Comparative evaluation of microalgae-activated sludge cocultivation for municipal wastewater treatment using four microalgal species under nonaerated batch conditions

ABSTRACT

Diagram illustrating evaluation of four microalgae species in activated sludge co-cultivation by comparing treatment performance, dissolved oxygen, nutrient removal, and biomass production.

Microalgae-activated sludge cocultivation is a promising low-energy alternative to conventional activated sludge processes; however, it remains unclear whether the beneficial functions of these systems are universal across phylogenetically different microalgal species or strongly species-specific. Here, we conducted controlled short-term comparative evaluation of four distinct microalgae (Chlorella vulgaris, Chlorella sorokiniana, Chlamydomonas reinhardtii, and Euglena gracilis) cocultivated with activated sludge in municipal wastewater under nonaerated conditions for 24 h. Across all species, all cocultures rapidly increased dissolved oxygen and removed approximately 90% of dissolved organic carbon, with removal rates of 0.087–0.091 h−1, comparable with activated sludge alone and significantly higher than microalgae alone. Ammonium was rapidly removed, and transient nitrite and nitrate accumulation decreased to low or undetectable levels, consistent with coupled bacterial nitrification and microalgal assimilation. Total nitrogen removal rates and total phosphorus removal rates were 0.07–0.14 and 0.14–0.20 h−1, respectively, and significantly higher than activated sludge alone. Biomass production was enhanced by 1.2–1.3 times within 24 h. Overall, all four phylogenetically distinct microalgae exhibited broadly similar functional behavior, suggesting functional universality of microalgae-activated sludge cocultivation under short-term controlled conditions and supporting its potential for low-carbon wastewater treatment.