Chlorella sp. growth under batch and fed-batch conditions with effluent recycling when treating the effluent of food waste anaerobic digestate

Anaerobic digestion (AD) of food waste diverts organic waste from landfills, generates sustainable baseload energy, and potentially an ecotoxic ammonia-rich digestate that requires post-treatment. Successful application of algal-based technology to treating high-ammonia AD effluents can be achieved by freshwater dilution. However, dilution of high-strength effluents with freshwater is currently unsustainable. Here, the feasibility of growing Chlorella sp. on the effluent of food waste anaerobic digestate with high ammonia content under recycling of the treated effluent was investigated for nutrient management and biomass production. The performance of the Chlorella sp. cultivated in repeated batch with effluent recycling (BR) and without recycling (BNR) was compared with repeated fed-batch mode with recycling (FR) and without recycling (FNR). Maximum cell density (6.1 × 10⁷ cells mL⁻¹) corresponding to the highest chlorophyll a (23.3 ± 1.3 mg L⁻¹) content was found in the FNR. Ammonia removal rates were not significantly different among all tested treatments. In all treatments, the analysis of the operating efficiency of PSII photochemistry (F_q′/F_m′) of the culture showed values > 0.5, indicating cells were not subjected to physiological stress. Harvested Chlorella biomass composition showed no variation in the contents of total protein, carbohydrate, and lipids. Turbidity increase in cultures with effluent recycling versus without recycling was negligible (5%), demonstrating the suitability of effluent recycling in the microalgae-based treatment of high-strength ammonia food waste digestate.