Engineering microalgal-activated sludge symbiosis in a 3D-printed biocarrier for advanced wastewater treatment

Conventional microbial immobilization technologies, such as biofilm media and sludge granules, are widely used in wastewater treatment. However, they rely on natural microbial attachment and aggregation, making them sensitive to environmental fluctuations. In contrast, 3D microbial printing enables the construction of complex, artificial structures embedded with living microorganisms, overcoming the limitations of conventional methods. This study provides a proof-of-concept demonstration of a strategically designed microalgae-activated sludge co-culture biocarrier fabricated via 3D bioprinting. Optimal printing conditions were a 22G needle and a speed of 10 mm/s. The carrier size was set to 5 layers based on mono-culture biocarrier characterization. To promote symbiotic interactions, microalgae were positioned in the outer layers to enhance photosynthesis and in the middle layer to facilitate oxygen supply to the activated sludge, which was distributed in the inner region, forming a stacked-type co-culture biocarrier. This biocarrier, constructed based on the selected conditions and strategy, provided a critical proof-of-concept for strategic microbial immobilization. Compared to the mixed-type, in which both microorganisms were homogeneously blended, it achieved a 7–8% increase in the removal efficiencies of soluble nitrogen and phosphorus and an approximately two-fold enhancement in microalgal growth. These results demonstrate that the structured configuration offers a more favorable environment for microbial interaction and proliferation.