Cultivation of Nordic Chlorococcum sp. in anaerobic digestion effluent: Effects of CO2 concentration and reactor configuration

The increasing discharge of untreated wastewater poses risks to ecosystems and public health, necessitating sustainable treatment strategies. Anaerobic digestion (AD) of sewage sludge offers several benefits including waste-volume reduction and sludge stabilization. However, it produces nutrient-rich effluents, requiring further treatment. Microalgae can remove nutrients while generating valuable biomass. This study aimed to evaluate the effect of CO2 concentration and reactor configuration on the performance of Chlorococcum sp. cultivated in AD effluent of municipal sewage sludge. Four CO2 levels (0.04, 3, 6, and 9%) was tested and 6% CO2 yielded the highest biomass (0.98 g L− 1) and CO2 fixation rate (162 mg L− 1 d− 1), while maintaining ammonium and phosphorous removal comparable to aeration with 3 and 9% CO2. This concentration was used in ALR, BC, and BC with carriers. The highest nutrient removal was achieved in BC, with 37.61% NH4⁺-N and 25.87% phosphorus reduction, whereas growth in ALR reached the highest cell density (81 × 106 cells mL− 1) in 9 days. Biomass composition was stable across reactors, with similar protein, carbohydrate, or fatty acid methyl esters content. These findings demonstrate that the Nordic Chlorococcum sp. grown in AD effluent can remove NH4⁺-N and phosphorus across a wide CO2 range (0.04–9%). Culturing in ALR is the preferred option for rapid growth. However, BC offered better nutrient removal and higher biomass production but required longer cultivation than ALR.