic performance of the algae recovered during the exponential growth phase, resulting in biomass with a high total amino acid content (53–59 % of dry weight), with all nutrients necessary for algae growth provided by the digestate. Microalgae cultivation resulted in substantial reductions of ammonium (26–28 %), chemical oxygen demand (43–53 %) and total organic carbon (31–42 %) concentrations within 5–10 days. These findings demonstrate the potential of microalgae cultivation as a combined digestate treatment and biomass production strategy for biogas systems based on fish sludge.